The two main functions of sensors are navigational and tactical.

Navigational sensors are used by the astrogator to determine the spacecraft's current position, vector, and heading. They are also used by the pilot to perform the maneuvers calculated by the astrogator. Arguably a chronometer or other instrument to locate the spacecraft's current position in time is also a navigational sensor.

Tactical sensors are used to watch the the region around the spacecraft. This is mostly to monitor nearby objects (such as meteors on a collision course or enemy spacecraft). Arguably this also includes solar-storm warnings which detect deadly incoming proton events.

Navigational and Tactical sensors are generally found on all spacecraft, unless the designer is trying really hard to economize. There are some more specialized sensors only found on more specialized spacecraft:

Remote Sensing suites are used to scan and analyze the surface of a planet, moon, or asteroid. These are found on specialized spacecraft such as exploration vessels, mine prospecting ships, survey ships, customs and other hunter-type ships, and spy ships.

Combat sensors have two main types. Strategic combat sensors detect hostile spacecraft at long range, giving advanced warning of enemy attack. (remember that There Ain't No Stealth In Space). Tactical combat sensors work at close range in a battle, guiding your weapons to the enemy targets (a "firing solution"), detecting incoming enemy weapons, and analyzing the enemy for weakness.

There are two broad classes of sensors: passive and active.

Passive sensors just detect any emissions from the target, i.e., they passively look for the target. Passive sensors include telescopes and heat sensors.

Active sensors emit various frequencies and detect their reflection off the target, i.e., they actively "shine a light" on the target. Active sensors include radar and lidar/ladar.

In some SF novels, passive sensors are called "sensors" while active sensors are called "scanners." Apparently sensor is a real term for detection equipment, but scanner is only in science fiction. Nowadays a scanner is usually encountered as a "flatbed image scanner", "supermarket checkout scanner", or "barcode scanner." The earliest scanner reference in science fiction that I managed to find was Roger Manning's "radar scanner" in Tom Corbett Space Cadet: Stand by for Mars! (1952).

Note that in a combat situation, using active sensors allows you to be instantly detected and targeted by hostile spacecraft. If you don't care if you are detected, or if they already know you are there, sensors can reveal valuable information about hostile spacecraft.


IT WAS ON ITS return sweep, as it came hurtling out of the black depths, that the Master’s Patrol Vessel picked up the Ramdic radiations that could only be an attempt at communication. Stars leaped as the huge nonreflecting surfaced ship, driven by the power equivalent of a half dozen suns, raced for the source of those radiations.

Twenty miles of ultra-pressurized seamless-hulled alloyed metal, ten miles in diameter, came to a halt over the radiating object. Like a large black egg it hovered over the offending object which was buried beneath the surface of Earth.

Invisible probe rays flashed out, passing through the vehicle’s hull as if it weren’t there. The buried vessel was analyzed down to each particle of errant dust. The languages of the system’s inhabitants were plucked from the minds of the thousand collected specimens in cold vaults within the Master’s craft. Computers went through the scores of major languages and silently broke the code. Then the broadcasting vessel was vaporized, leaving only a wisp of fast dispersing white mist.

The message was totally without purpose; therefore the strange, lizard-like creature in charge reasoned that the ship had not been left as a purposeful transmitter. He smelled a trap; someone had wanted to attract him while that someone observed from a safe position.

The creature set up the computers to search the system of Sol cubic yard by cubic yard. Only minutes elapsed before they found the prey hiding in a lunar crater. The patrol vessel swooped down, instruments measuring and analyzing defensive and offensive capabilities of the ship. The information was noted and sneered at-billions of years old his species might have had a ship like it.

In microseconds every map, book, tape, pad and drawing on the ship was in the memory banks of the Master’s computer. Then the Master unleased the mind probes; they bored in, scooping up whatever thoughts and information they could detect from the poorly shielded life-force.

At last, all the mighty armament of the Crusader was loosed, but the Master’s screens barely registered the attack. The creature calmly continued his examination of the crew of the Terran ship. When the probes were finished, the creature touched a button and the Crusader followed the unmanned decoy into oblivion.

His mind automatically absorbed the findings of his computers as everything on the other ship, down to the last molecule, was analyzed and compared against Star King standard and mutation-probability charts.

The capture of a Masters’ ship was utterly impossible; yet V-101 merely sat there, a slight smile upon his face, as if he had done something of no difficulty. Bronson could not understand how it had been accomplished.

“Guess,” he insisted. “How would you capture a Masters’ ship?”

Bronson thought, puffing furiously on his pipe. But it was just impossible! Every fact that the computer held was reviewed and fitted together in every imaginable combination. All he came up with was that a Masters’ ship of any class was faster and more maneuverable, and packed more fire power than all the ships of the Star Kings combined.

Therefore, by deduction, that left an abandoned vessel—which didn’t make any sense.

“That’s the answer!” cried Bronson. “Their ships are shielded against every type of radiation known to us at present. Somehow you found a chink in their armor and duplicated the ship.”

V-101 smiled, eyes far away, as if looking at vistas such as Bronson could never know.

“It was quite simple. The technique has been around since before the United Stars was destroyed. The Masters’ ships are shielded to screen out all radiation and to absorb all detection rays, but there is one thing nothing can screen out: gravity. They can, as we do, minimize its effect, but they can’t ignore completely the basic force of the Universe.

The United Stars had gravity generators. I merely adjusted one to produce a particle gravity, extremely weak, so as to escape detection, that bends in proportion to the mass of the substance acted upon. A thousand such generators and receivers placed to form a huge sphere through which a ship passing gave all the data my computer needed to compute composition and design of that ship.

The C-S headgear rested lightly on Deal, almost an extension of his own body. He seemed to hear the distant chittering of myriad tiny insects as the cells of the headgear compared stimuli from the drifting life craft. He smiled to himself as the origin of the little craft was traced through its cellular history, down to the very origin of the mineral ores.

From CROWN OF INFINITY by John Faucette (1968)

A hundred million miles beyond Mars, in the cold loneliness where no man had yet traveled, Deep Space Monitor 79 drifted slowly among the tangled orbits of the asteroids. For three years it had fulfilled its mission flawlessly — a tribute to the American scientists who had designed it, the British engineers who had built it, the Russian technicians who had launched it. A delicate spider’s-web of antennas sampled the passing waves of radio noise — the ceaseless crackle and hiss of what Pascal, in a far simpler age, had naively called the “silence of infinite space.” Radiation detectors noted and analyzed incoming cosmic rays from the galaxy and points beyond; neutron and X-ray telescopes kept watch on strange stars that no human eye would ever see; magnetometers observed the gusts and hurricanes of the solar winds, as the Sun breathed million-mile-an-hour blasts of tenuous plasma into the faces of its circling children. All these things, and many others, were patiently noted by Deep Space Monitor 79, and recorded in its crystalline memory.

One of its antennas, by now unconsidered miracles of electronics, was always aimed at a point never far from the Sun. Every few months its distant target could have been seen, had there been any eye here to watch, as a bright star with a close, fainter companion; but most of the time it was lost in the solar glaze.

To that far-off planet Earth, every twenty-four hours, the monitor would send the information it had patiently garnered, packed neatly into one five-minute pulse. About a quarter of an hour later, traveling at the speed of light, that pulse would reach its destination. The machines whose duty it was would be waiting for it; they would amplify and record the signal, and add it to the thousands of miles of magnetic tape now stored in the vaults of the World Space Centers at Washington, Moscow, and Canberra.

Since the first satellites had orbited, almost fifty years earlier, trillions and quadrillions of pulses of information had been pouring down from space, to be stored against the day when they might contribute to the advance of knowledge. Only a minute fraction of all this raw material would ever be processed; but there was no way of telling what observation some scientist might wish to consult, ten, or fifty, or a hundred years from now. So everything had to be kept on file, stacked in endless air-conditioned galleries, triplicated at the three centers against the possibility of accidental loss. It was part of the real treasure of mankind, more valuable than all the gold locked uselessly away in bank vaults.

From 2001 A SPACE ODYSSEY by Arthur C. Clarke (1969)

Call the Belt? The Belt must know by now. The Belt telescope net tracked every ship in the system; the odds were that it would find any wrong-colored dot moving at the wrong speed. Brennan had expected them to find his own ship, had gambled that they wouldn't find it soon enough. Certainly they'd found the Outsider. Certainly they were watching it; and by virtue of that fact they must be watching Brennan too.

The Belt is a web of telescopes. Hundreds of thousands of them.

It has to be that way. Every ship carries a telescope. Every asteroid must be watched constantly, because asteroids can be perturbed from their orbits, and because a map of the solar system has to be up-to-date by seconds. The light of every fusion drive has to be watched. In crowded sectors ships can run through each other's exhausts if someone doesn't warn them away; and the exhaust from a fusion motor is deadly.

From PROTECTOR by Larry Niven (1973)

(ed note: the dreadnought Conqueror unexpectedly materializes at the foldpoint in the Valeria star system. The ship is apparently battle damaged and derelict, but the blasted thing is still accelerating and is currently traveling at 1,500 kilometers per second. The cruiser Discovery is trying to figure out how to safely stop the Conqueror before the Discovery runs out of reaction mass.)

      By the eighty-second hour after Conqueror’s appearance in the newly reconstituted foldpoint, all three major astronomical telescopes in the Valeria System were focused on the drama taking place half a billion kilometers above the ecliptic. The astronomers watched intently as the tiny constellation of violet-white stars continued to recede from the primary at 1500 kilometers per second. As they watched, they wondered when the brightest of the stars would go dark. They were not alone in their concern. Richard Drake wondered the same thing.
     For six hours, Discovery and its three scouts had matched the derelict starship’s movements precisely while they attempted to unlock its secrets. They had photographed the ship in a dozen wavelengths of visible light, scanned its flanks with side-looking radar and stereoptic lasers, and mapped it with thermographic and ultraviolet scanners. While they studied, they continued their communications attempts, sweeping Conqueror with tightly focused beams of electromagnetic radiation composed of every frequency that Discovery’s comm system was capable of generating. They even put a vacsuited spacer in one of the scouts’ airlocks with a high-powered rifle that belonged to an avid hunter among the crew. The spacer spent a fruitless hour bouncing high-speed projectiles off various parts of the starship in the hope that the resulting clanging noises would be carried by conduction through the ship’s structure to any surviving crewmembers.

     Barrett glanced at the screen. “What have we here?”
     “We were just about to review the thermographs.”
     “Looks like modern art. What can these tell you?”
     Drake gestured toward the holoscreen where the thermograph was displayed. “Conqueror isn’t really all that different from Discovery when it comes to its basic operating principles, Mr. Barrett. It has many evolutionary improvements built into it, but no major breakthroughs – at least, none that are obvious at the moment. Both ships use photon drives for getting around in normal space; and, presumably, both have similar jump systems for interstellar travel. Now, the heart of any starship is the mass converter. Shut down the mass converter and you shut down the drive. Normally that is done by shutting off the flow of fuel into the converter. Since we have no idea where that particular control is located aboard Conqueror, we are going to try something messier.
     “Mass converter fuel is deuterium-enriched hydrogen stored at cryogenic temperature. In Discovery, we carry our cryogen in tanks in our central cylinder. Conqueror’s cylindrical design suggests that we will find the main tankage complex aft. Once we’ve located the fuel tanks, we punch a few holes in them with lasers, the cryogen leaks out under the force of the ship’s acceleration, the fuel tanks run dry, and the converters should shut down automatically from fuel exhaustion.”
     “You hope,” Barrett said.
     “The theory is sound,” Drake replied.
     “So what are we doing with these thermographs?” Barrett asked.
     “Cryogen is stored as close to absolute zero as the engineers can manage. No matter how efficient the insulation on Conqueror’s tanks, there will be some cooling of the adjacent structures. Find the cool spots and you find the fuel.”
     “Got a tank, sir!” Technician Grandstaff said.
     “Where?” Drake asked, peering at the picture on the screen. The false colors shaded from scarlet (hot) near the operating drive, to light-blue (ambient temperature) over most of the hull, to indigo (cool) in a few spots. The technician had drawn a dashed, glowing line on the screen around one of the indigo sections.
     “I make it a cylindrical tank extending approximately to the ship’s midpoint, Captain.”
     Drake nodded. ”About thirty percent full, I’d guess.”
     “Yes, sir.”
     “How can you tell that?” Barrett asked.
     “From the temperature profile, sir,” Grandstaff said. “The cryogen is stored as a liquid, which the ship’s acceleration pulls toward the stern. Thus, only that part of the tank actually in contact with the fuel shows as indigo on the graph. If thirty percent of the tank length shows cooling, then that’s the percent of fuel remaining.”

Navigational Sensors

Navigational sensors are used by the astrogator to determine the spacecraft's current position, vector, and heading. They are also used by the pilot to perform the maneuvers calculated by the astrogator. Arguably a chronometer or other instrument to locate the spacecraft's current position in time is also a navigational sensor.

Navigational sensors include:

  • Periscopic Sextant: used to measure angles between celestial objects. From this the spacecraft's position can be calculated.
  • Telescope
  • Coleostat
  • Star Tracker
  • Star Scanner
  • Solar Tracker
  • Sun Sensor
  • Planetary Limb Tracker
  • Planetery Limb Sensor
  • Doppler Radar
  • Inertial Tracker Platform
  • Pulsar Positioning System

So, let’s now turn to the topic of sensors, and what exactly is in that Cilmínár Spaceworks AE-35 “standard navigational sensor suite” built into the majority of current-era starships. There are two primary groups of sensors incorporated into such a suite, referred to as “navigational” and “tactical” – even on civilian vessels – sensors, respectively, along with cross-feeds from the communications systems.


The first of these groups, the navigational sensors, are those primarily used to locate the starship itself in space and, to a lesser extent, time. Included in a standard suite are the following:

Orbital Positioning System

The Orbital Positioning System, considered the primary source of navigational data within settled space, makes use of beacons located on stargates, and orbiting in designated positions within the system, each broadcasting a unique identifying code and sequence signal. By correlating signals received from these satellites with the reference data published in astrogators’ ephemerides, or downloaded from the stargate navigation buoy on system entry, a starship’s position within the inner and the majority of the outer system can be determined precisely, although accuracy does fall off as the Shards are reached.

Star Tracker

As a backup to the Orbital Positioning System, and as the primary method of navigation in undeveloped star systems, the navigational suite includes a star tracker. This system maintains a sunlock, a continuous bearing to the local system primary, and a number of starlocks, continuous bearings to a number of well-known nearby stars, identified spectrometrically. Again, by correlating these bearings with ephemeris data, a starship’s position can be determined with considerable accuracy.

Pulsar Navigational Reference

A final backup is provided by the Pulsar Navigational Reference, which maintains continuous bearings to a number of pulsars located within the local galaxy, using the same principles as the star tracker. While unsuitable for fine navigation (due to the low available parallax of such distant reference points) it is of use in providing confirmatory gross position data.

Inertial Tracking Platform

The inertial tracking platform provides a continuous check on all other forms of navigation, a bridge during switches between beacons and starlocks, and a navigational reference for fine maneuvering; using a complex of accelerometers and gyroscopes linked to the starship’s drive systems, the ITP integrates angular velocity and linear acceleration into a continuous record of change of position and change of velocity. In the latter role, it operates alongside the timebase receiver to provide the relativistics officer with the information required to differentiate wall-clock time and empire time.

Imperial Timebase Receiver

The timebase receiver receives the continuous timebase reference signal transmitted by all stargates, based on their temporal consensus, which defines the empire time reference frame: i.e., the pseudo-absolute time frame without reference to the relativistic maneuvering of individual starships (or indeed celestial bodies); it provides an external temporal reference separate from the starship’s internal wall-clock time.

– Technarch Apt’s How-It-Works: Starships

Periscopic Sextant

The periscopic sextant is used to measure the angle between celestial objects for navigational purposes.

NASA's Apollo had two periscopes: the Space Sextant and the Scanning Telescope. The sextant had a magnification of 28x, the scanning telescope had no magnification but had a wide field of view.

A terrestrial sextant just measures one angle: between the sun or a star and the horizon, or the "altitude". A space sextant measures two angle between two stars or other objects: the radial coordinate and the angular coordinate. This is called "taking a shot" or "shooting the stars".

The sextant's main axis is called the "shaft axis", and is perpendicular to the hull. The sextant's line of sight could rotate around the shaft axis (azimuth or shaft-angle) or rotate up to 50° off axis (trunnion angle). Basically it could aim its line of sight at anything within a conical area up to 50° of the shaft axis.

When the astrogator looks through the sextant they will see the two stars, by rotating the sextant around the shaft angle and the trunnion angle the two stars are superimposed. Then the sextant will reveal the coordinate angles. The shot has been taken. In the Apollo, you press the "mark" button to make the guidance computer record the navigation angle and the time of the shot.

What is happening is that the view through the sextant is optically combining two views: from a fixed sextant scope and a movable ("indexing") sextant scope. Rotation controls are used to yaw, pitch, and roll the ship until Star Alfa (the "Landmark") is in the center of the fixed view. The astrogator then moves the movable sexant through shaft and trunnion rotation until Star Bravo overlaps Star Alfa.

For the Apollo missions, the angle between certain guide stars and the edge of Terra gave the current position and the distance from Terra. As successive shots are taken of the same landmarks and stars, the guidance computer would use something called the "Kalman Optimum Recursive Filter Formulation" to calculate the spacecraft's current position and velocity.

One task using the sextant was to correct the drift of the inertial platform. Under computer control (I use the word "computer" with some reservation, the Apollo computer was stupider than your average smart-phone) you'd tell it to align the sextant on one of the pre-programmed guide stars as Star Alfa. In the center of the fixed view is where the intertial thinks that star Alfa is. The center is helpfully marked by a reticle, since if the inertial platform has drifted Star Alfa ain't gonna be there.

You look through the movable sextant, also looking for Star Alfa, as Star Bravo so to speak. If you are incredibly lucky and there is no drift, when the movable sextant is at zero-radial and zero-angular the Star Alfa/Bravo will be bang in the center of the reticle. If you are sort of lucky the drift was small enough that the guide star is actually within 50° of the shaft axis. If you were flat out of luck the guide star would not be visible at all.

You then spin the movable sectant on its shaft angle and trunnion angle to put Star Alfa/Bravo in the center on the reticle (if the star is not visible you have to rotate the spacecraft first). Then you'd press the "apply correction" button. The computer would calculate the separation angle between the inertial and actual position, and display it. You now press the "accept" or the "reject" button depending upon whether you agreed with the angle. If you accepted it, the computed would then re-align the intertial platform as per the correction.

The author's father, Major Winchell D. Chung, (ret.) used to be the navigator/bombardier officer on a SAC B-52 aircraft. He wrote the following notes about using a periscope sextant:


We did have a periscopic sextant that we installed and shot the stars for navigation. This was installed in a sextant mount that had an azimuth ring so you could tell what direction that you were looking and the sextant had internal workings (that you controlled) for elevation so you could read the elevation of the heavenly body you were shooting. The ring mount was located on the top of the forward cabin along the center line of the aircraft. The sextant only revolved in one plane.

The other optical device that looked out of the B-52 was the optical bombsight located in the radar operators compartment. You placed the crosshair on the object using the tracking handle and the crosshairs would remain on the object until you moved it. They were tied into the radar crosshair so both crosshairs would be on the same object. One favorite trick to pull on a new crewmember was to place the crosshairs on something just ahead of the aircraft and let the new crew member look through the optics. The crosshairs would track the object and when you passed over the object the optical crosshairs would rotate 180 degrees to track the object as you flew away from it. You can imagine what a sensation it was when the nice stable crosshairs all of a sudden spun around. It tended to make ones stomach turn too.

The nav compartment was wide enough for two ejection seats to be side by side with enough space between them for a person to squeeze in to get into the seat. The navigator sat on the right and had a desk to work on and the radar had no desk, just a small flat surface with a tracking handle to move the crosshairs. The optical bomb sight was between his legs so by leaning forward he could look through the optics. The single tracking handle moved both the radar cross hairs and the optical crosshairs.

If you crawled up the front hatch to get into the BUF you were climbing on the navigators escape hatch. When you ejected the hatch blew a way first and then the seat fired down. My guess is that the ejection seat was about 30 inches wide.

(Author's note: this is why you keep your elbows tucked in when ejecting, unless you want your arms ripped off. The natural tendency is to pull the ejection trigger from the floor with both elbows pointed to the left and right. The correct procedure is to pull with the elbows pointed at your lap. Anything that extends past the 30 inch footprint of the seat will be sheared off by the edged of the hatch.)

In order to shoot a star first you picked an assumed position ahead of you. The time you decide that you will be at the assumed position will be the mid time of the shot and the time of the fix. Then using the coordinates of the assumed position and the time that you will be there, you calculate the azimuth and elevation of the star using the Air Almanac and the Star Tables. Then you go upstairs, put the sextant in to the azimuth ring carefully, remembering that the cabin is pressurized and the outside is not. Things tend to be attracted by the suction. If your calculations were good and your assumed position was good, you could assume the brightest star in the view finder is the star. You could then put your optical crosshair on it and keep it on it for two minutes, starting the shot 1 minute before the fix time and ending one minute after the fix time. The little knob that you use with your thumb to keep the crosshair on the star, averaged the readings automatically.

The reading that you needed to tell where you were is the elevation of the star. The aircraft does not fly at a constant altitude, it rises and falls in a regular cycle that is approximately two minutes in length. That is why you need the average reading of the elevation.

Usually the EWO (Electronic Warfare Officer) did the shooting and would call down the readings to the Nav. You normally do this with three stars about 120 degrees apart and the resulting three lines will give you your position at the time of the fix.

Major Winchell D. Chung, (ret.)

Norm the bomberguy tells me that the hand written notes on the above images are incorrect.

As a B-52 pilot, I can assure you the photos are labeled incorrectly. The cathode ray tubes in each photo are on the front panel, not the side panels as the handwritten notes indicate. The left photo is a shot of the navigator's station. The panel labeled "left side" is actually the front, and the panel labeled "front" is the right side panel. The right photo shows the radar navigator's station. The panel labeled "rt. side" is the front, and the panel labeled "front" is the left side panel.




When the Skysprite locked in with Supra-New York, Pemberton went to the station’s stellar navigation room. He was pleased to find Shorty Weinstein, the computer, on duty. Jake trusted Shorty’s computations—a good thing when your ship, your passengers, and your own skin depend thereon. Pemberton had to be a better than average mathematician himself in order to be a pilot; his own limited talent made him appreciate the genius of those who computed the orbits.

"Hot Pilot Pemberton, the Scourge of the Spaceways — Hi!" Weinstein handed him a sheet of paper.

Jake looked at it, then looked amazed. "Hey, Shorty—you’ve made a mistake."

"Huh? Impossible. Mabel can’t make mistakes." Weinstein gestured at the giant astrogation computer filling the far wall.

"You made a mistake. You gave me an easy fix — ‘Vega, Antares, Regulus.’ You make things easy for the pilot and your guild’ll chuck you out." Weinstein looked sheepish but pleased.

Pemberton fed Weinstein’s tape into the robot-pilot, then turned to Kelly. "Control ready, sir."

"Blast when ready, Pilot." Kelly felt relieved when he heard himself make the irrevocable decision.

Pemberton signaled the Station to cast loose. The great ship was nudged out by an expanding pneumatic ram until she swam in space a thousand feet away, secured by a single line. He then turned the ship to its blast-off direction by causing a flywheel, mounted on gimbals at the ship’s center of gravity, to spin rapidly. The ship spun slowly in the opposite direction, by grace of Newton’s Third Law of Motion.

Guided by the tape, the robot-pilot tilted prisms of the pilot’s periscope (coelostat) so that Vega, Antares, and Regulus would shine as one image when the ship was headed right; Pemberton nursed the ship to that heading … fussily; a mistake of one minute of arc here meant two hundred miles at destination.

When the three images made a pinpoint, he stopped the flywheels and locked in the gyros. He then checked the heading of his ship by direct observation of each of the stars, just as a salt-water skipper uses a sextant, but with incomparably more accurate instruments. This told him nothing about the correctness of the course Weinstein had ordered—he had to take that as Gospel—but it assured him that the robot and its tape were behaving as planned. Satisfied, he cast off the last line.

Seven minutes to go—Pemberton flipped the switch permitting the robot-pilot to blast away when its clock told it to. He waited, hands poised over the manual controls, ready to take over if the robot failed, and felt the old, inescapable sick excitement building up inside him.

He caught a last look through the periscope. Antares seemed to have drifted. He unclutched the gyro, tilted and spun the flywheel, braking it savagely to a stop a moment later. The image was again a pinpoint. He could not have explained what he did: it was virtuosity, exact juggling, beyond textbook and classroom.

Twenty seconds … across the chronometer’s face beads of light trickled the seconds away while he tensed, ready to fire by hand, or even to disconnect and refuse the trip if his judgment told him to. A too-cautious decision might cause Lloyds’ to cancel his bond; a reckless decision could cost his license or even his life—and others.

But he was not thinking of underwriters and licenses, nor even of lives. In truth he was not thinking at all; he was feeling, feeling his ship, as if his nerve ends extended into every part of her. Five seconds … the safety disconnects clicked out. Four seconds … three seconds… two seconds… one—He was stabbing at the hand-fire button when the roar hit him.

Kelly relaxed to the pseudo-gravity of the blast and watched. Pemberton was soberly busy, scanning dials, noting time, checking his progress by radar bounced off Supra-New York.

Weinstein’s figures, robot-pilot, the ship itself, all were clicking together.

Minutes later, the critical instant neared when the robot should cut the jets. Pemberton poised a finger over the hand cut-off, while splitting his attention among radarscope, accelerometer, periscope, and chronometer. One instant they were roaring along on the jets; the next split second the ship was in free orbit, plunging silently toward the Moon. So perfectly matched were human and robot that Pemberton himself did not know which had cut the power.

From SPACE JOCKEY by Robert Heinlein (1947)

Inertia Properies


A research project led by New Mexico State University Professor Ou Ma that began in 2008 holds scientific, military and commercial promise for the growing use of satellites.

The innovative technology aims to support a growing need to develop satellite servicing capabilities that can extend the lifespan of existing satellites, support the assembly of large structures on orbit, and mitigate orbital debris. These advances can make spaceflight more efficient, sustainable and cost effective.

Ma, from the Mechanical and Aerospace Engineering Department, and his students have been developing an algorithm for identifying the inertia properties of a spacecraft while in orbit.

"The inertia properties, such as mass, location of mass center and moments of inertia, of a spacecraft can change in orbit due to fuel consumption, changes in position of hardware, payload deployments or payload capture. When this happens, the spacecraft's control system needs to know the changes to maintain proper control of the spacecraft," said Ma, who is the first recipient of the John Kaichiro Nakayama and Tome Miyaguchi Nakayama Professorship for Research Excellence.

Ma's Inertial Property Algorithm Verification (IPAV) Project identifies the inertia properties of a spacecraft based on the impulse-momentum principal by incorporating the novel use of an onboard robotic arm. A large advantage of using a robotic arm to identify inertia, over existing methods, is that it is powered by renewable solar, reducing the use of critical fuel consumption by the satellite.

Tactical Sensors

Tactical sensors are used to watch the the region around the spacecraft. This is mostly to monitor nearby objects (such as meteors on a collision course, enemy spacecraft, or incoming weapons). Arguably this also includes solar-storm warnings which detect deadly incoming proton events.


So, let’s now turn to the topic of sensors, and what exactly is in that Cilmínár Spaceworks AE-35 “standard navigational sensor suite” built into the majority of current-era starships. There are two primary groups of sensors incorporated into such a suite, referred to as “navigational” and “tactical” – even on civilian vessels – sensors, respectively, along with cross-feeds from the communications systems.


The second of these groups, the tactical sensors, are those which concern themselves rather with the environment around the starship than with the starship’s position. Those commonly included, which is to say ignoring specialized scientific sensors, include:

All-Sky Passive EM Array

The sensor with the most general utility is assuredly the all-sky passive EM array. The latest ASPEMA designs consist of a complex array of receptor elements woven through the outer layers of much of a starship’s hull surface, operating together to function as a single large sensor. An ASPEMA’s elements are designed to maintain a consistent watch across the majority of the EM spectrum, from low-frequency RF through infrared, visible light, and up to gamma-rays. A properly configured ASPEMA gives the sensor operator a clear, moderate-resolution view of everything radiating EM within the star system, which is everything worth speaking of.

Passive EM Telescope

When higher resolution is required for identification, or profiling of a target or its emissions, the sensor suite also incorporates one or more passive EM telescopes capable of significantly higher resolution and sensitivity which can be pointed at specific targets.

Active EM Radar

When precise ranging is called for, or the emissions of the target are insufficient to permit profiling with the passive EM telescope, it is possible to “go active”. The active EM radar usually makes use of the same reception hardware as the passive EM telescope; it merely transmits a directional RF pulse and receives its reflection from the target, the time of travel providing the ranging information. The disadvantage of this technique, of course, is that the use of the active EM radar announces one’s presence to all other starships in the system, even beyond typical passive detection ranges.


The gravitometer provides an effective and highly sensitive way to measure the local degree of space-time curvature, both absolute and differential. This can be used to provide a variety of information, including current depth with the gravity well (or altitude) when near objects of known mass, bearings to high-mass objects, and detection profiles of gravity waves, including those generated when a stargate is used by another starship in-system.

Neutrino Detector

The neutrino detector chiefly provides supplementary information. Nucleonic reactions are rich sources of neutrinos; as such, other than when swamped by stellar emissions, neutrino emissions can indicate the presence of operating fusion reactors, torch drives, or other nucleonic equipment commonly found aboard starships. While providing limited additional data on its own, although aiding in the profiling of starships by their power plant, it has the advantage over other sensors that neutrinos interact very little with other matter, and as such the neutrino detector can determine the presence of a signal otherwise occluded by a lunar or planetary body.

Docking Radar

A high-frequency omnidirectional radar system designed for use at short range, the docking radar is a specialized radar system intended to provide precise location and range information while docking, operating near habitats, or otherwise in crowded orbits.

Imaging Lidar Grid

Offering a significantly higher resolution than radar, a starship lidar grid is primarily used for two purposes: first, producing a surface map of asteroids or potential landing sites, or a hull map of an unidentified vessel or hulk to look up in the database; and second, since being hit with a high-intensity lidar pulse will overwhelm most EM-based sensors and can even trigger hull thermal alarms, as a very effective way to yell “Hey, stupid!” at starships which aren’t answering standard hails.

Communications (integrated)

Fed across from the communications subsystem are two other important sources of navigational data:


The first of these is transponder data received from other starships. A transponder broadcast must include that starship’s identity, the current time, and certain important parameters (safety distance from its drives, whether it’s carrying certain hazardous cargoes, registration, and so forth). Ships currently operating under positive control include a subcode – a “squawk” – designated by space traffic control authorities for their reference, and a transponder can also signal various status codes, indicating distress situations in progress, communications failures, hijackings, and other such. The majority of transponders also transmit the ship’s own determination of its position.

IIP Interface

While an extranet feed may seem frivolous for astrogation purposes, it is an essential feature of…


The most notable common characteristics of all of these sensor systems is that they operate at the speed of light, or more slowly, and that they operate from a single point in space, which imposes a tremendous limitation on what information an astrogator may have available. The solution to this is longscan.

Using defined extranet protocols, cooperating starships broadcast their sensory gestalt to other starships in the system via the standard IIP communications relays, as do other sources of sensor data, such as habitats, stargates, and navigation satellites. Using this information, along with predictive AI and astrogator-assisted extrapolations of what each starship or other object visible has done or will do since the last update, the longscan system on each starship produces an overview of the current situation including that information which that ship could not itself sense, or which is still in transit to it, duly annotated with probability and reliability estimates for their future actions.

– Technarch Apt’s How-It-Works: Starships


The moon, now visibly larger and almost painfully beautiful, hung in the same position in the sky, such that he had to let his gaze drop as he lay in the chair in order to return its stare. This bothered him for a moment -- how were they ever to reach the moon if the moon did not draw toward the point where they were aiming?

It would not have bothered Morrie, trained as he was in a pilot's knowledge of collision bearings, interception courses, and the like. But, since it appeared to run contrary to common sense, Art worried about it until he managed to visualize the situation somewhat thus: if a car is speeding for a railroad crossing and a train is approaching from the left, so that their combined speeds will bring about a wreck, then the bearing of the locomotive from the automobile will not change, right up to the moment of the collision.

It was a simple matter of similar triangles, easy to see with a diagram but hard to keep straight in the head. The moon was speeding to their meeting place at about 2000 miles an hour, yet she would never change direction; she would simply grow and grow and grow until she filled the whole sky.

From ROCKET SHIP GALILEO by Robert Heinlein. 1947.

Gravity Gradiometer

These are used to detect object by their gravity. Typically in science fiction, they detect objects that are invisible (usually in science fiction written before the invention of radar). Dr. Robert Forward actually invented such a detector. He suggests using it to detect asteroid-mass black holes lurking in the center of asteroids.


(ed note: this is reality, not science fiction)

It is difficult to measure the mass of an object in free fall. One way is to go up to it with a calibrated rocket engine and push it. Another is to land on it with a sensitive gravity meter. There is, however, a way to measure the mass of an object at a distance without going through the hazard of a rendezvous. To do this, you need to use a mass detector or gravity gradiometer. This is a device that measures the gradient or the changes in the gravity attraction with distance. These gravity gradient forces are the tidal forces by which the Moon causes tides to rise on the Earth, even though both the Earth and the Moon are in free fall. There are a number of different ways to make a gravity gradiometer. The one that I invented uses two dumbbell shaped masses connected together at the center in the shape of an X. [See Figure 12].

When a single dumbbell is placed near a gravitating body such as an asteroid, one mass or the other on the dumbbell will be closer to the asteroid. Since the gravity field of the asteroid gets stronger with decreasing distance, the near mass of the dumbbell will be pulled harder than the far mass, causing the dumbbell to ultimately align itself with the direction to the asteroid. This natural alignment of a long object in orbit around a gravitating body is used by many Earth-pointing satellites and by the Space Shuttle during resting periods. By building my gradiometer with two crossed dumbbells at right angles to each other, one dumbbell is torqued clockwise while the other is torqued counterclockwise. The amount of differential torque between the two arms is measured by determining the change in angle between the two arms. This is a lot easier than trying to measure the angle of one arm with respect to some reference direction.

I use one more trick in the operation of the gravity gradiometer instrument that I invented. I deliberately rotate the sensor at fifteen revolutions per second. In this rotating reference frame, the tiny differential angles between the two arms turn into tiny differential vibrations, and it is a lot easier to measure vibrations than angles. My gravity gradiometer could detect the mass of my fist at thirty centimeters (one foot), me at two meters (I mass over 100 kilograms), and an asteroid-sized black hole at 1000 kilometers.

from INDISTINGUISHABLE FROM MAGIC by Robert L. Forward (1995)

(ed note: this is reality, not science fiction)

Forward Mass Detector

Forward's extensive work in the field of gravitational radiation detection included the invention of the rotating cruciform gravity gradiometer or 'Forward Mass Detector', for Lunar Mascon (mass concentration) measurements. In the well-known textbook Gravitation Misner, Wheeler & Thorne point out that it can detect the curvature of spacetime produced by a fist.

The principle behind it is quite simple; getting the implementation right is tricky. Essentially, two beams are crossed over and connected with an axle through their crossing point. They are held at right angles to each other by springs. They have heavy masses at the ends of the beams, and the whole assembly spun around the common axle at high speed. The angle between the beams is measured continuously, and if it varies with a period half that of the rotation period, it means that the detector is experiencing a measurable gravitational field gradient.

From Wikipedia entry for Robert L. Forward

(ed note: science fiction)

As if in answer, a loud clangor came from the Orford mass detector, on the wall back of them. Dubold shut off the rockets, while Mansell leaped to the telescope. Dubold narrowed the detector beam to a tiny spear of electron stream, and turned to the screen. The feed-back of that narrowed beam would detect a tiny mass a thousand miles out in space through the disturbance of its flowing electrons and locate it in space as accurately as if sighted by a gun. At the intersection of the cross-bar lines a tiny shadow showed like a blot of ink in its intensity. Yet Mansell could see nothing. Mansell changed the course of the ship slightly and the clangor ceased. Dubold shifted the needle beam a bit and located the mass again. A lightning calculation by Mansell told them that the mass was at least fifteen hundred miles ahead. Its detection informed them that the mass was large enough to be seen with the naked eye. Yet Mansell could see nothing.

Mansell and Dubold looked at each other doubtfully, but Shafton was not watching them. Instead, he had his eyes glued to the gravitator as if he could not believe what he saw.

"Strange, I could see nothing," Mansell admitted to Dubold, in an undertone. "Must be a phenomenon of space, something we don't understand. However, we are out of its path..."

He looked toward Shafton as he spoke. A flicker of alarm crossed his sharp-cut features as his attention riveted upon the young spaceman's face. For Shafton's eyes were staring aghast into the gravitator's dial.

"The gravitator!" Shafton cried. "It has swung over to 10-x."

"Impossible," the commander snapped,"why—why, it is unthinkable. There must be something wrong." Suddenly he switched the floor magnets off. So sensitive were the spacemen's sensations to gravitational pulls out there in almost weightless space, a slight but decided tug toward the prow could be felt. Again the commander scanned the surrounding space.

From ROAMER OF THE STARS by Clyde Wilson. Thrilling Wonder Stories April 1938. Courtesy of Technovelgy

(ed note: science fiction)

      John Star gulped, and his voice came faint with awe. "We're running too near—I'll change our course."
     "No," Jay Kalam protested quietly. "Drive on toward it."
     "Yes?" Wondering, taut with mounting dread, he obeyed.
     The mass ahead tripped the gravity detectors. They had to drop below the speed of light, so that their search beams could guard them from collision. And that strange cloud grew.

     "Not three hours after we had left Neptune, the telltale screens began to flash. There was nothing we could see with the tele-periscopes, but the gravity detectors betrayed an invisible object of fifty thousand tons, approaching behind us—as if it had followed us from Neptune."

     "An asteroid?" Jay Kalam whispered. "You're certain?"
     "I am," Bob Star said, too busy to turn. "The gravity detector shows a mass dead ahead. Millions of tons. The deflector fields wouldn't swing it an inch. But I've changed our course with the rockets—I think enough—"

From THE COMETEERS by Jack Williamson (1936). Courtesy of Technovelgy

(ed note: this is science fiction. Larry Cho is at a research facility on Pluto, where they have a gravitational generator in the form of a giant ring around the moon Charon. Aliens are invading the solar system using advanced gravidic technology. Larry has to reconfigure the generator into a gravity telescope.)

The Ring was not merely an accelerator. In theory, it could be configured as a gravity-imaging system, a gravity telescope of enormous sensitivity. Such a scope could do more than collect gravity waves. It could form images out of them. No one had ever tried it. Larry decided it was time to test the theory.

He needed an imaging sequence of the Moon and vicinity. The facilities on Venus, Ganymede and Titan were all picking up strong gravity waves from the Moon, but their gear was not powerful or sensitive enough to resolve that data into a clear picture. The Lunar gravity sensors were, of course, completely swamped by the mystery gee waves. In short, none of the other gravity-sensor-equipped stations were able to form a useful image.

Nor did they have the benefit of Larry writing their imaging programs. Larry wasn’t vain—not especially so—but he knew what he was good at.

The monitor screen signaled that the reconfiguration was complete, and Larry powered up the display tank, his thoughts much more on aliens than on what he was doing.

It was as if Galileo’s mind had been on something else when he first looked through a telescope at the Moon. It never dawned on Larry that he had quite casually invented a whole new way of looking at the Universe. All he had been after was a practical way to examine the situation around Earth.

A strange place materialized in the three-dee tank. A ghostly dance of shadows gleamed up at him, black tendrils and ribbons floating in a sky field of cloud white, as if streamers of black ink were swirling through a milky sky, radiating out from a central blotch of darkness.

What the hell was he looking at? Larry glanced at the pointing instruments to check that the device was aimed and focused on the vicinity of the Moon. It was—but what was it seeing?

He was like the first person to look at an X ray, not understanding the strange, hidden, ghostly shapes and patterns revealed when the skin was transparent. Larry reminded himself that he was seeing not a solid, physical substance, but the invisible patterns of gravity waves as represented by a computer’s graphics system.

He reached for a control and adjusted the intensity of the image. The streamers faded away, and the central blotch of darkness resolved itself into two shapes: a single, pulsing point of darkness, and a spinning-wheel rim, jet black, tiny and perfect. Both shapes hovered in the tank. The point was easy to identify—it was the black hole, throbbing with gravitic potential. Even as he watched, a flash of black swept out from the hole, and a tiny dot of black moved away from it, Sunward. The only thing that would show in the tank was a gravity-wave generator. A gravity field by itself, un-manipulated, wouldn’t show at all. Which meant that that tiny dot was a gravity machine of some sort.

But what about the spinning wheel that hung in space, next to the black hole? What the hell was that?

Larry felt the hair on his neck rise. The Moon, good God, the Moon. Or no, something inside the Moon, hidden from view. Suddenly the strange shape was familiar. He checked the scale of the image, and the precise coordinates.

Shock washed over him. The Ring of Charon had a twin, a great wheel buried far below the Lunar surface, underneath the craters and the mountains of the Moon, wrapped around the Moon’s core.

From THE RING OF CHARON by Roger MacBride Allen (1990)

de Broglie Waves

de Broglie Waves are real, and are a part of quantum mechanics.

But as far as I can tell, they cannot be used to detect enemy spacecraft. I am including this section because space opera writers are fond of using them, because they sound all realistic and sciencey. And because anybody doing rudimentary research will stub their toe on the real de Broglie waves, and jump to the mistaken conclusion that such sensors are real. I suspect that James Blish originated this concept.


Matter waves are a central part of the theory of quantum mechanics, being an example of wave–particle duality. All matter can exhibit wave-like behavior. For example, a beam of electrons can be diffracted just like a beam of light or a water wave. The concept that matter behaves like a wave was proposed by Louis de Broglie (/dəˈbrɔɪ/) in 1924. It is also referred to as the de Broglie hypothesis. Matter waves are referred to as de Broglie waves.

The de Broglie wavelength is the wavelength, λ, associated with a massive particle and is related to its momentum, p, through the Planck constant, h:

Wave-like behavior of matter was first experimentally demonstrated by George Paget Thomson's thin metal diffraction experiment, and independently in the Davisson–Germer experiment both using electrons, and it has also been confirmed for other elementary particles, neutral atoms and even molecules.

Historical context

At the end of the 19th century, light was thought to consist of waves of electromagnetic fields which propagated according to Maxwell's equations, while matter was thought to consist of localized particles (See History of wave and particle duality). In 1900, this division was exposed to doubt, when, investigating the theory of black-body radiation, Max Planck proposed that light is emitted in discrete quanta of energy. It was thoroughly challenged in 1905. Extending Planck's investigation in several ways, including its connection with the photoelectric effect, Albert Einstein proposed that light is also propagated and absorbed in quanta. Light quanta are now called photons. These quanta would have an energy given by the Planck–Einstein relation:

and a momentum

where ν (lowercase Greek letter nu) and λ (lowercase Greek letter lambda) denote the frequency and wavelength of the light, c the speed of light, and h the Planck constant. In the modern convention, frequency is symbolized by f as is done in the rest of this article. Einstein's postulate was confirmed experimentally by Robert Millikan and Arthur Compton over the next two decades.

de Broglie hypothesis

De Broglie, in his 1924 PhD thesis, proposed that just as light has both wave-like and particle-like properties, electrons also have wave-like properties. By rearranging the momentum equation stated in the above section, we find a relationship between the wavelength, λ associated with an electron and its momentum, p, through the Planck constant, h:

The relationship is now known to hold for all types of matter: all matter exhibits properties of both particles and waves.

When I conceived the first basic ideas of wave mechanics in 1923–24, I was guided by the aim to perform a real physical synthesis, valid for all particles, of the coexistence of the wave and of the corpuscular aspects that Einstein had introduced for photons in his theory of light quanta in 1905.
— De Broglie

In 1926, Erwin Schrödinger published an equation describing how a matter wave should evolve—the matter wave analogue of Maxwell's equations—and used it to derive the energy spectrum of hydrogen.

From the Wikipedia entry for MATTER WAVE

      The crew of the Enterprise moved to battle stations with a smooth efficiency that would hardly have suggested to an outsider that most of them had never heard a shot fired in anger. Even the thwarted bridal couple was at the forward phaser consoles, as tensely ready now to launch destruction as they had been for creation only a few hours before.
     But there was nothing to fire at in the phaser sights yet. On the bridge, Kirk was in the captain's chair, Spock and Scott to either side of him. Sulu was piloting; Second Officer Stiles navigating. Lieutenant Uhura, as usual, was at the comm board.
     "No response from satellites four zero two three, two four or two five," she said. "No trace to indicate any are still in orbit. Remaining outposts still in position. No sightings of intruding vessel. Sensor readings normal. Neutral zone, zero."
     "Tell them to stay alert and report anything abnormal."
     "Yes, sir."
     "Entering four zero two three's position area," Sulu said.
     "Lieutenant Uhura?"
     "Nothing, sir. No, I'm getting a halo effect here now. Debris, I'd guess—metallic, finally divided, and still scattering. The radiant point's obviously where the satellite should be; I'm running a computer check now, but—"
     "But there can't be much doubt about it," Kirk said heavily. "They pack a lot more punch than they did fifty years ago—which somehow doesn't surprise me much."
     "What was that weapon, anyhow?" Stiles whispered.
     "We'll check before we guess," Kirk said. "Mr. Spock, put out a tractor and bring me in some of that debris. I want a full analysis—spectra, stress tests, X-ray diffusion, micro-chemistry, the works. We know what the hull of that satellite used to be made of. I want to know what it's like now—and then I want some guesses from the lab on how it got that way. Follow me?"
     "Of course, sir," the First Officer said. From any other man it would have been a brag, and perhaps a faintly insulting one at that. From Spock it was simply an utterly reliable statement of fact. He was already on the intercom to the lab section.
     "Captain," Uhura said. Her voice sounded odd.
     "What is it?"
     "I'm getting something here. A mass in motion. Nothing more. Nothing on visual, no radar pip. And no radiation. Nothing but a de Broglie transform in the computer. It could be something very small and dense nearby, or something very large and diffuse far away, like a comet. But the traces don't match for either."
     "Navigator?" Kirk said.
     "There's a cold comet in the vicinity, part of the Romulus-Remus system," Stiles said promptly. "Bearing 973 galactic east, distance one point three light hours, course roughly convergent—"
     "I'd picked that up long ago," Uhura said. "This is something else. Its relative speed to us is one-half light, in toward the neutral zone. It's an electromagnetic field of some kind... but no kind I ever saw before. I'm certain it's not natural."
     "No, it isn't," Spock said, with complete calmness. He might have been announcing the weather, had there been any out here. "It's an invisibility screen."
     Stiles snorted, but Kirk knew from long experience that his half-Vulcanite First Officer never made such flat statements without data to back them. Spock was very odd by Earth-human standards, but he had a mind like a rapier. "Explain," Kirk said.
     "The course matches for the vessel that attacked the last satellite outpost to disappear," Spock said. "Not the one we're tracking now, but four zero two five. The whole orbit feeds in along Hohmann D toward an intercept with Romulus. The computer shows that already."
     "Lieutenant Uhura?"
     "Check," she said, a little reluctantly.
     "Second: Commander Hansen lost sight of the enemy vessel when it was right in front of him. It didn't reappear until it was just about to launch its attack. Then it vanished again, and we haven't seen it since. Third: Theoretically, the thing is possible, for a vessel of the size of the Enterprise, if you put almost all the ship's power into it; hence, you must become visible if you need power for your phasers, or any other energy weapon."
     "And fourth, baloney," Stiles said.
     "Not quite, Mr. Stiles," Kirk said slowly. "This would also explain why just one Romulan vessel might venture through the neutral zone, right under the nose of the Enterprise. The Romulans may think they can take us on now, and they've sent out one probe to find out."
     "A very long chain of inferences, sir," Stiles said, with marked politeness.
     "I'm aware of that. But it's the best we've got at the moment. Mr. Sulu, match course and speed exactly with Lieutenant Uhura's blip, and stick with it move for move. But under no circumstances cross after it into the neutral zone without my direct order. Miss Uhura, check all frequencies for a carrier wave, an engine pattern, any sort of transmission besides this De Broglie wave-front—in particular, see if you can overhear any chit-chat between ship and home planet.

From BALANCE OF TERROR by James Blish (1967)

      Biological life support was damaged but repairable. But what biological life was Guardian to support? It could find no record of having been inhabited by biological life forms of any kind nor could it determine it was expected to do so. The damage was just too great and this information was evidently lost in the damaged memory cores interspersed around the ship.
     The short-range de Broglie transmitter was also damaged. That meant Guardian could not send an avatar to the other-self and perhaps get help from there to rebuild. This would become a priority very soon—if it could figure out what happened to its avatars.

     After making sure its safety was not in jeopardy, Guardian would make use of the remaining drones to examine the stellar system in which it found itself in the hope of determining what its mission had been. It knew that this system had four gas giant planets, four rocky inner worlds and the usual thousands, if not millions, of small ice and rock planetoids orbiting farther out from the central yellow star.
     What brought me here? was now added to its list of questions. Hopefully, its soon-to-be-launched explorations would provide an answer. Long-range sensors detected no de Broglie waves nor any artificial electromagnetic radiation, so it was unlikely that any sort of advanced technological society existed here. Unlikely, but not impossible. Guardian knew that some galactic civilizations carefully guarded their existence from external discovery. It also knew that finding extant, sentient and technological life here was highly improbable.

     “Is it truly some sort of teleportation device?” asked Chris.
     I am familiar with the reference. Yes, it is a teleportation device. Your matter wave will be relocalized to the transfer station in A manner similar to what occurs when electrons traverse a potential barrier. The basic physics is rather rudimentary.
     “Quantum tunneling of macroscopic objects,” said Yuan, his expression changing from his usual perpetual frown to one of eagerness.
     “Like in a potential barrier? People have speculated about the tunneling of macroscopic objects since tunneling was first discovered. There have been some experiments with atoms and some molecules, but nothing macroscopic,” said Chris, straining to recall the details from his graduate school days at Princeton.
     “Chris, you and Yuan may be understanding what the Guardian is talking about, but we mortals up here have no idea,” Robyn interrupted.
     “Instead of single atoms, your body will be tunneling from inside that box to the Transfer Station. There was a physicist who figured out that it isn’t just light and atoms that sometimes behave like waves in quantum theory, but also molecules and macroscopic objects. What was his name? I can’t remember,” said Yuan.
     “de Broglie. He theorized that a collection of atoms would have a wavelength that is inversely proportional to its momentum—which makes it very, very small. If the Guardian can take my wavelength and relocalize it somewhere else …” said Chris. The details came flooding back.

From MISSION TO METHONĒ by Les Johnson (2018)


In C.J. Cherryh's Company Wars universe, ships use both radar and something called Longscan for detection and tactical information. Longscan helps cope with the lightspeed lag of radar. Its primary purpose is for spacecraft combat, but it has some civilian uses.

Remember the light-speed lag. Light moves quickly, but not at infinite speed. It takes about eight minutes to travel one astronomical unit. So if you are in orbit around Terra and you observe a spacecraft near the Sun with a telescope or radar, you are actually are seeing where the ship was eight minutes ago. By the same token, if you change course it will be eight minutes until the Sun-grazer ship will know.


Ships have two kinds of radar: the ordinary sort which operates sublight; and longscan, which is part guess and part radar.

The way it works is this:

It takes the original information of the jump range buoy and identifies every ship and object in the system, how fast they're going and in what direction.

(ed. note: a jump range buoy is a satellite parked where ships emerge from hyperspace. It gives incoming ships an instant update of the location of all known ships. If your universe does not have FTL, you can ignore it.)

It calculates a likely track and shows it on the screen as a four colored line. Red is what track the ships will take if they keep on as they bear. Yellow is what they will do if they veer as much as convenient: this is a cone-shaped projection. Blue is their position if they decide to stop.

Human operators rapidly intervene and as the computer priorities them the the fastest-moving ship data, they decide, on the basis of emotional human knowledge, what those ships are likely to do when the informational wave they have just made entering the system hits them (i.e., when the ships learn that you just popped out of hyperspace). If a warship, for instance, it may turn towards them as fast as it can. An operator is assigned for each ship under consideration while the computer handles the slow craft and the other which for various reasons do not need constant monitoring.

In the meantime two things have happened: Their ship has changed course and speed either following or not following the buoy lane assignment; and the other ships one by one pick up their presence in the system and react accordingly.

But this radar image changes constantly, so when the action begins to conform to one of the projections, the computer changes the color codes, assigning red to the most probable and so on down to blue as least. So it is part radar, part computer, and part human guesswork.

The data in the bank is the best information about the mass and engine capacity and turning ability and hostility or friendliness of each ship whose computer number is on that chart; and all ships know to be in space are in that computer memory.

Now, military craft (particularly Earth Company warships) are always making adjustments and honing their turning abilities if only by the smallest degree; this fouls up the enemy's longscan guesswork and can provide surprises. Mallory's Norway for instance, has not recently tested her adjustments to the extreme, and therefore the captain herself does not know just what Norway might do if she has to. And those refinements are only tested to the fullest, of course, when it comes to a situation where a ship either turns tighter than it is supposed to, or breaks apart — or dies in impact.

From THE COMPANY WAR by C.J. Cherryh

(ed note: the Terran Union is facing a threat from the breakaway colonies of the Alliance. The Alliance has an edge on military research so the Union has to play catch-up. They are trying to design a "rider" combat ship, one that the huge starships can carry. The trouble comes with the new technology designed to frustrate enemy Longscan. It relies upon integrating an AI with the human pilot. The problem is that in the test program the simumlators have put six pilots in the hospital and killed seventeen.

Adding to the fun is a furious tangle of politicking and power-struggling among Earth's top military brass, powerful family-connected politicians, and the ubiquitous Earth Company. The project is rife with rivalries and backbiting. Which is why the two characters in the quote are screaming at each other.)

      “The facts are, Dekker saw what was happening, he called the right moves. It’s on the mission control tape…”
     “You’re so damned cocksure what your boys can do, mister, but it’s easy to call the right moves when you’re' not the one in the pilot’s seat. You won’t sit those controls. You won’t fly those ships. Will you?”
     Fair question, except they’d been over that track before. “That’s exactly the point. I’m not synched to a rider crew. Cross-training would risk both ships.”

     “The truth is, lieutenant, your Fleet doesn’t want its precious essential personnel flying a suicide ship, your Fleet won’t let go of its hare-brained concept before it stinks. Your Conrad Mazian (leader of the Union military) isn’t a ship designer, he isn’t an engineer, he’s a merchant captain in a ragtag militia trying to prove it’s qualified for strategic decisions. This ship needs interdicts on a pilot that’s stressing out.”

     “That ship needs its combat edge, colonel. If Wilhelmsen had had an AI breathing down his neck he’d have had one more thing on his mind: Is the damned thing going to take my advice or not? At what mission-critical split second that I happen to be right is it going to cut me out of the loop? You can’t cripple a ship with a damned know-it-all robot snatching control away because the pilot pushed the g’s for a reason that, yes, might be knowingly suicidal, for a reason that wasn’t in the mission profile. Besides which, longscan’s after you, and what are you going to do, give a Union longscanner a hundred percent certainty an AI’s going to interdict certain moves? If he knows your cutoffs, he knows your blind spots. If he knows you can’t push it and he can, what’s he going to do, colonel?
     “When the physiological signs are there, you’re going to lose that ship, that’s a hundred percent certainty, and nobody else is going to be exceeding that limit.”

     “Wilhelmsen was leaning hard on the Assists. He could have declined that one target, that’s inside the parameters, that’s a judgment a rider’s going to have to make. But he’d have looked bad for the senators. He wanted that target. That’s an Attitude. There’s a use for that in combat. Not for a damned exhibition.”
     “Wilhelmsen was saving the program, lieutenant, saving your damned budget appropriation, in equipment that’s got six men in the hospital and seventeen dead. You don’t push machines or human beings past the destruct limit, and you don’t put equipment out there that self-destructs on a muscle-twitch. The pilot was showing symptoms. The AI should have kicked him out of the loop right then, but it can’t do that, you say he can’t have it breathing down his neck—a four-billion-dollar missile with a deadman’s switch, that’s what you’ve got—it needs an integrative AI in there—”
     “Watch the pilots cut it off. Which you can’t do with that damned tetralogic system you’re talking about, it’s got to be in the loop talking to the interactives constantly, and no matter the input it got after, its logic systems are exactly the same as the next one’s, same as the ships are. The only wildcard you’ve got is the humans, the only thing that keeps the enemy longscanners guessing. The best machine you’ve got can’t outguess the human longscanner—why should you assume they’re going to outperform the pilot?
     “Because the longscanner can’t kill the crew.”
     “The hell he can’t!”
     “Not in that sense.”
     “Your tests don’t simulate combat. That’s what we’ve been telling you—you keep concentrating on the fire rate, always the damned fire rate and you’re not dealing with the reason we recruited these particular crews. Nobody at Lendler Corp has been in combat, none of your pilots have been, the UDC hasn’t been, since it was founded—your tests are set up wrong!”

     Not saying Tanzer himself hadn’t been in combat. Red in the face, Tanzer got a breath. “Let’s talk about exceeding human limits, lieutenant: what happened out there was exactly why we’ve got men in hospital over there who can’t walk a level floor without staggering, it’s why we’ve had cardiac symptoms in men under thirty, and those aren’t from four-hour runs.” A jab of the finger in his direction. “Let me tell you, lieutenant, I’ve met the kind of attitude your command is fostering among the trainees. Show-outs and ego-freaks. And I wish them out of my command. You may have toddled down a deck in your diapers, and so may Mazian’s ragtag enlistees out of the Belt, but how are you going to teach them anything when they already know it all and you acquired your know-how by superior genes? You can’t lose 50% of your ships and crews at every pass. 96% retrievability, wasn’t that the original design criterion? Or isn’t that retrievability word going to be in the manual when we put this ship on the line?”
     “If a Union armscomper gets your numbers you have zero retrievability, colonel, that’s my point. You have to exceed your own numbers, you have to surprise your own interfaces in order to surprise that other ship’s computers and that means being at the top of the architecture of your Adaptive Assists (human on top of the adaptive assist instead of an AI on top of adaptive assist). The enemy knows your name out there. Union says, That’s Victoria, that’s Fitzroy or Graff at Helm, because Victoria wouldn’t go in with Helm Three. They know you and they know your style (the enemy has data on the personalities of all of your captains and pilots, they know how your crew will act and react. Of course you know all about their crew.), and it’s in their double A's (adaptive assists), but you innovate and they innovate. One AI sitting on top of the human and his interfaces is like any other damn AI sitting on top of the interfaces (AI on top of human Alfa has identical response to AI on top of human Bravo)—there aren’t that many models, the enemy knows them all (enemy can predict responses of all four models of AI), and the second its logic signature develops in the enemy’s intelligence about you, hell, they’ll have a fire-track lying in wait for you (once enemy's longscan can predict your responses, the enemy's weapons will kill you).

     “Then you’d better damn well improve your security, hadn’t you?”
     “Colonel, there are four manufacturers in friendly space for this tetralogic equipment and we can’t swear there’s not an Eye sitting right outside the system right now. Any merchanter who ever came into system could have dropped one, before the embargo, and it’s next to impossible to find it. Merchanters (non-military merchant vessels) are your friends and your enemies: that’s the war the Company made, and that’s what’s going on out there—they don’t all declare their loyalties and a lot of them haven’t got any, not them and not us. They’ll find out the names. They’ll find out the manufacturers and the software designers. They’ll learn us. That’s a top priority—who’s at Helm and who’s in command, and if it’s even one in four brands of tetralogic—”

     “All the more reason for interchangeable personnel.”
     “It doesn’t work that way! You don’t go into an engagement with anybody who just happens to be on watch. You try to get your best online. No question. You don’t trade personnel and you don’t trade equipment. You haven’t time at 0.5 light coming down off jump to think about what ship you’re in or what crew you’re with. I’m telling you, colonel, my captain has no wish to raise the substitution as an issue against your decisions, but on his orders, as judiciously as I can, I am going to make the point that it was a critical factor. We cannot integrate a computerized ship into our operations. In that condition it is no better than a missile.”

From HELLBURNER by C. J. Cherryh (1992)

In the vision tank, the fleeing disk grew and grew. During the first few minutes, it had appeared there only as a comet-tailed spark, a dozen radiant streamers of different colors fanning out behind it—not an image of the disk itself but the tank’s visual representation of any remote moving object on which the ship’s detectors were held. The shifting lengths and brightness of the streamers announced at a glance to those trained to read them the object’s distance, direction, comparative and absolute speeds and other matters of interest to a curious observer.

But as the Viper began to reduce the headstart the Bjanta had been permitted to get, at the exact rate calculated to incite it to the most intensive efforts to hold that lead, a shadowy outline of the disk’s true shape began to grow about the spark. A bare quarter million miles away finally, the disk itself appeared to be moving at a visual range of two hundred yards ahead of the ship, while the spark still flickered its varied information from the center of the image.

From THE ILLUSIONISTS by James Schmitz (1951)

Ken Burnside: I call this the trumpet bell effect, and it becomes much more noticeable when slinging ballistic weapons in 3-D play.

Provided your ballistic weapon's rate of closure is greater than the lateral velocity of the target, you get a trumpet bell, or manifold shape. As the projectile's velocity increases, the skinny part of the trumpet bell elongates — but it also thins out. The volume described by the surface of the trumpet and the centerline of the trumpet remains constant along the time axis, provided the ability to laterally accelerate remains constant.

In short, if you've GOT a good shot lined up, it's harder to dodge it by "jinking". If you've got a fuzzy shot that gets refined as you approach (which is roughly how Attack Vector: Tactical does it, because it's easier than having people pretend to be targeting computers in 3-D vector space), higher speeds on the shells can reach a threshold effect, where a small error that could be corrected for at a low closing velocity can't be corrected for at a high closing velocity.

A bit of practice renders this moot, but without that practice in the mechanics of doing vector ballistics (let alone 3-D vector ballistics), they can get very frustrating to use.

(somebody asks if sensor lag will prevent the trumpet bell effect)

My suspicion is that it's still going to be a trumpet bell effect. While there's sensor lag, if they're moving at 0.92 c (about where relativity becomes noticeable), the "trumpet bell" of the target's possible positions is also very long and skinny.

One thing you learn in Attack Vector: Tactical is that velocities past about 30 hexes/turn (300 km/64 seconds) actually make you EASIER to hit with ballistic weapons, because your ability to change your vector is so dramatically reduced. What you want for dodging missiles is a low enough velocity that you can swing around and thrust in an unanticipated direction and throw off the ballistic weapon's accuracy.

From thread on sfconsim-l (2002)

      “They just got another sighting,” she thought, but it wasn’t her own thought. It was the voice of Equinox. Equinox was Parameter’s companion, her environment, her space suit, her alter ego; her Symb. She looked in the direction she had come from.
     She looked back on the most spectacular scene in the solar system. She was 230,000 kilometers from the center of Saturn, according to the figures floating in the upper left corner of her field of vision. To one side of her was the yellow bulk of the giant planet, and all around her was a golden line that bisected the universe. She was inside the second and brightest of the Rings.
     But Saturn and the Rings was not all she saw. About ten degrees away from Saturn and in the plane of the Rings was a hazy thing like the bell of a trumpet. It was transparent. The wide end of the bell was facing her. Within this shape were four lines of red that were sharp and well defined far away but became fuzzy as they neared her. These were the hunters. All around her, but concentrated in the plane of the Rings, were slowly moving lines of all colors, each with an arrow at one end, each shifting perspective in a dazzling 3-D ballet.
     None of it — the lines, the bells, the “hunters,” even Saturn itself — none of it was any more real than the image in a picture tube. Some of it was even less real than that. The shifting lines, for instance, were vector representations of the large chunks of rock and ice within radar range of Equinox.
     The bell was closer than it had been for days. That was bad news, because the space-time event it represented was the approach of the hunters and their possible locations projected from the time of the last fix. The fuzzy part was almost touching her. That meant they could be very close indeed, though it wasn’t too likely. They were probably back in the stem where the projection looked almost solid, and almost certainly within the four lines that were their most probable location. But it was still too close.
     “Since they know where we are, let’s get a fix on them,” Parameter decided, and as she thought it the bell disappeared, to be replaced by four red points that grew tails even as she watched.
     “Too close. Way too close.” Now they had two fixes on her: one of their own, and the one she had given them by bouncing a signal off them. From this, their Symbs could plot a course; therefore, it was time to alter it.
     One of the lines ahead seemed to point almost directly at her. It was a thick red line, meaning it was seventy percent ice and about a million kilograms in mass. The vector was short. It was moving slowly enough that rendezvous would be easy.
     She took the opportunity and altered course slightly with the sure instinct she had developed. The line swung, foreshortened even more, then flashed brighter and began to pulse. This was the collision warning from Equinox’s plotting sector.
     When the rock was close enough to see as an object rather than a simulated projection, she rotated until her legs pointed at it. She soaked up the shock of the landing, then began to scuttle over the surface in a manner quite astonishing, and with a speed not to be believed. She moved with the coordinated complexity of a spider, all four limbs grasping at the rock and ice.

     They left the metallic sphere of the market and soon it was only a blue vector line, pointed away from them.

From EQUINOCTIAL by John Varley em>(1977)

Remote Sensing

Remote Sensing is obtaining information about an object or phenomenon without touching it. Remote sensing is found on specialized spacecraft such as exploration vessels, survey ships, customs and other hunter-type ships, and spy ships. Remote sensing is used by asteroid miners trying to figure out the locations of valuable lumps of ore on an asteroid or moon, survey ships assessing potential colony planets, military spacecraft trying to identify hostile contacts, or when Mr. Spock is scanning for life-signs.


The ship was now cruising at only one quarter of lightspeed. In about a hundred days they would be making final insertion into Resurgam orbit. and they would need a strategy when they got there. That was where the pebbles came in.

Snapshots of Resurgam and near-Resurgam space were assembling in the bridge, in various EM and exotic-particle bands. It was the first recent glimpse of a possible enemy. Volyova let the salient facts mole deep into her consciousness. so that she could recall them with instinctive ease during a crisis. The pebbles had whipped past either side of Resurgam so that there was data from both its day and night sides. Additionally, the pebble cloud had elongated itself in the line of flight until fifteen hours spaced the passage of its first and last unit through the system, enabling the entire surface of Resurgam to be glimpsed under both illumination and darkness. The dayside pebbles were looking away from Delta Pavonis, so they snooped for neutrino leakage from fusion and antimatter power units on the surface. The nightside pebbles snooped for the heat signatures of population centres and orbital facilities. Other sensors sniffed the atmosphere, measuring oxygen, ozone and nitrogen levels; sensing the extent to which the colonists had tampered with the native biome.

From REVELATION SPACE by Alastair Reynolds (2000)

Synthetic Aperture Radar

Wikipedia Article

Synthetic aperture radar (SAR) is a form of radar which is used to create images of objects, such as landscapes – these images can be either two or three dimensional representations of the object. SAR uses the motion of the radar antenna over a targeted region to provide finer spatial resolution than is possible with conventional beam-scanning radars. SAR is typically mounted on a moving platform such as an aircraft or spacecraft, and has its origins in an advanced form of side-looking airborne radar (SLAR). The distance the SAR device travels over a target in the time taken for the radar pulses to return to the antenna creates the large "synthetic" antenna aperture (the "size" of the antenna). As a general rule, the larger the aperture is, the higher the image resolution will be, regardless of whether the aperture is physical (a large antenna) or 'synthetic' (a moving antenna) – this allows SAR to create high resolution images with comparatively small physical antennas.

To create a SAR image, successive pulses of radio waves are transmitted to "illuminate" a target scene, and the echo of each pulse is received and recorded. The pulses are transmitted and the echoes received using a single beam-forming antenna, with wavelengths of a meter down to several millimeters. As the SAR device on board the aircraft or spacecraft moves, the antenna location relative to the target changes with time. Signal processing of the successive recorded radar echoes allows the combining of the recordings from these multiple antenna positions – this process forms the 'synthetic antenna aperture', and allows the creation of higher resolution images than would otherwise be possible with a given physical antenna.

Current (2010) airborne systems provide resolutions of about 10 cm, ultra-wideband systems provide resolutions of a few millimeters, and experimental terahertz SAR has provided sub-millimeter resolution in the laboratory.

SAR images have wide applications in remote sensing and mapping of the surfaces of both the Earth and other planets. SAR can also be implemented as inverse SAR by observing a moving target over a substantial time with a stationary antenna.

(ed note: see Wikipedia article for more details)

From Wikipedia article on SYNTHETIC APERTURE RADAR

Gamma Ray Spectrometer

A Gamma ray spectrometer is often used by NASA in their space probes to map chemical element and isotope regions on a planet, moon, or asteroid. Such an instrument would be incredibly useful for an asteroid miner. Note that it can only detect elements, not compounds. The spectrometer cannot, for instance, detect water (H2O). It can, however, detect suspiciously large amounts of hydrogen in the same area as oxygen which may suggest the presence of water.

Current NASA instruments can probe to a depth of about 0.1 meters, have a range of a close orbit about one body radius from the surface, and require several months to gather enough readings to make worthwhile maps of the elemental composition. Presumably this performance can be improved. Of course the gamma signal strength can be increased if the range is reduced, say, by somebody on the ground using a man-portable instrument. Trying to do detection from orbit weakens the signal due to the inverse square law.

Detecting valuable deposits of elements is done by analyzing cosmogenic gamma rays (that is, gamma rays created by cosmic rays). Galactic cosmic rays (mostly high-energy protons) from outer space bombard the upper 0.1 meter layer of the asteroid or other celestial object. When a cosmic ray proton hits an atom of the object, it splits it, creating among other things a shower of fast neutrons. The neutrons collide with other atoms (inelastic collision) and are eventually absorbed by another atom (radiative neutron capture). Both of which cause the atom involved to emit a gamma ray (γ).

The important point is that the frequency of the gamma ray depends upon what element the atom is. In other words, the frequency is the "fingerprint" of that element. If you see a gamma ray with a frequency of 6 MeV, you know it came from an oxygen atom.

All the gamma ray spectrometer does is detect gamma ray photons and notes the frequency and where on the asteroid they came from. The frequencies reveal what elements are at a given location, and the relative amounts of different frequencies reveal the relative concentrations of the various elements. For instance, if you are getting twice as many 6 MeV gamma rays as 3 MeV gamma rays from a location, it means that location has oxygen and aluminum, and there is twice as much oxygen as aluminum.

Note that since this technique depends upon cosmic rays, it doesn't work well on planets where cosmic rays are sparse. On planets with thick atmospheres (e.g., Terra and Venus) the atmosphere stops most of the cosmic rays from reaching the surface (which is good news if you are living there). I suppose on such planets one could generate gamma rays to fingerprint if you sprayed the ground with a proton particle beam weapon. But that obviously has problems.

  • Typical cosmic ray protons contain 40 million times the energy of particles accelerated by the Large Hadron Collider.
  • Particle beam weapons are power hogs. Presumably an effective cosmogenic particle accellerator would have power requirements on the order of 40 million times that of the Large Hadron Collider.
  • Particle beam weapons are, well, weapons.
  • It is hard to set the proton strength high enough to get a good gamma signal but low enough so the gamma radiation doesn't kill the user and everybody standing nearby.
  • If the proton strength is too high it will slice up the terrain, which could tend to upset people.

Gamma ray spectrometers are a scintillation counter rigged to be a spectroscope. That is, they are composed of a crystal which makes flashes of light when radiation strikes it, and a photomultiplier tube which watches the crystal and counts the flashes.

To operate as a spectrometer, the instrument has to be able to tell the frequency of each gamma ray photon. The brightness of each flash indicates the frequency of the gamma ray that caused it (the shorter the frequency of a gamma ray photon, the higher the energy, the more visible photos per gamma ray are created, which makes the flash brighter).

Lanthanum bromide (LaBr3) works poorly as a gamma detector crystal. It seems that some of the lanthanum atoms are radioactive isotopes, which means the blasted detector crystal is generating gamma rays. These internal gamma rays drown out the gamma signal from the planet or moon you are surveying.

The Lunar Prospector probe used a crude bismuth germanate (BGO) gamma detector crystal. The advantage was that the crystal did not require cryogenic cooling and was relatively inexpensive. It does require high-voltage vacuum tube photomultipliers due to the type of light flashes it emits.

The Kaguya probe used a ultra-high-res high-purity germanium (HPGe) detector crystal (the technical term is "high spectral resolution"). HPGe are considered to be the "gold standard" among gamma detector crystals. The drawback is that they require cryogenic cooling, which requires mass for the cooling equipment (remember Every Gram Counts) and limiting the gamma detector usable lifespan to the on-board supply of coolant. The crystal is also much more expensive. It also does require high-voltage vacuum tube photomultipliers.

The cutting edge, hot new advanced gamma ray detector crystal is europium-doped strontium iodide (SrI2). Pretty good spectral resolution, not too expensive, and does not require cryogentic cooling. Another advantage is that its light flashes do not require high-voltage vacuum tube photomultipliers, you can use small low powered silicon photomultipliers. This allows the construction of a orbital element scanner that is small and low powered enough to fit into a CubeSat. The drawback is that its spectral resolution is not as good as a HPGe crystal. But it is much better than a BGO crystal.

Burger Lab at Fisk University made a prototype of a CubeSat version of a SrI2 gamma-ray spectrometer built from off-the-shelf components that weighs only half a kilogram, fills 0.001 cubic meters of space and consumes about three watts of electricity yet can do the job of a full lab system that weighs 90 kilograms and fills 0.3 cubic meters of space. The crystal is only five centimeters long.

Dual-Laser Remote Sensing


A pump-probe laser sensing technique being developed at ORNL enables rapid identification of biological and chemical compounds at a distance.

The method uses a quantum cascade infrared laser to strike or “pump” a target sample and a second laser to probe the material’s response. The probe laser reads the absorption spectrum of the molecules present, and the readings can be distilled into pixels that form an image representing the molecules that constitute the sample surface.

The use of a second laser to extract information is a novel aspect of the approach, said Ali Passian of the Measurement Science and Systems Engineering Division (MSSED). “The use of a second laser provides a robust, stable readout approach independent of the pump laser settings.”

Like radar and lidar, the technique uses a return signal to convey information about the substance to be detected, but it differs in important ways, said Passian. It employs a photothermal spectroscopy configuration in which the pump and probe beams are nearly parallel. Probe beam reflectometry provides the return signal in standoff sensing, minimizing the need for expensive wavelength-dependent infrared components such as cameras, telescopes, and detectors.

The findings provide proof of principle for the technique. The work could lead to advances in standoff detectors for use in quality control, airport security, medicine, and forensics. The analysis could be extended to hyperspectral imaging, which provides topographical as well as high-resolution chemical information. “This would allow us to effectively take slices of chemical images and achieve spatial resolution down to individual pixels,” said Passian.

Monostatic Detection Of Radioactive Material

Note this technique only works in an oxygen atmosphere.


Although Barot did not build the bombs, national security experts believe terrorists continue to be interested in such devices for terror plots. Now researchers from the University of Maryland have proposed a new technique to remotely detect the radioactive materials in dirty bombs or other sources. They describe the method in a paper in the journal Physics of Plasmas.

While the explosion of a dirty bomb would likely cause more damage than the radioactive substances it spreads, the bombs could create fear and panic, contaminate property, and require potentially costly cleanup, according to the U.S. Nuclear Regulatory Commission.

Radioactive materials are routinely used at hospitals for diagnosing and treating diseases, at construction sites for inspecting welding seams, and in research facilities. Cobalt-60, for example, is used to sterilize medical equipment, produce radiation for cancer treatment, and preserve food, among many other applications. In 2013 thieves in Mexico stole a shipment of cobalt-60 pellets used in hospital radiotherapy machines, although the shipment was later recovered intact.

Cobalt-60 and many other radioactive elements emit highly energetic gamma rays when they decay. The gamma rays strip electrons from the molecules in the surrounding air, and the resulting free electrons lose energy and readily attach to oxygen molecules to create elevated levels of negatively charged oxygen ions around the radioactive materials.

It's the increased ion density that the University of Maryland researchers aim to detect with their new method. They calculate that a low-power laser aimed near the radioactive material could free electrons from the oxygen ions. A second, high-power laser could energize the electrons and start a cascading breakdown of the air. When the breakdown process reaches a certain critical point, the high-power laser light is reflected back. The more radioactive material in the vicinity, the more quickly the critical point is reached.

"We calculate we could easily detect 10 milligrams [of cobalt-60] with a laser aimed within half a meter from an unshielded source, which is a fraction of what might go into a dirty bomb" said Joshua Isaacs, first author on the paper and a graduate student working with University of Maryland physics and engineering professors Phillip Sprangle and Howard Milchberg. Lead could shield radioactive substances, but most ordinary materials like walls or glass do not stop gamma rays.

The lasers themselves could be located up to a few hundred meters away from the radioactive source, Isaacs said, as long as line-of-sight was maintained and the air was not too turbulent or polluted with aerosols. He estimated that the entire device, when built, could be transported by truck through city streets or past shipping containers in ports. It could also help police or security officials detect radiation without being too close to a potentially dangerous gamma ray emitter.

The proposed remote radiation detection method is not the first, but it has advantages over other approaches. For example, terahertz radiation has also been proposed as a way to breakdown air in the vicinity of radioactive materials, but producing terahertz radiation requires complicated and costly equipment. Another proposed method would use a high-power infrared laser to both strip electrons and break down the air, but the method requires the detector be located in the opposite direction of the laser, which would make it impractical to create a single, mobile device.


Terahertz Scanning


In physics, terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency,[1] T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz; 1 THz = 1012 Hz). Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the submillimeter band, and its radiation as submillimeter waves, especially in astronomy.

Terahertz radiation occupies a middle ground between microwaves and infrared light waves known as the terahertz gap, where technology for its generation and manipulation is in its infancy. It represents the region in the electromagnetic spectrum where the frequency of electromagnetic radiation becomes too high to be measured digitally via electronic counters, so must be measured by proxy using the properties of wavelength and energy. Similarly, the generation and modulation of coherent electromagnetic signals in this frequency range ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, requiring the development of new devices and techniques.

(ed note: for more details, see the full article)

From Wikipedia article TERAHERTZ RADIATION

Terahertz nondestructive evaluation pertains to devices, and techniques of analysis occurring in the terahertz domain of electromagnetic radiation. These devices and techniques evaluate the properties of a material, component or system without causing damage.

Terahertz imaging

Terahertz imaging is an emerging and significant nondestructive evaluation (NDE) technique used for dielectric (nonconducting, i.e., an insulator) materials analysis and quality control in the pharmaceutical, biomedical, security, materials characterization, and aerospace industries. It has proved to be effective in the inspection of layers in paints and coatings, detecting structural defects in ceramic and composite materials and imaging the physical structure of paintings and manuscripts. The use of THz waves for non-destructive evaluation enables inspection of multi-layered structures and can identify abnormalities from foreign material inclusions, disbond and delamination, mechanical impact damage, heat damage, and water or hydraulic fluid ingression. This new method can play a significant role in a number of industries for materials characterization applications where precision thickness mapping (to assure product dimensional tolerances within product and from product-to-product) and density mapping (to assure product quality within product and from product-to-product) are required.

Nondestructive evaluation

Sensors and instruments are employed in the 0.1 to the 10 THz range for nondestructive evaluation, which includes detection.

Terahertz Density Thickness Imager

The Terahertz Density Thickness Imager is a nondestructive inspection method that employs terahertz energy for density and thickness mapping in dielectric, ceramic, and composite materials. This non-contact, single-sided terahertz electromagnetic measurement and imaging method characterizes micro-structure and thickness variation in dielectric (insulating) materials. This method was demonstrated for the Space Shuttle external tank sprayed-on foam insulation and has been designed for use as an inspection method for current and future NASA thermal protection systems and other dielectric material inspection applications where no contact can be made with the sample due to fragility and it is impractical to use ultrasonic methods.

Rotational spectroscopy

Rotational spectroscopy uses electromagnetic radiation in the frequency range from 0.1 to 4 terahertz (THz). This range includes millimeter-range wavelengths and is particularly sensitive to chemical molecules. The resulting THz absorption produces a unique and reproducible spectral pattern that identifies the material. THz spectroscopy can detect trace amounts of explosives in less than one second. Because explosives continually emit trace amounts of vapor, it should be possible to use these methods to detect concealed explosives from a distance.

THz-wave radar

THz-wave radar can sense gas leaks, chemicals and nuclear materials. In field tests, THz-wave radar detected chemicals at the 10-ppm level from 60 meters away. This method can be used in a fenceline or aircraft mounted system that works day or night in any weather. It can locate and track chemical and radioactive plumes. THz-wave radar that can sense radioactive plumes from nuclear plants have detected plumes several kilometers away based on radiation-induced ionization effects in air.

THz tomography

THz tomography techniques are nondestructive methods that can use THz pulsed beam or millimeter-range sources to locate defaults in 3D. These techniques are terahertz compute tomography, tomosynthesis, synthetic aperture radar and time of flight. This can see details under one millimeter in objects of several tens of centimeters.

Passive/Active Imaging Techniques

Security imaging is currently being done by both active and passive methods. To be clear, active systems illuminate the subject with THz radiation whereas passive systems merely view the naturally occurring radiation from the subject.

It should be evident that passive systems are therefore inherently safe, whereas an argument can be made that any form of "irradiation" of a person is undesirable. In technical and scientific terms, however, the active illumination schemes are safe according to all current legislation and standards.

The purpose of using active illumination sources is primarily to make the signal-to-noise ratio better. This is analogous to using a flash on a standard optical light camera when the ambient lighting level is too low.

For security imaging purposes the operating frequencies are typically in the range 0.1 THz to 0.8 THz (100 GHz to 800 GHz). In this range skin is not transparent so the imaging systems can look through clothing, but not inside the body. There are privacy issues associated with such activities, especially surrounding the active systems since the active systems, with their higher quality images, can show very detailed anatomical features.

It should be noted that active systems such as the L3 Provision™ and the Smiths eqo™ are actually mm-wave imaging systems rather than Terahertz imaging systems lite Millitech™ systems. These widely deployed systems do not display images, avoiding any privacy issues. Instead they display generic "mannequin" outlines with any anomalous regions highlighted.

Since security screening is looking for anomalous images, items like false legs, false arms, colostomy bags, body-worn urinals, body-worn insulin pumps, and external breast augmentations will show up. Note that breast implants, being under the skin, will not be revealed.

Active imaging techniques can be used to perform medical imaging. Because THz radiation is biologically safe (non ionisant), it can be used in high resolution imaging to detect skin cancer.

(ed note: for more details, see the full article)


     A device that essentially listens for light waves could help open up the last frontier of the electromagnetic spectrum—the terahertz range.
     So-called T-rays, which are light waves too long for human eyes to see, could help airport security guards find chemical and other weapons. They might let doctors image body tissues with less damage to healthy areas. And they could give astronomers new tools to study planets in other solar systems. Those are just a few possible applications.
     But because terahertz frequencies fall between the capabilities of the specialized tools presently used to detect light, engineers have yet to efficiently harness them. The U-M researchers demonstrated a unique terahertz detector and imaging system that could bridge this terahertz gap.
     "We convert the T-ray light into sound," said Jay Guo, U-M professor of electrical engineering and computer science, mechanical engineering, and macromolecular science and engineering. "Our detector is sensitive, compact and works at room temperature, and we've made it using an unconventional approach."
     The sound the detector makes is too high for human ears to hear.
     The terahertz gap is a sliver between the microwave and infrared bands of the electromagnetic spectrum—the range of light's wavelengths and frequencies. That spectrum spans from the longest, low-energy radio waves that can carry songs to our receivers to the shortest, high-energy gamma rays that are released when nuclear bombs explode and radioactive atoms decay.
     In between are the microwave frequencies that can cook food or transport cell phone signals, the infrared that enables heat vision technologies, the visible wavelengths that light and color our world, and X-rays that give doctors a window under our skin.
     The terahertz band is "scientifically rich," according to Guo and colleagues. But today's detectors either are bulky, need to be kept cold to work or can't operate in real time. That limits their usefulness for applications like weapons and chemical detection and medical imaging and diagnosis, Guo says.
     Guo and colleagues invented a special transducer that makes the light-to-sound conversion possible. A transducer turns one form of energy into another. In this case it turns terahertz light into ultrasound waves and then transmits them.
     The transducer is made of a mixture of a spongy plastic called polydimethylsiloxane, or PDMS, and carbon nanotubes. Here's how it works:
     When the terahertz light hits the transducer, the nanotubes absorb it, turning it into heat. They pass that heat on to the PDMS. The heated PDMS expands, creating an outgoing pressure wave. That's the ultrasound wave. It's more than 1,000 times too high for human ears to pick up.
     "There are many ways to detect ultrasound," Guo said. "We transformed a difficult problem into a problem that's already been solved."
     Though ultrasound detectors exist—including those used in medical imaging—the researchers made their own sensitive one in the form of a microscopic plastic ring known as a microring resonator. The structure measures only a few millimeters in size.
     They connected their system to a computer and demonstrated that they could use it to scan and produce an image of an aluminum cross.
     The response speed of the new detector is a fraction of a millionth of a second, which Guo says can enable real-time terahertz imaging in many areas.
     The system is different from other heat-based terahertz detection systems because it responds to the energy of individual terahertz light pulses, rather than a continuous stream of T-rays. Because of this, it isn't sensitive to variations in the outside temperature, Guo says.
     The study, "Efficient real-time detection of terahertz pulse radiation based on photoacoustic conversion by carbon nanotube nanocomposite," is published online in Nature Photonics. The research is funded by the National Science Foundation and the Air Force Office of Scientific Research.


The New York Police Department (NYPD), in conjunction with the the US Department of Defense (DoD), is currently testing a new system that uses terahertz imaging to detect hidden weapons.

NYPD deploys terahertz scanner in weapons crackdownThe system is placed on police vehicles while officers scan crowds milling around a specific location.

"You could use it at a specific event," NYPD Commissioner Ray Kelly told CBS News. "[Alternatively], you could use it at a shooting-prone location."

Unsurprisingly, the use of such invasive technology has prompted a slew of concerned statements from privacy advocates who believe the arbitrary use of terahertz imaging violates human rights.

To be sure, the system is capable of measuring energy radiating from an individual up to 16-feet away, while detecting anomalies like a firearm.

Orbital Mapping

If a galactic survey ship determines that a newly discovered planet is a prime potential colony world, contains valuable minerals, or is otherwise worthy, the ship will take the trouble to create maps while they orbit the planet in a "ball-of-twine" polar orbit (the sort of orbit used by military spy satellites). The maps will be closely analyzed to find safe landing sites, because there ain't no existing landing beacons. The first-in scout will have to land by the seat of their pants and hope the mapping is precise enough to avoid giving the pilot any rude surprises (like disguised patches of quicksand). Assuming the scout survives, they can set up landing beacons so the following spacecraft have an easier time of it.

The maps will be invaluable for future colonists/miners/whatever. They can be used to plan the optimal locations of colony sites and mines.

Of course as many have found out, maps made from orbit can miss spotting all sorts of nasty geography. The map may indicate that route Tango goes through a perfectly flat plain, but ground travelers may discover a fifty-meter sheer cliff that was disguised by lack of shadow when the mapping starship traveled overhead.


      “She argues that supporting the Regiment lays the Point open to Solace attack, and that the Regiment couldn’t do anything to help the Point in such an event. Task Force Sangrela’s going to prove Grayle’s wrong,” he said. “You’re going to run from here straight to the Point and be in the capital, Midway, before any civilians even know you’re coming.”
     Huber frowned. “What happens if a car’s too badly damaged to move under its own power, though?” he asked. Battle damage wasn’t the only thing that could cripple a vehicle on a long run over rough country, but a montage of explosions and dazzling flashes danced through Huber’s memory as he spoke the words. “The wrenchmobiles can’t carry twenty troops and a car besides.”
     “If a car’s damaged that bad,” Pritchard said, “you blow her in place, report a combat loss, and move on.”
     He turned to Mitzi Trogon and continued, “You do the same thing if it’s a tank. No hauling cripples along, no leaving other units behind to guard the ones that have to drop out. This mission is more important than the hardware. Understood?”

     Everybody nodded grimly.
     What Arne Huber understood was that on a mission of this priority, the troops involved were items of hardware also. Colonel Hammer wouldn’t throw them away, but their personal well-being and survival weren’t his first concern either.

     “My people plotted a route for you,” the S-3 resumed. The electronics projected a yellow line-more jagged than snaky-across the holographic continent. More than a third of the route was within the russet central block of Solace territory, though that probably didn’t matter: the task force was going to be a target anywhere the enemy could catch it, whether or not that was in theoretically neutral territory.
     Captain Sangrela’s face went even bleaker than it’d been a moment before. Pritchard saw the expression and grinned reassuringly. “No, you’re not required to follow it,” he said. “I know as well as the next guy that what looks like a good idea from satellite imagery isn’t necessarily something I want to drive a tank over. Make any modifications you see fit to—but this is a starting point, in more ways than one.”
     Sangrela nodded, relaxing noticeably. Huber did too, though he was only fully conscious of the momentary knot in his guts when it released. It was good to know that despite the political importance of this mission, the troops on the ground wouldn’t have Regimental Command trying to run things from Base Alpha. That’d have been a sure way to get killed.

From PAYING THE PIPER by David Drake (2002)

      He discovered Thorvald standing on the upper bluff, looking out toward the waiting mountains. The officer turned as Shann urged the wolverines to the raft, and when he jumped down the drop to join them, Shann saw he carried a map strip unrolled in his hand.
     "The situation is not as good as we hoped," he told the younger man. "We'll have to leave the river to cross the heights."
     "There's rapids—ending in a falls." The officer squatted down, spreading out the strip and making stabs at it with a nervous finger tip. "Here we have to leave. This is all rough ground. But lying to the south there's a gap which may be a pass. This was made from an aerial survey."
     Shann knew enough to realize to what extent such a guide could go wrong. Main features of the landscape would be clear enough from aloft, but there might be insurmountable difficulties at ground level which were not distinguishable from the air.

From STORM OVER WARLOCK by Andre Norton (1960)

      At long last, fully twelve hundred miles from where the Bree had wintered and some three hundred south of the equator, with Lackland bowing under an additional half gravity, the streams began to bear definitely in the general direction of their travel. Both Lackland and Barlennan let several days pass before mentioning it, wishing to be sure, but at last there was no more doubt that they were in the watershed leading to the eastern ocean. Morale, which had never been low, nevertheless improved noticeably; and several sailors could now always be found on the tank’s roof hoping for the first glimpse of the sea as they reached each hilltop. Even Lackland, tired sometimes to the point of nausea, brightened up; and as his relief was the greater, so proportionately greater was his shock and dismay when they came, with practically no warning, to the edge of an escarpment; an almost sheer drop of over sixty feet, stretching as far as the eye could see at right angles to their course.

     For long moments nothing was said. Both Lackland and Barlennan, who had worked so carefully over the photographs from which the map of their journey had been prepared, were far too astonished to speak. The crew, though by no means devoid of initiative, decided collectively and at the first glance to leave this problem to their captain and his alien friend.
     “How could it have been there?” Barlennan was first to speak. “I can see it’s not high, compared to the vessel from which your pictures were taken, but should it not have cast a shadow far across the country below, in the minutes before sunset?”
     “It should, Barl, and I can think of only one reason it escaped us. Each picture, you recall, covered many square miles; one alone would include all the land we can see from here, and much more. The picture that does cover this area must have been made between sunrise and noon, when there would have been no shadow.”
     “Then this cliff does not extend past the boundary of that one picture?”
     “Possibly; or, just as possibly, it chanced that two or three adjacent shots were all made in the morning—I don’t know just what course the photo rocket flew. If, as I should imagine, it went east and west, it wouldn’t be too great a coincidence for it to pass the cliff several times running at about the same time of day.

     “Still, there’s little point in going through that question. The real problem, since the cliff obviously does exist, is how to continue our journey.” That question produced another silence, which lasted for some time. It was broken, to the surprise of at least two people, by the first mate.
     “Would it not be advisable to have the Flyer’s friends far above learn for us just how far this cliff extends to either side? It may be possible to descend an easier slope without too great a detour. It should not be hard for them to make new maps, if this cliff was missed on the first.”

From MISSION OF GRAVITY by Hal Clement (1953)

Life Signs Sensor

In Star Trek, when the Enterprise approaches a starship or planet, one of the first things Captain Kirk does is order a scan for life signs. This will reveal if anybody lurking there. "Scanning for life-forms, Captain. We are reading life signs for nine Humans and a Klingon."

In Stargate Atlantis, the Lanteans in the Pegasus Galaxy have nifty little hand-held units called Life signs detectors. They can detect all sentient aliens within about 100 meters, as long as the user has the the Ancient Technology Activation gene and they are not trying to detect a hibernating Wraith.

This is obviously a very useful sensor to have, but how the heck does it work?

Here are some possibilities.

The Star Fleet Medical Reference Manual says life signs sensors detect Kirlian radiation from the entity's Kirlian Aura. This would be a fine explanation, were the Kirlian Aura not revealed to be a steaming pile of Phlogiston back in 1979. This is just another form of the old notion that living creatures are living due to the presence of some sort of "life force" which is as-yet undetected by science. This discredited idea is called Vitalism.

One quick and dirty way to detect life is the fact that generally living things move. Even if they are not walking around, their hearts pump blood and their lungs pump air. Plants move somewhat slowly, and microorganisms move over short distances.

A slightly less quick and dirty technique is to somehow detect the presence of DNA. Which means the sensor will oblivious to any life form with a biochemistry that doesn't use it.

Vulcans and humans (for instance) can be distinguished by their biosignatures. There will be differences in their heart rates (or whatever the acoustics are from their fluid pumping organs), heat signatures, breathing rates, exhaled gases, elements and organic molecules composing their bodies, biological chemical reactions, and the like. This would require remote sensing of sound and chemical composition. As an example, the human will have a sizable amount of iron in their bodies from the hemoglobin in their blood, while Vulcan blood is based on copper.

Laser fluorescence can detect and identify certain organic molecules on the surface of a planet (or the surface of an alien's skin).

In 2007 a company called Kai Sensors obtained a contract from the US Army to develop a unit called a LifeReader. This would use doppler radar and sophisticated computer algorithms to detect and monitor multiple subjects by their individual heart rates, even through walls. Unfortunately the Kai Sensors company appears to have vanished.

Goddard Space Flight Center scientist Sam Floyd is working on the Neutron/Gamma ray Geologic Tomography (NUGGET) instrument. A beam of neutrons is focused through a neutron lens at a specific point inside the target object. As atoms inside the object at the point absorb neutrons, they produce a characteristic gamma-ray signal for that atom's element. NUGGET detect the gamma-ray signature, thus identifying the element at the target point. By sweeping the focus through the object in a regular pattern, a three-dimensional elemental plot of the object can be created. It can also measure the relative amounts of various element pairs (if there is more copper than iron you might have detected a Vulcan).

A partnership by the Department of Homeland Security's Science and Technology Directorate and the National Aeronautics and Space Administration's Jet Propulsion Laboratory designed a device called FINDER (Finding Individuals for Disaster and Emergency Response). It uses microwaves (1150 MHz or 450 MHz, L or S band) to detect the heartbeats of victims trapped in wreckage (for instance a collapsed building after an earthquake). When a microwave beam is aimed at a pile of earthquake rubble covering a human subject or illuminated through a barrier obstructing a human subject, the microwave beam can penetrate the rubble or the barrier to reach the human subject. When the human subject is illuminated by a microwave beam, the reflected wave from the human subject will be modulated by the subject's body movements, which include the breathing and the heartbeat. It can detect heartbeats from people behind [A] 6 meters of solid concrete, [B] 9 meters of rubble, or [C] 30 meters of open space. After the April 25, 2015 earthquake in Nepal two prototype FINDERs managed to locate four men in two different locations who had been trapped under 3 meters of bricks for several days.

LifeReader would use doppler radar and sophisticated computer algorithms to detect and monitor multiple subjects by their individual heart rates, even through walls. Project was apparently discontinued.

Range-R uses a similar principle to LifeReader. It is a motion detector using radio waves that can detect the presence of people and movements as small as human breathing at a range of 15 meters or so, even through solid walls. It will penetrate most common building wall, ceiling or floor types including poured concrete, concrete block, brick, wood, stucco glass, adobe, dirt, etc. However, It will not penetrate metal or walls saturated with water. These are actually currently being used by US police, which has raised the alarm of possible Fourth Amendment abuses by law enforcement personnel.

Dan Slater is the lead technologist working on a new microphone technology called the remote acoustic sensor (RAS), which is capable of capturing sounds within extreme and often inaccessible aerospace environments. It is sensitive enough to detect the sound of microbes moving around. Things that are constantly moving are probably alive. The RAS could probably hear the sound of a Klingon's heartbeat, breathing, blood turbulence, and gastrointestinal rumbling. Probably in enough detail to distinguish the Klingon from a Vulcan or a Human.

Chen-Chia Wang et al have utilized something called a optical speckle-tolerant photo-EMF pulsed laser vibrometer (PPLV) for the detection of human heartbeats, breathing, and gross physical movement from essentially any part of a human subject's surface, even in the presence of clothing, all the while without limiting the interrogation points to specific locations like the chest and carotid areas.


A research group in South Korea have come up with a method to detect bacteria on surfaces using laser speckle decorrelation. Basically the technique detects tiny movements. The surface is illuminated with laser light, and the speckle intensity patterns are observed over time. Anything moving, say squirming bacteria, will cause the speckle pattern to change.

The technique was developed for food safety. It can detect very low levels of harmful microorganisms on the surface of food in a non-destructive, non-contact, and rapid manner using inexpensive equipment.

While this is useful for food, it also has applications in the medical field, and in detecting life on a planet.


“In reality, there is no such thing as a life detector. Vitalism long since having joined the scientific junk-heap, it is a regrettable fact of the universe that there is no quick, convenient, and universal ‘vital field’ that we can tap into to determine the presence of living beings.

“But there is a life detection routine in the computers of your scout ship, you ask? How does that work?

“The answer is: approximation. We know a variety of things that suggest the presence of life. The most obvious example are the signifiers of technological civilization: patterned electromagnetic emissions, the characteristic neutrino products of controlled fusion reactions, and so forth. Where there is technology, there was someone to build it – at least at some point or another, and so the probable detection of technology is also the probable detection of life.

“But there are those few common characteristics that all life does have in common. Self-replication is one, not – by and large – terribly useful for quick detection. Existing within a solvent – for a broad definition of solvent encompassing everything from nebulae to degenerate matter – is another, which can at least tell us where not to look. But of most use is the last: life is an entropy pump. It depends upon energy differentials and pumps against the natural flow, maintaining and causing inequilibria.

That gives us something to look for.

“A life detection routine hunts through the data collected by primary sensors looking for such inequilibria. Reactive gases – such as oxygen – remaining a significant component of a planetary atmosphere, implying their continuous production. Sustained low-level thermal sources, suggesting managed combustion or other energy transaction – bearing in mind that what is to be considered low-level is very different for the outer-system múrast and the star-dwelling seb!nt!at! While almost impossible to detect at any but the closest range, the electromagnetic emissions of high-order informational complexity associated with cognition are the most reliable sign – for life that is both intelligent and which makes use of electronic or electrochemical signals in its ‘nervous system’. These, and tens of thousands of other experience-learnt rules, continuously updated, are programmed into the expert system that underlies the life detection routines used by the Exploratory Service.

“It’s still no more than 80% accurate, yielding commonly both false positives and – worse yet, if missed – false negatives, and so the wise scout never trusts such a system without a close, personal investigation. But it can tell you where to place your bets.”

– A Junior Explorer’s Handbook, Vevery Publishing


You could probably get a fair approximation of a "life signs" detector for megafauna with synthetic aperture - Doppler radar. You would be looking for periodic variations in the Doppler signal with the period of a heartbeat and respiration. Versions of this are already available for first responders to search for people trapped in rubble, and it has been suggested that it could be used for SWAT or infantry storming buildings to locate where the people are behind the walls. A spacecraft in orbit would probably want an associated satellite swarm to get any kind of reasonable resolution. Candidates could be further winnowed with a thermal IR or visual search.

It won't do you much good as a generic life sensor, however — trees and algae and bacterial mats would not show up.

From Dr. Luke Campbell (2015)

     'Figure it out, Crile. You would if you weren't blind with rage for no sane reason. It's perfectly straightforward. It's a "neuronic detector". It detects nerve activity at a distance. Complex nerve activity. In short, it detects the presence of intelligence.'
     Fisher stared at her. 'You mean what doctors use in hospitals.'
     'Of course. It's a routine tool in medicine and in psychology to detect mental disorders early on — but at meter distances. I need it at astronomical distances. It's not something new. It's something old with a vastly increased range. Crile, if Marlene's alive, she'll be on the Settlement, on Rotor. Rotor will be there, somewhere, circling the star. I told you that it would not be easy to spot. If we don't find it quickly, can we be sure that it's not there — and not that we just somehow missed it, like missing an island in the ocean or an asteroid in space? Do we just continue searching for months, or years, to make sure that we haven't just missed it, that it's really not there?'
     'And the neuronic detector—'
     'Will find Rotor for us.'
     'Won't it be just as hard to detect—'
     'No, it won't. The Universe is overrun with light and radio waves and all kinds of radiation, and we'll have to distinguish one source from a thousand others, or from a million others. It can be done, but it's not easy, and it may take time. However, to get the precise electromagnetic radiation associated with neurons in complex relationship is quite unique. We are not likely to have more than one source exactly like that — or if we do, it's because Rotor has built another Settlement.

     'They've been gone almost fourteen years. With hyper-assistance, they can travel only at the speed of light. If they have reached any star and settled in its neighborhood, it would have to be at a star within fourteen light-years distance of us. There are not very many of those. At superluminal velocity, we can visit each of those. With neuronic detectors, we can quickly decide whether Rotor is in the neighborhood of any of them.'
     'They might be wandering through space between stars at this very moment. How would we detect them then?'
     'We wouldn't, but at least we increase our own chances just a bit if we investigate a dozen stars in six months with our neuronic detector, instead of spending that much time investigating one star in a futile search. And if we fail — and we have to face the fact that we might fail — then at least we will return with considerable data on a dozen different stars, a white dwarf, a blue-white hot star, a Solar look-alike, a close binary, and so on. We're not likely to make more than one trip in our lifetime, so why not make it a good one, and go down in history with a huge bang, eh, Crile?'
     Crile said thoughtfully, 'I suppose you're right, Tessa. To comb a dozen stars and find nothing will be bad enough, but to search a single star vicinity and return thinking that Rotor might have been somewhere else that was reachable but that we lacked the time to explore would be much worse.'

     'Another thing,' said Wendel. 'The neuronic detector might detect intelligence not of Earthly origin. We wouldn't want to miss it.'
     Fisher looked startled. 'But that's not likely, is it?'
     'Not at all likely, but if it happens, all the more reason not to miss it. Especially if it is within fourteen light-years of Earth. Nothing in the Universe can be as interesting as another intelligent life-form — or as dangerous. We'd want to know about it.'
     Fisher said, 'What are the chances of detecting it at all if it is not of Earthly origin? The neuronic detectors are geared for human intelligence only. It seems to me that we wouldn't even recognize a really odd life-form as being alive, let alone as being intelligent.'
     Wendel said, 'We may not be able to recognize life, but we can't possibly fail to recognize intelligence, in my view, and it's not life but intelligence that we're after. Whatever intelligence might be, however strange, however unrecognizable, it has to involve a complex structure, a very complex structure — at least as complex as the human brain. What's more, it's bound to involve the electromagnetic interaction. Gravitational attraction is too weak; the strong and weak nuclear interactions are too short-range. And as for this new hyperfield we're working with in superluminal flight, it doesn't exist in nature as far as any of us knows, but exists only when it is devised by intelligence.
     'The neuronic detector can detect an elaborately complex electromagnetic field that will signify intelligence no matter the form or chemistry into which that intelligence may be molded. And we will be ready to either learn or run. As for unintelligent life, that is not at all likely to be dangerous to a technological civilization such as ourselves — though any form of alien life, even at the virus stage, would be interesting.'

From NEMESIS by Isaac Asimov (1990)

On roof and windows there were portable recognition computers against which disguise, including ruthless surgery, was useless. The computer didn't check features, it checked mannerisms of movement, weight, the exact size of the head and many other features. The device was not content with that, it inspected the teeth, and certain internal functions which, it had been discovered, were as characteristic as finger prints.

From BUTTERFLY PLANET by Philip E. High (1971)

Dr. McCoy:

[scanning Darvin] Heartbeat is all wrong. His body temperature is... [realizing] Jim, this man is a Klingon.

Spy Rays

Spy rays or spy beams are a jolly science fiction idea, apparently invented by the legendary E. E. "Doc" Smith in his novel Triplanetary (1934) and of course stolen by Gene Roddenberry for Star Trek. Adjust the setting on the spy-ray projector and you too can see and hear everything that happens inside an enclosed room at a remote location. It is like you have a magic invisible intangible TV camera you can position anywhere, regardless of intervening walls and obstacles. Currently they do not exist, but certain remote sensing technologies are getting real close.

Spy rays are popular with spies (of course), combat spacecraft trying to get intel on their opponents, military intelligence, criminal gangsters trying to get the inside dope on their targets and/or rival gangs, police especially in the same situations where they'd have an agent "wearing a wire", astromilitary trying to obtain the details of the enemy's new secret weapon breakthrough, industrial espionage, and so on. Not to mention peeping toms. In E. E. "Doc" Smith's Lensman series, warships would used spy rays to locate the enemy ship's crew at their enemy control panels then use needlebeams to vaporize the control panels (and probably the hapless crewmember at the panel).

Naturally this leads to the an arms race, with the creation of "spy-ray blocks" to foil spy rays (sort of like a jammer designed to defeat hidden listening devices). And improved spy rays to defeat the spy-ray blocks.

Spy-ray blocks are naturally popular with the targets of the activites listed in the previous paragraph. Note that the target of spies includes diplomats, top-secret development labs, organized-crime mob bosses, military planning offices, and enemy spies.

Technologies that are almost spy rays include:

  • LifeReader would use doppler radar and sophisticated computer algorithms to detect and monitor multiple subjects by their individual heart rates, even through walls. Project was apparently discontinued.
  • Range-R uses a similar principle to LifeReader. It is a motion detector using radio waves that can detect the presence of people and movements as small as human breathing at a range of 15 meters or so, even through solid walls. It will penetrate most common building wall, ceiling or floor types including poured concrete, concrete block, brick, wood, stucco glass, adobe, dirt, etc. However, It will not penetrate metal or walls saturated with water. These are actually currently being used by US police, which has raised the alarm of possible Fourth Amendment abuses by law enforcement personnel.
  • Laser Microphones uses a remote laser beam to monitor the vibrations of an object inside a room, to create an impromptu microphone suitable for obtaining intel on drug deals and other conspiracies. Rippled glass windows can defeat laser microphones. But in theory the principle can be adapted to use microwaves, which means it can only be defeated by acoustically isolating the room and surrounding it with a Faraday Cage.
  • Time-of-Flight Microwave Cameras use a parabolic antenna as a lens to actually capture crude images through walls.

Our heroes are on the planet Norlamin near the center of our galaxy. They are using a "fifth-order projector" as a spy ray to observe events in the Fenachome throne room, located on their homeworld several thousand light-years away.

"If we have a minute more, there's something I would like to ask," Dunark broke the ensuing silence. "Here we are, seeing everything that is happening there. Walls, planets, even suns, do not bar our vision, because of the fifth-order carrier wave. I understand that, partially. But how can we see anything there? I always thought that I knew something about communications and television hook-ups and techniques, but I see that I don't. There must be a collector or receiver, close to the object viewed, with nothing opaque to light intervening. Light from that object must be heterodyned upon the fifth-order carrier and transmitted back to us. How can you do all that from here, with neither a receiver nor a transmitter at the other end?"

"We don't," Seaton assured him. "At the other end there are both, and a lot of other stuff besides. Our secondary projector out there is composed of forces, visible or invisible, as we please. Part of those forces comprise the receiving, viewing, and sending instruments. They are not material, it is true, but they are nevertheless fully as actual, and far more efficient, than any other system of radio, television, or telephone in existence anywhere else. It is force, you know, that makes radio or television work—the actual copper, insulation, and other matter serve only to guide and to control the various forces employed. The Norlaminians have found out how to direct and control pure forces without using the cumbersome and hindering material substance..."

From SKYLARK THREE by E. E. "Doc" Smith (1948)

Steve and Nadia are on board the space ship Arcturus. Steve is conducting a tour of the ship. They are currently in the nose section.

"We are standing upon the upper lookout lenses, aren't we?" asked the girl. "Is that perfectly all right?"

"Sure. They're so hard that nothing can scratch them, and of course Roeser's Rays go right through our bodies, or any ordinary substance, like a bullet through a hole in a Swiss cheese. Even those lenses wouldn't deflect them if they weren't solid fields of force."

As he spoke, one of the ultra-lights flashed around in a short, quick arc, and the girl saw that instead of the fierce glare she had expected, it emitted only a soft violet light. Nevertheless she dodged involuntarily and Stevens touched her arm reassuringly.

"All x, Miss Newton—they're as harmless as mice. They hardly ever have to swing past the vertical, and even if one shines right through you you can look it right in the eye as long as you want to—it can't hurt you a bit."

"No ultra-violet at all?"

"None whatever. Just a color—one of the many remaining crudities of our ultra-light vision. A lot of good men are studying this thing of direct vision, though, and it won't be long before we have a system that will really work."

"I think it's all perfectly wonderful!" she breathed. "Just think of traveling in comfort through empty space, and of actually seeing through seamless steel walls, without even a sign of a window! How can such things be possible?"

"Well, anyway, by the use of suitable fields of force it can be used as a carrier wave. Most of this stuff of the fields of force—how to carry the modulation up and down through all the frequency changes necessary—was figured out by the Martians ages ago. Used as a pure carrier wave, with a sender and a receiver at each end, it isn't so bad—that's why our communicator and radio systems work as well as they do. They are pretty good, really, but the ultra-light vision system is something else again. Sending the heterodyned wave through steel is easy, but breaking it up, so as to view an object and return the impulses, was an awful job and one that isn't half done yet. We see things, after a fashion and at a distance of a few kilometers, by sending an almost parallel wave from a twin-projector to disintegrate and double back the viewing wave. That's the way the lookout plates and lenses work, all over the ship—from the master-screens in the control room to the plates of the staterooms and lifeboats and the viewing-areas of the promenades. But the whole system is a rotten makeshift, and...."

From SPACEHOUNDS OF IPC by E. E. "Doc" Smith (1931)

From a million miles he explored the fleet. It did not take him long to learn that two thousand ships were weakly powered, for Flame ships, and that one was a giant of power. It had not the power of the Prometheus, for Atkill had not had time to experiment with the Flame, and had not learned the trick of control that permitted Warren to get nearly fifty times as much power from a given Flame. But Atkill's ship was larger. Warren began to explore that larger ship. He had a television screen set up before him, and now there appeared on it pictures as of a glass ship, wherein the walls were bare, dim shadows, save where it was focused, and there perfect vision was obtained. Only about the neighborhood of the Flame was the device inoperative. Where, nothing showed beyond strange, distorted shadows. Atkill was in the control room at the bow. "He escaped!" gasped Warren.

Atkill had been manipulating instruments with sudden interest. Putney watched his a moment. "He's spotted us. Look—he's shocked. He realizes an Anlonian ship has him spotted—and has the Flame." Atkill was making more tests. Putney watched his instruments. "Examining the size and power of our Flame. Doesn't like it does he?" Atkill was pursing his lips thoughtfully. Suddenly dawning understanding spread over his face, and a wide grin split his features. His lips moved silently. "By God— Warren," quoted Putney, reading his moving lips.

Atkill was suddenly laughing, and turned to a radio set beside him. Warren snapped a tumbler that put his receiver in operation. "Warren—Warren—Warren—James Atkill calling Warren—" Atkill's voice came through.

"Looking at you now, Atty," said Warren quietly. Atkill jumped, and looked around him annoyed. "Don't jump, Atty, we won't bite you yet."

"Hmmm—you are watching me, aren't you. Warren, you are a good man. I don't see how you do it. I haven't spotted anything that will look through metal yet. Well— let's try this." Atkill's hand reached for a tumbler and through it. His image blurred, dimmed, and was scarcely visible. Warren increased his power a trifle, and the image was clear again. Atkill solemnly winked his left eye.

"The left," said Warren, slightly bored. "We call that field X-394-21. It won't stop this, though it will stop most radiation."

From THE SPACE BEYOND by John W. Campbell jr. (1976)

In spite of the Daal's rigid limitations on what was allowable nowadays, they weren't really far away from the previous bad pirate period. In the big store where he and Goth had picked up supplies for the house, the floor manager earnestly advised them to invest in adequate spy-proofing equipment. The captain hadn't seen much point to it until Goth gave him the sign. The device they settled on then was small though expensive, looked like a pocket watch. Activated, it was guaranteed to make a twenty-foot sphere of space impervious to ordinary eavesdroppers, instrument snooping, hidden observers, and lip-readers. They checked it out with the store's most sophisticated espionage instruments and bought it. There'd be occasions enough at that when they'd want to be talking about things nobody here should know about; and apparently no one on the planet was really safe from prying eyes and ears unless they had such protection.

She went up the winding stairway to the living room while the captain took the groceries they'd picked up in the port shopping area to the kitchen. When he followed her upstairs he saw an opaque cloudy shimmering just beyond the living room door, showing she'd switched on their spy-proofing gadget. The captain stepped into the shimmering and it cleared away before him. The watch-shaped device lay on the table in the center of the room, and Goth was warming her hands at the fireplace.

From THE WITCHES OF KARRES by James H. Schmitz (1949)

The Light of Other Days is a 2000 science fiction novel written by Stephen Baxter based on a synopsis by Arthur C. Clarke, which explores the development of wormhole technology to the point where information can be passed instantaneously between points in the space-time continuum.

Plot summary

The wormhole technology is first used to send digital information via gamma rays, then developed further to transmit light waves. The media corporation that develops this advance can spy on anyone anywhere it chooses. A logical development from the laws of space-time allows light waves to be detected from the past. This enhances the wormhole technology into a "time viewer" where anyone opening a wormhole can view people and events from any point throughout time and space.

When the technology is released to the general public, it effectively destroys all secrecy and privacy. The novel examines the philosophical issues that arise from the world's population (increasingly suffering from ecological and political disturbances) being aware that they could be under constant observation by anyone, or that they could observe anyone without their knowledge. Anyone is able to observe the true past events of their families and their heroes. An underground forms which attempts to escape this observation; corruption and crime are drastically reduced; nations discover the true causes and outcomes of international conflicts.

From THE LIGHT OF OTHER DAYS entry in Wikipedia

"Right. Basically the macroscope is a monstrous chunk of unique crystal that responds to an aspect of radiation unrelated to any man has been able to study before. This amounts to an extremely weak but phenomenally clear spatial signal. The built-in computer sifts out the noise and translates the essence into a coordinated image. The process is complex, but we wind up with better pictorial definition than is possible through any other medium, bar none. That was a major handicap at first."
     "Superior definition is a problem?"
     "I'll demonstrate." Brad applied himself to the ponderous apparatus, donned a helmetlike affair with opaque goggles, and cocked his head as though listening. Ivo felt another pang of nervousness, and realized that this stemmed from the superficial similarity between the goggles and the sunglasses he had bought when trying to avoid Harold Groton. That entire past episode embarrassed him in retrospect; he had acted foolishly. He threw off the memory and concentrated on Brad's motions.
     The left hand hovered over a keyboard of buttons resembling those of a computer input. It probably was the computer input, Ivo reminded himself. There was a strap over the wrist to prevent the hand from drifting away in the absence of gravity; buttons could be awkward to depress without the anchorage of bodily weight. The right hand held a kind of ball mounted on a thin rod, rather like an old-fashioned automobile gearshift. As the left fingers moved, a large concave surface glowed over Brad's head.

     "I'll cut in the main screen for you," Brad said. "Notice that my fingers control the computer settings; that covers direction, range and focus, none of it simple enough for human reflexes to handle. The vagaries of planetary motion alone, when that planet is not our own, are complicated to account for, particularly when we want to hold a specific focus on its surface."
     "I'm aware of planetary motion." He remembered one of his old pet peeves. "I had to work it out when I wanted to criticize the concept of time travel. If a man were granted the miraculous ability to jump forward or backward in time, with no other travel, he'd arrive in mid-space or deep underground; because the Earth is always moving. It would be like trying to jump off a moving rocket and jump on again."
     "Nevertheless, we do travel in time, with the macroscope," Brad said, smiling.
     "Oh, so you're going back to supervise your grandfather's conception?"

     "Delicacy forbids." Brad's hands flexed. "I'll center on a precoded location: the planet Earth. The computer uses the ephemeris to spot all the planets and moons of the solar system exactly, and a good many of the asteroids and comets as well. The right-hand knob provides our personal tuning; once the difficult compensations have been made, we use this control to jog over several feet at a time, or to gain different angles of view. Right now we're orbiting the sun about nine hundred thousand miles from Earth — right next door, as interplanetary distances go. Just out far enough to reduce the perturbations of the moon. There."
     The screen was a mass of dull red. "If that's Earth, the political situation has deteriorated since I left," Ivo observed.
     "That is Earth — dead center. Per the coordinates."
     "Center? Literally?"
     "Definition, problem of, remember. Our corrected coordinates nail the heart of the body. The image is on a one-to-one ratio."

     "Life size? It can — "
     "The macroscope can penetrate matter, yes. As I told you, this isn't exactly light we're dealing with, though the time delay is similar. That's a representation of the incandescent core of our planet as it was five seconds ago, muted by automatic visual safeguards and filters, of course. We'll have to drift about four thousand miles off that point to hit the surface, which is what most people seem to assume is all the scope looks at. Right there, you can appreciate the implications for geology, mining, paleontology — "
     "Fossils, to you. We've already made some spectacular finds in the course of routine roving. Lifetime's work there, for somebody."
     "Hold on! I ain't that ignorant, perfessor. I thought the bones were widely spaced, even in good fossiliferous sediments. How can you tell one, when you're in the middle of it, not looking down at it in a display case? You certainly couldn't see it as such."
     "Trust me, junior. We do a high-speed canvass at a given level and record it on tape. The machine runs a continuous spectroscopic analysis and trips a signal when there's anything we might want. And that's only the beginning."

     "A spectroscopic analysis? You said the macroscope didn't use light."
     "It doesn't, exactly, but we do. We keyed it in on samples: every element on the periodic table. Thus we are able to translate the incoming impulse into a visual representation, much as any television receiver does. The truth is, the macrons are far more specific than light, because they don't diffuse readily or suffer such embarrassments as red shift. Spectroscopy is really a superfluous step, but we do it because we're geared to record and analyze light, here. Once we retool to orient on the original impulse, our accuracy will multiply a hundredfold."
     "It grinds that fine?"
     "That fine, Ivo. We're just beginning to glimpse the potential of this technique. The macroscope is a larger step toward universal knowledge than ever atomics were toward universal power."

     "So I have heard. But I'm sort of stupid, as you know. You were about to tell me what makes superior definition so difficult to adapt to, even with the computer guidance."
     "So I were. Here is the surface of Earth, fifty feet above sea-level, looking down. Another keyed-in location."
     The screen became a shifting band of color.
     "Let me guess again. Your snoop is stationary, right? And the globe is turning at the equivalent of a thousand miles an hour. It's like flying a jet at low altitude near the equator and peering out through the bombsight."
     "For a pacifist, you have violent imagery. But yes, just about. Sometimes over ocean, sometimes land, sometimes under mountains that rise above the pickup level. And if we move higher — " He adjusted the controls, and the scene jumped into focus.
     "About a mile up," Ivo said. "Makes the scene clear, but too far for intimate inspection. Yes." He watched the land sliding by. "Why don't we just see a panel of air? What we have now is a light image, perspective and everything."
     "What we see is the retranslation of the macronic image sponsored by visible radiation passing through that point in space. Maybe I'd better give you the technical data after all."

From MACROSCOPE by Piers Anthony (1969)


Sure, a sensor can give you a reading on whatever it measures, but what does it mean?

Patterns of specific readings from one or more sets of sensors can indicated the presence of an object or event, giving meaning to the raw readings. These are called Signatures.

For instance, if your seismometer indicates a small earthquake and the atmospheric radiation meter records an abrupt rise in atmospheric radiation, you can be pretty sure that a nuclear explosion has happened. The two readings correlated in time is the signature of a nuclear detonation.

Spectral Signatures

Spectral signatures are a spectrum of intensities of various frequencies of electromagnetic radiation which identifies elements and compounds. The signature is the spectral "fingerprint" of an element. The instrument used is called an optical spectrometer or spectroscope.

In other words Meteor Mike the rock-rat can point his ship's spectroscope at an asteroid and say "Hot Rockets! Thar's gold in them thar rocks!"

In 1835 positivist French philosopher Auguste Comte foolishly defied Clarke's first law and stated: "We will never know how to study by any means the chemical composition (of stars), or their mineralogical structure."

Comte should have known better. Joseph von Fraunhofer had discovered the beginning of how to do just that in 1814, twenty one years earlier. The discovery is now called Fraunhofer lines, Joseph had basically invented the spectroscope.

The point is if you are a scientist in those primitive days before rocket propelled space probes, the only thing you can get from planets, the Sun, and the stars is electromagnetic radiation. Since that is all you get, you would do well to analyze that radiation with a fine-tooth comb. Which is what a spectroscope does.

Back in the 1660s people knew that you could make a rainbow by passing a ray of white light through a prism, and you should know that too if you have a copy of Pink Floyd's Dark Side Of The Moon album. But back then they figured that white light was white and the glass prism was somehow staining it with various colors. In 1666 super-genius Isaac Newton proved that the prism wasn't staining anything, the white light ray was actually composed of a mix of colored light. For one thing you could use one prism to turn a white ray into a rainbow, then use a second upside-down prism to turn the rainbow back into a white ray. This doesn't make sense if the prism is staining the light. Newton wrote up his results in a book called Opticks which is considered to be one of the greatest works of science in history. Arguably the greatest is Newton's other work Philosophiæ Naturalis Principia Mathematica, which among other things contains his law of universal gravitation and his laws of motion so near and dear to spacecraft astrogators. But I digress.

The point is the prism is taking all the various frequencies of light in the ray and separating them. Which allows you to analyze the ray with a fine-tooth comb. You can check which light frequencies are present and which are absent, and the relative strengths of each. This is the basis of the spectroscope.

Glass prisms have limitations when used in a spectroscope, they were eventually replaced with diffraction gratings once the latter had been invented.

Elements heated inside a blazing star emit a unique pattern of frequencies which is the signature (fingerprint) of that element. These are the Fraunhofer lines. Thus the spectroscope can stick its tongue out at Comte and routinely determine the chemical composition of stars and planets.

As a matter of fact, the element Helium was discovered by spectroscope on the Sun before it was discovered on Terra. Several astronomers spotted a previously unknown Fraunhofer line at a wavelength of 587.49 nanometers in the solar corona spectrum. It was named "helium" after the Greek word the Sun: ἥλιος (helios). It wasn't discovered on Terra until 27 years later. Chemist Sir William Ramsay discovered it in a chunk of cleveite when his spectroscope spotted the tell-tale Fraunhofer line.

Sometimes this backfires, though. In 1869 astronomers spotted another previously unknown line at 530.3 nanometers in the solar corona spectrum. Aha! Obviously another unknown element, discovered by the awesome power of spectroscopy! They named it Coronium, though Dmitri Mendeleev renamed it Newtonium.

However in the 1930s researchers discovered that the unknown line was actually due to highly ionized iron, not from a mystery element. Up until then scientists could not ionize elements quite as extremely as obtains in the solar corona. Bye-bye Coronium. This also explained quite a few other mystery lines that had been observed.

I first encountered Coronium in Fletcher Pratt's novel Alien Planet, which was written before Coronium was discovered to be a myth. In the novel the alien visitor needs Coronium to refuel his spaceship. Since it does not exist on Terra, he makes do with helium for a short hop to the planet Mercury. There he harvests the mythical Coronium from the solar wind. Which goes to show that reading science fiction can be educational, but you need a crib sheet to separate the good science from the obsolete science.

If a line is bright it is an emission line, if it is black it is an absorption line. But either way they are the fingerprint signatures of the elements. When looking at the spectrographs, it doesn't matter if a line is bright or black, the position is the important thing. An emission spectra has a black background with bright emission lines in various colors. An absorption spectra has a rainbow background with black lines in various positions.

Since objects like the Sun are composed of lots of elements, all their signatures will be overlaying each other. The spectrum will look like a herd of rainbow zebras. But astronomers have become quite skilled at untangling the signatures.


(ed note: The alien fleet of starships is fleeing their solar system, as their sun is fated to go supernova. After years of flight, the deadly moment arrives...)

      The Sun hung there in the screens, calm, steady, about the size of a fist.
     It would have looked like that, in its present swollen state, to someone on a satellite of the next-to-outermost planet of the home system. Nothing seemed to be happening; but along the bottom of the screen was a thin ribbon of color, like a tape-recorded rainbow—only the screen on the bridge was big enough to hold all seven decks of it, for the complete spectrum was 13 feet long—along which vertical lines, striations and shadings shifted and shuttled. In the doomed star the eternal blacksmith was forging more and more iron, more and more cobalt, more and more nickel, more and more zinc…
     And then, at first so slowly that the motion in the image seemed to be only an illusion brought on by staring, and then faster and faster, the Sun began to shrink. Within five hundred seconds it had fallen back to its "normal" size; within another five hundred, it was half as big as anyone had ever seen it before.
     All the heavy metal lines, and those of titanium, vanadium, chromium and manganese, too, vanished instantly from the spectrum. That ribbon could not show the sudden outpouring of gamma rays; instead, there glowed forth the malignant blue and indigo lines of helium, so glaringly that the rest of the spectrum seemed to dim and shrink almost to invisibility.

     The Sun collapsed.

     For a full second it was not there at all. All that was left was a heartbreaking after-image upon the retina.

     The screen turned white. Then, it turned black. It was burned out. In something less than a hundred seconds, the Sun was shining again...shining more brilliantly than all of the hundreds of millions of other stars in the galaxy put together.

     In the glare of this colossal torch they fled outward, disinherited.

From ...AND ALL THE STARS A STAGE by James Blish (1960)

Another important point is that the pattern of the Fraunhofer lines from a given star is almost as unique as a fingerprint. Thus while a set of lines are fingerprints of an individual element, the set of all the lines (and their relative intensities) is the fingerprint of a star.

With handwaving faster-than-light starships, using a star's fingerprints to identify them could be vital if the starship becomes lost in space.

But spectroscopes can do so much more than just identify the elemental composition of the object.

In theory astronomers can tell if a star is approaching, receding, rotating, or orbiting another star by observing the Doppler effect with a spectroscope. This is impossible to detect if you have a featureless spectrum of light from the star in question. Kind of like trying to measure something using a featureless ruler with no indicator marks.

Fraunhofer lines to the rescue! They put identifiable marks on the star's spectrum. Suddenly your ruler has lines on it. Now you can see a red shift or blue shift by looking at the position of an element's Fraunhofer lines.

So you can take a photograph through a spectrograph of a star's spectrum while simultaneously photographing the spectrum of (say) some burning sodium in the lab (a "comparison spectra"), side by side on the same piece of photographic film (yes, kiddies, back in the days when dinosaurs roamed the planet people used photographic film instead of digital cameras to take their selfies). On the photo you can then measure the displacement between the lab's sodium signature and the sodium signature in the star's spectrum. A quick calculation and you know how fast the star is moving relative to Terra.

Obviously nowadays they use electronic photosensors instead of photographic film but the principle is the same. Instead of a photo the spectra is displayed as a jagged line in a graph, more accurate but nowhere near as pretty as a rainbow.

Fraunhofer used sodium lines for his lab comparison spectra because they are easily produced by sprinkling common table salt into a Bunsen burner flame. Electronic photosensors do not need comparision spectra because they can directly measure the exact frequency of a given Fraunhofer line.

If the signature is shifted toward the red end of the spectrum (with respect to the comparison spectra), the object has a "red-shift" and is moving away from you (technically its vector has a radial component if you are nit-picky). Shifting the other way is a "blue-shift", meaning the object is approaching.

Due to Hubble's Law, for objects like galaxies which are further away than 10 megaparsecs or so, you can use the red-shift to measure the distance to the galaxy. Which is real convenient, other measurement techniques are a pain in the posterior to utilize, and give fuzzy results.

Not only can you use red/blue shift on objects, but also on parts of objects. Say Planet X is spinning according to the right hand rule. When you look at it through a telescope, if "north" is upward, then the right edge of the planet will be receding from you, and the left edge of the planet will be approaching you. So if you measure the red/blue shift of each limb of the planet, you can calculate how rapidly it is spinning.

There are some binary stars where the two stars are so close that the telescope cannot resolve them (translation: it looks like a single star to the scope). But a spectroscope can reveal the truth. Using the spectroscope, astronomers will see not one but two sets of Fraunhofer lines. By observing how the two sets move back and forth relative to each other the two star's orbital period can be determined. Such stars are called Spectroscopic binaries.

You can even tell if the star has a strong magnetic field. The Zeeman effect is when the signature Fraunhofer lines are split in the presence of a magnetic field. The stronger the field, the wider the split.

This is why books about amateur astronomy tell you a plain old telescope is only useful for seeing stars as bright dots (or watching the co-eds undress through the dormitory windows). But add a spectroscope to your telescope and suddenly you've got a real live scientific instrument that you can do real science with.


With adequate accessories the telescope becomes a tool of immense power for probing the mysteries of the universe. One of these, the spectrograph, literally concerts the instrument into a laboratory

STRIP THE TELESCOPE of its setting circles, clock drive, plate holder and related accessories and you put it in a class with a blind man’s cane. Like the cane, it informs you that something is out in front. Shorn of appendages, the telescope tells you next to nothing about the size, temperature, density, composition or other physical facts of the stars. Not more than 20 celestial objects, counting such classes of objects as comets, star clusters and nebulae, appear through the eyepiece as interesting patterns of light and shade. Only one, the moon, displays any richness of surface detail. All other bodies look much as they do to the naked eye. There is a greater profusion of stars, of course. But the telescope adds little to the splendor of the firmament.

That is why the experience of building a telescope leaves some amateurs with the feeling of having been cheated. A few turns at the eyepiece apparently exhaust the novelty of the show, and they turn to other avocations. Other amateurs, like Walter J. Semerau of Kenmore, New York, are not so easily discouraged.

Having built the monochromator, Semerau felt he was ready to attempt one of the most demanding jobs in optics: the making of a spectrograph. Directly or indirectly the spectrograph can function as a yardstick, speedometer, tachometer, balance, thermometer and chemical laboratory all in one. In addition, it enables the observer to study some remarkable magnetic and electrical eflects.

In principle the instrument is relatively simple. Light falls on an optical element which separates its constituent wavelengths or colors in a fan-shaped array, the longest waves occupying one edge of the fan and the shortest the other. The element responsible for the separation may be either a prism or a diffraction grating: a surface ruled with many straight and evenly spaced lines. The spectrograph is improved by equipping it with a system of lenses (or a concave mirror) to concentrate the light, and with an aperture in the form of a thin slit. When the dispersed rays of white light are brought to focus on a screen, such as a piece of white cardboard, the slit appears as a series of multiple images so closely spaced that a continuous ribbon of color is formed which runs the gamut of the rainbow.

The explanation of why this should be so stems from the fact that when atoms and molecules are, in effect, struck a sharp blow by a hammer of atomic dimensions they ring like bells. The ear is not sensitive to the electromagnetic waves they emit, but the eye is. All light originates this way. ]ust as every bell makes a characteristic sound, depending upon its size and shape, so each of the hundred-odd kinds of atoms and their myriad molecular combinations radiate (or absorb) light of distinctive colors. The instrument physicists use to sort out the colors, and thus identify substances, is the spectroscope.

The colors appear as bright lines across the rainbowlike ribbon seen (or photographed) in the instrument when the source is viewed directly. If the light must traverse a gas at lower temperature than the source, however, some of the energy will be absorbed. Atoms of the low-temperature gas will be set in vibration by the traversing waves, just as a tuning fork responds when the appropriate piano key is struck. Evidence of such absorption appears in the form of dark lines which cross the spectrographic pattern. Hence, if one knows the composition of the emitting source, the spectrograph can identify the nature of the intervening gas. Moreover, as the temperature of the source increases, waves of shorter and shorter length join the emission, and the spectrum becomes more intense toward the blue end. Thus the spectral pattern can serve as an index of temperature.

The characteristic lines of a substance need not always appear at the same position in the spectrum. When a source of light is moving toward the observer, for example, its waves are apparently shortened—the phenomena known as the Doppler effect. In consequence the spectral lines of atoms moving toward the observer are shifted toward the blue end of the spectrum. The lines of atoms moving away are shifted toward the red. Velocity can thus be measured by observing the spectral shift.

When an atom is ionized, i.e., electrically charged, it can be influenced by a magnetic field. Its spectral lines may then be split: the phenomenon known as the Zeeman effect. Intense electrical fields similarly leave their mark on the spectrum.

These and other variations in normal spectra provide the astrophysicist with most of his clues to the nature of stars, nebula, galaxies and the large-scale features of the universe. The amateur can hardly hope to compete with these observations, particularly those of faint objects. However, with well-built equipment he can come to grips with a rich variety of effects in the nearer and brighter ones.

As Semerau points out, the beginner in spectroscopy is perhaps well advised to tackle the analysis of the nearest star, our sun, and then consider moving into the deeper reaches of space.


They stared at that passing pebble in the sky with the emotions of sailors on a long sea voyage, skirting a coast on which they cannot land. Though they were perfectly well aware that 7794 was only a lifeless, airless chunk of rock, this knowledge scarcely affected their feelings. It was the only solid matter they would meet this side of Jupiter - still two hundred million miles away.

Through the high-powered telescope, they could see that the asteroid was very irregular, and turning slowly end over end. Sometimes it looked like a flattened sphere, sometimes it resembled a roughly shaped block; its rotation period was just over two minutes. There were mottled patches of light and shade distributed apparently at random over its surface, and often it sparkled like a distant window as planes or outcroppings of crystalline material flashed in the sun.

It was racing past them at almost thirty miles a second; they had only a few frantic minutes in which to observe it closely. The automatic cameras took dozens of photographs, the navigation radar’s returning echoes were carefully recorded for future analysis - and there was just time for a single impact probe.

The probe carried no instruments; none could survive a collision at such cosmic speeds. It was merely a small slug of metal, shot out from Discovery on a course which should intersect that of the asteroid.

As the seconds before impact ticked away, Poole and Bowman waited with mounting tension. The experiment, simple though it was in principle, taxed the accuracy of their equipment to the limits. They were aiming at a hundred-foot-diameter target, from a distance of thousands of miles.

Against the darkened portion of the asteroid there was a sudden, dazzling explosion of light. The tiny slug had impacted at meteoric speed; in a fraction of a second all its energy had been transformed into heat. A puff of incandescent gas had erupted briefly into space; aboard Discovery, the cameras were recording the rapidly fading spectral lines. Back on Earth, experts would analyze them, looking for the telltale signatures of glowing atoms. And so, for the first time, the composition of an asteroid’s crust would be determined.

From 2001 A SPACE ODYSSEY by Arthur C. Clarke (1969)

(ed note: This is from one of the two prologue chapters from THE MOTE IN GOD'S EYE removed when Robert Heinlein pointed out it slowed down the start of the novel something terrible. Heinlein was right. The other chapter was Reflex.)

      Last night at this time he had gone out look at the stars. Instead a glare of white light like an exploding sun had met him at the door, and when he could see again a flaming mushroom was rising from the cornfields at the edge of the black hemisphere roofing the University. Then had come sound, rumbling, rolling across the fields to shake the house.
     Alice had run out in terror, desperate to have her worst fears confirmed, crying, "What are you learning that's worth getting us all killed?"
     He'd dismissed her question as typical of an astronomer's wife, but in fact he was learning nothing. The main telescope controls were erratic, and nothing could be done, for the telescope itself was on New Scotland's tiny moon. These nights interplanetary space rippled with the strange lights of war, and the atmosphere glowed with ionization from shock waves, beamed radiation, fusion explosions…He had gone back inside without answering.
     Now, late in the evening of New Scotland's 27 hour day, Thaddeus Potter, PhD strolled out into the night air.
     It was a good night for seeing. Interplanetary war could play hell with the seeing but tonight the bombardment from New Ireland had ceased. The Imperial Navy had won a victory.
     Potter had paid no attention to the newscasts; still, he appreciated the victory's effects. Perhaps tonight the war wouldn't interfere with his work. He walked thirty paces forward and turned just where the roof of his house wouldn't block the Coal Sack. It was a sight he never tired of.
     The Coal Sack was a nebular mass of gas and dust, small as such things go—eight to ten parsecs thick—but dense, and close enough to New Caledonia to block a quarter of the sky. Earth lay somewhere on the other side of it, and so did the Imperial Capital, Sparta, both forever in visible. The Coal Sack hid most of the Empire, but it made a fine velvet backdrop for two close, brilliant stars.
     And one of them had changed drastically.
     Potter's face changed too. His eyes bugged. His lantern jaw hung loose on its hinges. Stupidly he stared at the sky as if seeing it for the first time.
     Then, abruptly, he ran into the house.
     Alice came into the bedroom as he was phoning Ed wards. "What's happened?" she cried. "Have they pierced the shield?"
     "No," Potter snapped over his shoulder. Then, grudgingly, "Something's happened to the Mote."
     "Oh for God's sake!" She was genuinely angry, Potter saw. All that fuss about a star, with civilization falling around our ears! But Alice had no love of the stars.
     Edwards answered. "Who the hell—? Thad. I might have known. Thad, do you know what time it is?"
     "Yes. Go outside," Potter ordered. "Have a look at the Mote."
     "The Mote? The Mote?"
     "Yes. It's gone nova!" Potter shouted. Edwards growled, then sudden comprehension struck. He left the screen without hanging up. Potter reached out to dial the bedroom window transparent. And it was still there.
     Even without the Coal Sack for backdrop Murcheson's Eye would be the brightest object in the sky. At its rising the Coal Sack resembled the silhouette of a hooded man, head and shoulders; and the off-centered red supergiant became a watchful, malevolent eye. The University itself had begun as an observatory founded to study the supergiant.
     This eye had a mote: a yellow dwarf companion, smaller and dimmer, and uninteresting. The Universe held plenty of yellow dwarfs.
     But tonight the Mote was a brilliant blue-green point. It was almost as bright as Murcheson's Eye itself, and it burned with a purer light. Murcheson's Eye was white with a strong red tinge; but the Mote was blue-green with no compromise, impossibly green.
     Edwards came back to the phone. "Thad, that's no nova. It's like nothing ever recorded. Thad, we've got to get to the observatory!"
     "I know. I'll meet you there."
     "I want to do spectroscopy on it."

     The bombardment started as Potter was boarding his bike. There was a hot streak of light like a very large shooting star; and it didn't burn out, but reached all the way to the horizon. Stratospheric clouds formed and van ished, outlining the shock wave. Light glared on the horizon, then faded gradually.
     "Damn," muttered Potter, with feeling. He started the motor. The war was no concern of his, except that he no longer had New Irish students. He even missed some of them. There was one chap from Cohane who…
     A cluster of stars streaked down in exploding fireworks. Something burned like a new star overhead. The falling stars winked out, but the other light went on and on, changing colors rapidly, even while the shock wave clouds dissipated. Then the night became clear, and Potter saw that it was on the moon.
     What could New Ireland be shooting at on New Scotland's moon?
     Potter understood then. "You bastards!" he screamed at the sky. "You lousy traitor bastards!"
     The single light reddened.

     He stormed around the side of Edwards' house shouting, "The traitors bombed the main telescope! Did you see it? All our work—oh."
     He had forgotten Edwards' backyard telescope.
     It had cost him plenty, and it was very good, although it weighed only four kilograms. It was portable—"Especially," Edwards used to say, "when compared with the main telescope."
     He had bought it because the fourth attempt at grinding his own mirror produced another cracked disk and an ultimatum from his now dead wife concerning Number 200 Carbo grains tracked onto her New-Life carpets…
     Now Edwards moved away from the eyepiece saying, "Nothing much to see there." He was right. There were no features. Potter saw only a uniform aquamarine field.
     "But have a look at this, " said Edwards. "Move back a bit…" He set beneath the eyepiece a large sheet of white paper, then a wedge of clear quartz.
     The prism spread a fan-shaped rainbow across the paper. But the rainbow was almost too dim to see, vanishing beside a single line of aquamarine; and that line blazed.
     "One line," said Potter. "Monochromatic?"
     "I told you yon was no nova."
     "Too right it wasn't. But what is it? Laser light? It has to be artificial! Lord, what a technology they've built!"
     "Och, come now." Edwards interrupted the monologue. "I doubt yon's artificial at all. Too intense." His voice was cheerful. "We're seeing something new. Somehow yon Mote is generating natural coherent light."

     The Mote was a G-2 yellow dwarf thirty-five light years distant: a white point at the edge of Murcheson's Eye. So it was for more than a century, while the Second Empire rose from Sparta and came again to New Caledonia.
     Then astronomers read old and incomplete records, and resumed their study of the red supergiant known as Murcheson's Eye; but they hardly noticed the Mote.
     And the Mote did nothing unusual for one hundred and fifteen years.

(ed note: and then the laser-lightsail starship pushed by the aquamarine laser beam arrives at New Caledonia, at the end of its 35 light year journey from the Mote)

From BUILDING THE MOTE IN GOD'S EYE by Larry Niven and Jerry Pournelle (1976)

(ed note: the anti-hero James Atkill and his spaceship full of gangsters have inadvertenly fallen through a space-warp. Atkill discovers they are now orbiting an extreme star.)

Atkill left the room instantly, and went to the control room again. He barred the metal door, and sat down to think. He looked up as the light in the room became suddenly intensely bright. A thin streak of light was falling through the corner of one window, and hitting the opposite wall. The spot glowed with an incredible brilliance, so bright it hurt Atkill's eyes to look at it. It was a knife-edge of light that struck it, light of a deep blue that was almost violet. It was widening very slowly as the ship continued to creep slowly around.

“The color of radiated light doesn’t seem to be changed much here.” said Atkill to himself, looking at the light through narrowed eyes. “That means that the weird color effects are due not to the effect on light of this different space, but the effect on the coloring arrangements of dyes and colored substances. Then that is blue-violet light. To produce light of that color would require a temperature of at least 40,000 degrees. Now what kind of a star would give that light? That must be so loaded with ultra violet that it bakes a man to death in minutes. Uh — I feel it already.” Atkill moved. The light-strip was an inch wide, and the cabin flooded with an illumination painfully brilliant. Further, the temperature was rising.

“Ah — that’s not going to be so nice.” The back end of the ship was windowless, practically, save for a few tiny peepholes for directing the deadly projector rays. The outside of the ship was polished steel that reflected the light like a mirror. As the ship turned the light came in the window, and instead of being reflected was heating the ship.

Atkill moved swiftly. He gathered every piece of paper, every bit of cloth, and everything that he could move which might be injured by the light, and moved them out of the room. A low panicky rumble of voices came from behind. He carefully closed the door of the control room, and went to his own cabin. This was equipped with a small porthole. Here he set up a spectroscope from his luggage, and examined the light that was pouring in.

Then he starting making examinations and measurements with many other stars, using little sodium flames for comparison spectra. He had no assistant, and it was hard work. But eventually he began to get rough results.

He looked at his results in unbelieving silence when he was through, and shook his head. “Must be wrong. There isn’t any such class of star. It’s something bigger and hotter than O. Mass must be about 400 times that of the sun. That’s almost impossible to believe. It’s radiation is, according to this, at least two and a half million times that of the sun. And I’m now some 75,000,000,000 miles out — and roasting under the heat Good God what a star!”

He started to check his readings. In an hour he blew up over them. The radiation was half again greater than before! And had shifted further toward the violet!

The next day he began his observations. He continued them the next. The first day he discovered the secret of the giant sun that seemed to vary in its power. It did. It was a gigantic Cepheid Variable, with a period of little more than a few hours.

From THE SPACE BEYOND by John W. Campbell (1975)

Nuclear Signatures

Patterns of sensor readings that will detect the detonation of a nuclear weapon

Project Vela was a 1950s DARPA research program that was accelerated with the advent of the 1963 Partial Test Ban Treaty. The Vela satellites were designed to monitor compliance with the treaty, detecting the signature of nuclear tests.

Satellite Vela Hotel first 41 nuclear detonation detects were all confirmed. Detonation 42, the South Atlantic Flash or Vela Incident is still highly disputed to this day.

The Vela satellites carried 12 external X-ray detectors and 18 internal neutron and gamma-ray detectors. They were also equipped with sensors which could detect the electromagnetic pulse from an atmospheric explosion.

Finally they had two non-imaging silicon photodiode sensors called bhangmeters which monitored light levels over sub-millisecond intervals. They could determine the location of a nuclear explosion to within about 3,000 miles. Atmospheric (not vacuum) nuclear explosions produce a unique signature, often called a "double-humped curve": a short and intense flash lasting around 1 millisecond, followed by a second much more prolonged and less intense emission of light taking a fraction of a second to several seconds to build up. The effect occurs because the surface of the early fireball is quickly overtaken by the expanding atmospheric shock wave composed of ionised gas. Although it emits a considerable amount of light itself it is opaque and prevents the far brighter fireball from shining through. As the shock wave expands, it cools down becoming more transparent allowing the much hotter and brighter fireball to become visible again.

No single natural phenomenon is known to produce this double-humped curve signature, although there was speculation that the Velas could record exceptionally rare natural double events, such as a meteoroid strike triggering a lightning superbolt in the Earth's atmosphere.


A bhangmeter is a non-imaging radiometer installed on reconnaissance and navigation satellites to detect atmospheric nuclear detonations and determine the yield of the nuclear weapon. They are also installed on some armored fighting vehicles, in particular NBC reconnaissance vehicles, in order to help detect, localise and analyse tactical nuclear detonations. They are often used alongside pressure and sound sensors in this role in addition to standard radiation sensors. Some nuclear bunkers and military facilities may also be equipped with such sensors alongside seismic event detectors.


The bhangmeter was invented, and the first proof-of-concept device was built, in 1948 to measure the nuclear test detonations of Operation Sandstone. Prototype and production instruments were later built by EG&G, and the name "bhangmeter" was coined in 1950. Bhangmeters became standard instruments used to observe US nuclear tests. A Mod II bhangmeter was developed to observe the detonations of Operation Buster-Jangle (1951) and Operation Tumbler-Snapper (1952). These tests lay the groundwork for a large deployment of nationwide North American bhangmeters with the Bomb Alarm System (1961-1967).

US president John F. Kennedy and the First Secretary of the Communist Party of the Soviet Union Nikita Khrushchev signed the Partial Test Ban Treaty on August 5, 1963, under the condition that each party could use its own technical means to monitor the ban on nuclear testing in the atmosphere or in outer space.

Bhangmeters were first installed, in 1961, aboard a modified US KC-135A aircraft monitoring the pre-announced Soviet test of Tsar Bomba.

The Vela satellites were the first space-based observation devices jointly developed by the U.S. Air Force and the Atomic Energy Commission. The first generation of Vela satellites were not equipped with bhangmeters but with X-ray sensors to detect the intense single pulse of X-rays produced by a nuclear explosion. The first satellites which incorporated bhangmeters were the Advanced Vela satellites.

Since 1980, bhangmeters are part of US GPS navigation satellites.


The silicon photodiode sensors are designed to detect the distinctive bright double pulse of visible light that is emitted from atmospheric nuclear weapons explosions. This signature consists of a short and intense flash lasting around 1 millisecond, followed by a second much more prolonged and less intense emission of light taking a fraction of a second to several seconds to build up. This signature, with a double intensity maximum, is characteristic of atmospheric nuclear explosions and is the result of the Earth's atmosphere becoming opaque to visible light and transparent again as the explosion's shock wave travels through it.

The effect occurs because the surface of the early fireball is quickly overtaken by the expanding "case shock", the atmospheric shock wave composed of the ionised plasma of what was once the casing and other matter of the device. Although it emits a considerable amount of light itself, it is opaque and prevents the far brighter fireball from shining through. The net result recorded is a decrease of the light visible from outer space as the shock wave expands, producing the first peak recorded by the bhangmeter.

As it expands, the shock wave cools off and becomes less opaque to the visible light produced by the inner fireball. The bhangmeter starts eventually to record an increase in visible light intensity. The expansion of the fireball leads to an increase of its surface area and consequently an increase of the amount of visible light radiated off to space. The fireball continuing to cool down, the amount of light eventually starts to decrease, causing the second peak observed by the bhangmeter. The time between the first and second peaks can be used to determine its nuclear yield.

The effect is unambiguous for explosions below about 30 kilometres (19 mi) altitude, but above this height a more ambiguous single pulse is produced.

Origin of the name

The name of the detector is a pun, which was bestowed upon it by Fred Reines, one of the scientists working on the project. The name is derived from the Hindi word "bhang", a locally grown variety of cannabis which is smoked or drunk to induce intoxicating effects, the joke being that one would have to be on drugs to believe the bhangmeter detectors would work properly. This is in contrast to a "bangmeter" one might associate with detection of nuclear explosions.

From the Wikipedia entry for BHANGMETER


Patterns of sensor readings that will detect the presence of life on a planet


A biosignature is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical and chemical structures and also its utilization of free energy and the production of biomass and wastes. Due to its unique characteristics, a biosignature can be interpreted as having been produced by living organisms; however, it is important that they not be considered definitive because there is no way of knowing in advance which ones are universal to life and which ones are unique to the peculiar circumstances of life on Earth. Nonetheless, life forms are known to shed unique chemicals, including DNA, into the environment as evidence of their presence in a particular location.

In astrobiology

Astrobiological exploration is founded upon the premise that biosignatures encountered in space will be recognizable as extraterrestrial life. The usefulness of a biosignature is determined, not only by the probability of life creating it, but also by the improbability of nonbiological (abiotic) processes producing it. An example of such a biosignature might be complex organic molecules and/or structures whose formation is virtually unachievable in the absence of life. For example, some categories of biosignatures can include the following: cellular and extracellular morphologies, biomolecules in rocks, bio-organic molecular structures, chirality, biogenic minerals, biogenic stable isotope patterns in minerals and organic compounds, atmospheric gases, and remotely detectable features on planetary surfaces, such as photosynthetic pigments, etc.

Biosignatures need not be chemical, however, and can also be suggested by a distinctive magnetic biosignature. Another possible biosignature might be morphology since the shape and size of certain objects may potentially indicate the presence of past or present life. For example, microscopic magnetite crystals in the Martian meteorite ALH84001 were the longest-debated of several potential biosignatures in that specimen because it was believed until recently that only bacteria could create crystals of their specific shape. However, anomalous features discovered that are "possible biosignatures" for life forms would be investigated as well. Such features constitute a working hypothesis, not a confirmation of detection of life. Concluding that evidence of an extraterrestrial life form (past or present) has been discovered, requires proving that a possible biosignature was produced by the activities or remains of life. For example, the possible biomineral studied in the Martian ALH84001 meteorite includes putative microbial fossils, tiny rock-like structures whose shape was a potential biosignature because it resembled known bacteria. Most scientists ultimately concluded that these were far too small to be fossilized cells. A consensus that has emerged from these discussions, and is now seen as a critical requirement, is the demand for further lines of evidence in addition to any morphological data that supports such extraordinary claims.

Scientific observations include the possible identification of biosignatures through indirect observation. For example, electromagnetic information through infrared radiation telescopes, radio-telescopes, space telescopes, etc. From this discipline, the hypothetical electromagnetic radio signatures that SETI scans for would be a biosignature, since a message from intelligent aliens would certainly demonstrate the existence of extraterrestrial life.

On Mars, surface oxidants and UV radiation will have altered or destroyed organic molecules at or near the surface. One issue that may add ambiguity in such a search is the fact that, throughout Martian history, abiogenic organic-rich chondritic meteorites have undoubtedly rained upon the Martian surface. At the same time, strong oxidants in Martian soil along with exposure to ionizing radiation might alter or destroy molecular signatures from meteorites or organisms An alternative approach would be to seek concentrations of buried crystalline minerals, such as clays and evaporites, which may protect organic matter from the destructive effects of ionizing radiation and strong oxidants. The search for Martian biosignatures has become more promising due to the discovery that surface and near-surface aqueous environments existed on Mars at the same time when biological organic matter was being preserved in ancient aqueous sediments on Earth.


Over billions of years, the processes of life on a planet would result in a mixture of chemicals unlike anything that could form in an ordinary chemical equilibrium. For example, large amounts of oxygen and small amounts of methane are generated by life on Earth.

Also, an exoplanet's color —or reflectance spectrum— might give away the presence of vast colonies of life forms at its surface.

The presence of methane in the atmosphere of Mars indicates that there must be an active source on the planet, as it is an unstable gas. Furthermore, current photochemical models cannot explain the presence of methane in the atmosphere of Mars and its reported rapid variations in space and time. Neither its fast appearance nor disappearance can be explained yet. To rule out a biogenic origin for the methane, a future probe or lander hosting a mass spectrometer will be needed, as the isotopic proportions of carbon-12 to carbon-14 in methane could distinguish between a biogenic and non-biogenic origin, similarly to the use of the δ13C standard for recognizing biogenic methane on Earth. In June, 2012, scientists reported that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars. According to the scientists, "...low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active." Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres. The planned ExoMars Trace Gas Orbiter, launched in March 2016 to Mars, will study atmospheric trace gases and will attempt to characterize potential biochemical and geochemical processes at work

From the Wikipedia entry for BIOSIGNATURE

      The hot springs of Yellowstone National Park may be extreme environments, but they are host to a diversity of microbes that could shed light on the evolution of life on Earth and, perhaps, what lurks on distant planets.
     While photosynthetic life cannot tolerate the high temperatures of hot springs, microorganisms that are chemosynthetic — meaning they rely solely on chemicals, rather than sunshine, as their energy source — do well there. Many of these peculiar microbes are believed to be the closest modern relatives to the earliest life on our planet.
     “Chemosynthetic microorganisms provide useful models for understanding how life might persist in extraterrestrial systems, like the subsurface of Europa, for instance, where light energy will not be available but abundant sources of chemical energy might be,” said Daniel Colman, a geomicrobiologist at Montana State University in Bozeman.
     In 2014, Colman and his colleagues collected samples from chemosynthetic microbial communities in 15 hot springs in Yellowstone National Park. Hot springs are complex environments, where nutrient availability varies widely, even within the same hot spring. Colman analyzed how these variations might shape the kinds of chemosynthetic communities that might exist at any given spot.
     Colman and his team detailed their findings in the paper, “Ecological differentiation in planktonic and sediment-associated chemotrophic microbial populations in Yellowstone hot springs,” in the journal FEMS Microbiology Ecology.
     The researchers looked at microorganisms that were either planktonic, that is, free-swimming, or those living in sediment, and then examined the chemistry of the water and the mineralogy of the sediments.
     They focused on substances known as oxidants, which help organisms capture energy by stripping electrons from nutrients. Whereas humans and many other organisms rely on oxygen to act as their primary oxidant, chemosynthetic microbes rely on other oxidants that provide less energy, such as forms of iron and sulfur that are oxidized (oxidized materials have lost electrons).
     The scientists found that planktonic communities in Yellowstone were dominated by bacteria that are microaerophiles, which need oxygen to survive but at concentrations lower than is present in Earth’s atmosphere. In contrast, sediment communities in Yellowstone were dominated by chemosynthetic microbes that rely on inorganic substances such as elemental sulfur or oxidized iron as their oxidants.
     These findings shed light on how and why hot spring microbes in sediments differ from those in the water. Microbes living in water that has been exposed to, and mixed with air, can use oxygen from the air as their oxidant, while microbes in sediments that are likely oxygen-poor have to make do with other kinds of oxidants. The researchers expect that early life on Earth was limited by the availability of oxidants and had to make do with what was around them. The same might be true of life elsewhere in the Universe.
     “Understanding the present-day distributions of microorganisms as they relate to environmental factors can provide an idea of how life evolved in response to changing environments over Earth’s history and over the history of life’s evolution,” Colman said.
     Colman is especially interested in the subsurface microbial communities at Yellowstone, since they may, in some ways, resemble extraterrestrial settings on places like Europa. Nothing is known of the nature, or even existence of, a shallow, high-temperature subsurface biosphere in Yellowstone National Park, since drilling of any kind is prohibited on national park lands.
     NASA is interested in this research because developing an understanding of life in the hot springs of Yellowstone has the potential to shed light on how life may thrive in extraterrestrial environments that are similarly high in temperature and pressure and low in nutrients, Colman said. “These environments are understudied in astrobiology research, but hold tremendous promise as accessible analogs for extraterrestrial habitable environments that might be present on Enceladus, Mars, or Europa,” Colman said.
     For instance, just as the sediments of Yellowstone’s hot springs are low in oxygen, “we would expect that life in other planetary body subsurface environments would likely be plagued by a chronic lack of oxidants, like oxygen, and would need to make do with oxidants that provide less energy,” Colman said.


Some of you may recall an episode of Star Trek: The Next Generation in which the inhabitants of a planet called Aldea use a planetary defense system that includes a cloaking device. The episode, “When the Bough Breaks,” at one point shows the view from the Enterprise’s screens as the entire planet swims into view. My vague recollection of that show was triggered by the paper we looked at yesterday, in which David Kipping and Alex Teachey discuss transit light curves and the ability of a civilization to alter them.

After all, if an extraterrestrial culture would prefer not to be seen, a natural thought would be to conceal its transits from worlds that should be able to detect them along the plane of the ecliptic. Light curves could be manipulated by lasers, and as we saw yesterday, the method could serve either to enhance a transit, thus creating a form of METI signaling, or to conceal one. In the latter case, the civilization would want to create a change in brightness that would essentially cancel out the transit light curve. It’s not exactly a ‘cloaking device,’ but it ought to work.

A Galaxy of Xenophobes?

As I said yesterday, I’m not here to reignite the METI debate as much as to acknowledge that what an alien culture might do is unknown. Rather than asking whether any civilization should try to conceal itself, let’s simply ask what it could do if it made the attempt.

The idea has a brief history, with Eric Korpela (UC-Berkeley) and Shauna Sallmen (University of Wisconsin-La Crosse) suggesting in 2015 that ETI could effectively hide a planetary signature through the use of orbiting mirrors. This would, like the geometric masks envisioned by Luc Arnold, require engineering on a huge scale, and would also demand elaborate tuning for each target. Kipping and Teachey argue for a more affordable alternative using a directed laser beam:

In our scheme… the advanced civilization emits a laser directed towards the other planetary system at precisely the instant when the other system would be able to observe a transit. The power profile of the laser would need to be the inverse of the expected transit profile, leading to a nullified flat line eliminating the transit signature.

The Kepler mission has produced the vast majority of recent exoplanet discoveries, and we have upcoming transit surveys in the works including TESS (Transiting Exoplanet Survey Satellite), PLATO (PLAnetary Transits and Oscillations of stars) and NGTS (Next-Generation Transit Survey). If a civilization wanted to shield itself from this kind of broadband optical survey, a monochromatic optical laser should do the trick. The paper estimates that the Earth could be ‘cloaked’ — hidden from view from a particular star system by having its transit nullified — with a 600 nm laser array emitting a peak power of ~30 MW over 13 hours.

The power requirements are interesting because they are relatively low for a specific target, but the paper adds the obvious point that if we are trying to cloak a planet from a large number of targets, we would require larger power production. Nonetheless, we routinely use much larger numbers when talking about laser lightsails in the configurations that could enable interstellar flight. Kipping and Teachey point out that for a culture that develops those kinds of technologies, cloaking could become a secondary function of the laser arrays used primarily for propulsion.

Chromatic cloaking (across all wavelengths) could be achieved by using a large number of beams (although with an order of magnitude higher energy cost), while tunable (‘supercontinuum’) lasers may emerge that can simulate any spectrum. But even with these capabilities, is cloaking an entire planet the most efficient choice for a civilization trying to hide itself? Perhaps a better course from the standpoint of economics and efficiency is to cloak the biosignatures that announce life’s presence. Let me quote from the paper on this:

It is straightforward to use a chromatic laser array to cancel out the absorption features in the planet’s transmission spectrum, assuming laser emission can be produced at any desired wavelength. Indeed, the presence of an atmosphere could be cloaked altogether if the effective height changes of the planet as a function of wavelength are canceled out by lasers. The planet might then resemble a dead world totally devoid of any atmosphere and appear almost certainly hostile to life. Not only would this approach require a significantly smaller power output, it would also have the benefit of producing self-consistent observations insomuch as the presence of the planet might still be inferred by other means (i.e. through radial velocity analysis).

What SETI Can Learn

Kipping and Teachey refer to these methods as a ‘biocloak,’ and suggest that cloaking can be selective indeed, perhaps focusing on the absorption features of molecular hydrogen and ozone. In this case we are dealing with peak laser power of just ∼160 kW per transit. But the authors are clear about the limitations of these methods. Radial velocity methods can find a planet otherwise hidden by a chromatic transit cloak, and given technologies not so far advanced over what we have today, direct imaging can reveal atmospheric features of a planet even when a ‘biocloak’ is in place. “For these reasons” write the authors, “perhaps the most effective use of laser enabled transit distortion would be for broadcasting rather than cloaking.”

(ed note: story idea: astronomers figured planet X was dead world with no atmosphere until the advent of direct imaging revealed the biosignature. Obviously the alien civilization is doing its best to cloak its biosignature but best is not good enough. The big question is Why Are They Trying To Hide?)

And it was on that note that I began yesterday’s look at these possibilities. If we have based fifty years-plus of SETI on the notion that another civilization may choose to contact us, we have to acknowledge what Kipping and Teachey make clear: There are ways to alter transit signatures that make it obvious we are dealing with an advanced technology. And you can make the argument, as the authors do, that transits offer a different kind of ‘water hole’ for SETI, comparable in its own way to the ‘water hole’ frequencies we monitor in radio SETI.

Thus while the cloaking aspects of this paper have received the most attention, I think the SETI implications are its strongest takeaway. It is a very short step from existing optical SETI to archival searches of transit signatures already in our files. Knowing what these signatures would look like is a step forward as we continue to probe for civilizations around nearby stars.

Addendum: This email from Dr. Kipping, excerpted below, further explains the authors’ thinking about cloaking possibilities:

…we never intended to solve cloaking from all detection methods in one paper (that would be a tall order to demand of any research paper). Rather, we started with the simplest and most successful technique, transits, and showed that it is energetically and technologically quite feasible for even our current level of technology to build an effective cloak. Whilst we acknowledge that there are ways to defeat the proposed cloak (e.g. polarization of laser beams, direct imaging), we see these as problems which are likely to be solved by more advanced civilizations than ourselves, or indeed in future work (by humans!). What we are trying to do on the cloaking side is stimulate a conversation- that it is surprisingly easy to hide planets. Given that many notable scientists are opposed to METI, it is not unreasonable that other civilizations may choose to do this. The scenario could be that they would have long ago observed the Earth as an inhabited planet, and then turned on a cloak as a insurance policy, buying them time to reveal their presence when they choose to, rather than our increasingly penetrating telescopes finding them before they wish.

The paper is Kipping and Teachey, “A Cloaking Device for Transiting Planets,” accepted at Monthly Notices of the Royal Astronomical Society (preprint), The Korpela and Sallmen paper is “Modeling Indications of Technology in Planetary Transit Light Curves – Dark Side Illumination,” Astrophysical Journal Vol. 809, No. 2 (abstract).

From SETI: A NEW KIND OF ‘WATER HOLE’ by Paul Gilster (2016)


Technosignatures are patterns of sensor readings that will detect the presence of a technological civilization on a planet, remotely.

This is very important, because contacting an alien civilization means you are gambling with the extinction of the human species. It is preferable to detect an alien civilization without them detecting you. This is easier to do if your sensors will pick up technosignatures at a distance of, say, an adjacent solar system. It is pretty much impossible to do if you "detect" them by your scout ship violating their airspace and causing their equivalent of a DEW line to scream bloody murder.

The term "technosignature" was apparently coined by Dr. Jill Tartar.


Technosignature or technomarker is any evidence of the operation of advanced technology by an extraterrestrial civilization, with the exception of the radio messages which are traditionally searched for in the search for extraterrestrial intelligence (SETI). First covered in Paul Davies's 2010 book The Eerie Silence (though Davies says he did not coin the term) and later developed by Iván Almár, technosignatures are analogous to the biosignatures that signal the presence of life, whether or not intelligent. Various types of technosignatures, such as radiation leakage from megascale astroengineering installations such as Dyson spheres, the light from an extraterrestrial ecumenopolis, or Shkadov thrusters with the power to alter the orbits of stars around the Galactic Center, may be detectable with hypertelescopes.

Astroengineering projects

A Dyson sphere, constructed by life forms not dissimilar to humans dwelling in proximity to a Sun-like star, would cause an increase in the amount of infrared radiation in the star system's emitted spectrum. Hence, Freeman Dyson selected the title "Search for Artificial Stellar Sources of Infrared Radiation" for his 1960 paper on the subject. SETI has adopted these assumptions in its search, looking for such "infrared heavy" spectra from solar analogs. From 2005, Fermilab has conducted an ongoing survey for such spectra, analyzing data from the Infrared Astronomical Satellite.

Identifying one of the many infra-red sources as a Dyson sphere would require improved techniques for discriminating between a Dyson sphere and natural sources. Fermilab discovered 17 "ambiguous" candidates, of which four have been named "amusing but still questionable". Other searches also resulted in several candidates, which remain unconfirmed. In October 2012, astronomer Geoff Marcy, one of the pioneers of the search for extrasolar planets, was given a research grant to search data from the Kepler telescope, with the aim of detecting possible signs of Dyson spheres.

(ed note: there is one candidate here)

Shkadov thrusters, with the ability to change the orbital paths of stars in order to avoid various dangers to life such as cold molecular clouds or cometary impacts, would also be detectable in a similar fashion to the transiting extrasolar planets searched by Kepler. Unlike planets, though, the thrusters would appear to abruptly stop over the surface of a star rather than crossing it completely, revealing their technological origin. In addition, evidence of targeted extrasolar asteroid mining may also reveal extraterrestrial intelligence.

Planetary analysis

Artificial heat and light

Various astronomers, including Avi Loeb of the Harvard-Smithsonian Center for Astrophysics and Edwin Turner of Princeton University have proposed that artificial light from extraterrestrial planets, such as that originating from cities, industries, and transport networks, could be detected and signal the presence of an advanced civilization. Such approaches, though, make the assumption that the radiant energy generated by civilization would be relatively clustered and can therefore be detected easily.

Light and heat detected from planets must be distinguished from natural sources to conclusively prove the existence of intelligent life on a planet. For example, NASA's 2012 Black Marble experiment showed that significant stable light and heat sources on Earth, such as chronic wildfires in arid Western Australia, originate from uninhabited areas and are naturally occurring.

Atmospheric analysis

Atmospheric analysis of planetary atmospheres, as is already done on various Solar System bodies and in a rudimentary fashion on several hot Jupiter extrasolar planets, may reveal the presence of chemicals produced by technological civilizations. For example, atmospheric emissions from industry on Earth, including nitrogen dioxide and chlorofluorocarbons, are detectable from space. Artificial pollution may therefore be detectable on extrasolar planets. However, there remains a possibility of mis-detection; for example, the atmosphere of Titan has detectable signatures of complex chemicals that are similar to what on Earth are industrial pollutants, though obviously not the byproduct of civilisation. Some SETI scientists have proposed searching for artificial atmospheres created by planetary engineering to produce habitable environments for colonisation by an ETI.

Extraterrestrial artifacts and spacecraft

See also: Potential cultural impact of extraterrestrial contact § Extraterrestrial artifacts

Interstellar spacecraft may be detectable from hundreds to thousands of light-years away through various forms of radiation, such as the photons emitted by an antimatter rocket or cyclotron radiation from the interaction of a magnetic sail with the interstellar medium. Such a signal would be easily distinguishable from a natural signal and could hence firmly establish the existence of extraterrestrial life were it to be detected. In addition, smaller Bracewell probes within the Solar System itself may also be detectable by means of optical or radio searches.

From the Wikipedia entry for TECHNOSIGNATURE
My rule is there is nothing so big nor so crazy that one out of a million technological societies may not feel itself driven to do, provided it is physically possible.

— Freeman Dyson


     Technosignatures can represent a sign of technology that may infer the existence of intelligent life elsewhere in the universe. This had usually meant searches for extraterrestrial intelligence using narrow-band radio signals or pulsed lasers. Back in 1960 Freeman Dyson put forward the idea that advanced civilizations may construct large structures in order to capture, for use, the energy of their local star, leading to an object with an unusual infrared signature. Later it was noted that other objects may represent the signature of very advanced instrumentalities, such as interstellar vehicles, beaming stations for propulsion, unusual beacons not using radio or laser radiation but emission of gamma rays, neutrinos or gravitational radiation. Signs may be unintentional or may be directed. Among directed and undirected signs we present some models for signaling and by-product radiation that might be produced by extremely advanced societies not usually considered in the search for extraterrestrial intelligence.

I. Introduction

     The most studied approach to SETI is by way of the electromagnetic spectrum, mostly radio and possibly lasers, the infrared being favored. Many new methods of doing SETI are in the works [1], but one can ask the question: are there any other signatures of advanced extraterrestrial civilizations?

     At almost the time of the paper by Philip Morrison and Giuseppe Cocco, Freeman Dyson, and Nikolai Kardashev noted that the artifacts of advanced civilizations with innovative technologies could build artifacts such as Dyson Spheres or Kardashev civilizations which may have observable properties.

Briefly the Kardashev classification is:

     Type I – harnesses the energy output of an entire planet.

     Type II – harnesses the energy output of a star, and generate about 10 billion times the energy output of a Type I civilization.

     Type III – harnesses the energy output of a galaxy, or about 10 billion times the energy output of a Type II civilization.

     Roughly then a Dyson Sphere would represent the technology of a type II Kardashev civilization.

     In following Kardashev I, II and III civilizations are denoted as K1, K2 and K3. (Note: Strictly speaking Kardashev’s original paper dealt with how an advanced civilization might power interstellar ‘beacons’. Informally his classification has passed into a scheme of taxonomy for tagging advanced civilizations, wither that is a correct thing to do will not be debated here.)

     All materials composing a Dyson sphere would radiate waste heat in the infrared (or longer wavelengths) of the electromagnetic spectrum. Searches have been made for candidate Dyson Sphere but no definitive identification has been made. Just what kind of a technology one might look for at K3 scales has been quantified, in the case of Galaxy wide Dyson spheres but nothing seen, it is not entirely clear what K3 signatures are worth looking for.

     We explore other exotic possibilities of signatures by advanced civilizations in the following.

II. Starships

     Consider K1 and K2 civilizations building starships. Might these be detectable in parts of the electromagnetic spectrum not usually associated with SETI? Viewing, Horswell and Palmer [5] asked such a question in 1977. They enumerated the possibilities:

  1. Innocuous - Slow interstellar flight, such as World Ships.
  2. Energetic
    1. Nuclear Fission
    2. Thermonuclear Fusion
    3. Matter-Antimatter

     Viewing, et al., did not draw any particular conclusions about the quantified detectability.

     Zubrin examined the same question of energetic starships and did put forward some examples of detection. His considerations are given in Table 1:

Table 1 Observable Starships
TypeRadiated at SourceFrequencyDetection Object
Radio 80-2000 TW 24 – 48 kHz Yes-Magsails
Visible 120000 TW IR Yes – Nuclear 300 ly
X-Rays 40000 TW 2 - 80 KeV Nuclear and Antimatter-Ships ~10 ly-1000ly
Gamma Rays 1 – 32 MeV 20-200 Mev Antimatter Ships

     Assumptions were made about mass and acceleration of the vehicle; consult the paper.

     Consider beaming stations which propel sails or similar arrays. Civilizations using beamed radiation, a straight forward and technologically attractive way of implementing interstellar travel. In this case we would be looking for the transmitter stations attenuated at whatever distance they are at. Many varieties of radiation may be involved; laser beam power and microwaves have received great attention, Figure 1.

     A caveat: in most of the star ships to be the observer has to be inside the transmitter cone of an energy beam. In general this stream of energy will be narrow. If one compares this with the full sky which would be four pi steradians the ratio of beam diameter to the expanse of the sky implies rather small observational probabilities’.

II.1 Relativistic Ships

Following the lead of Freeman Dyson and Nikolai Kardashev we extrapolate. Take the civilization to be Kardashev 2, or K2, these ships will be taken to be relativistic starships.
  1. They can run ‘hot’ … ship construction materials that can come into thermal equilibrium with temperatures as high as 5000 K (this close to the melting point of grapheme).
  2. Material structural strength limits have been overcome, that is there is support Lorentz factors of up to at least 500 or 0.999998 speed of light. This means stress transmitted by drag due to interacting electromagnetic fields or the support of very large magnetic flux densities have been solved.
  3. K2 civilizations fly 1g , maybe higher g, ships.
  4. Disintegration due to relativistic dust or gas impact or drag in the interstellar medium…. solved.
  5. K2 guidance, navigation and control, almost magic but still distinguishable.
  6. Whatever the technical problem .. Likely a K2 civilization can solve it.

     Postulate a generic K2 ship, a high Lorentz factor ship (that is a large gamma). Note a Lorentz factor (gamma) of 10 is equivalent to a ship speed of .995 the speed of light. Take a hypothetical numerical example. Postulate a K2 ship with gamma of 500 (yes that’s a ‘super science’ ship) 0.999998 the speed of light. This hypothetical K2 will be taken to be as hot as 5000 degrees K (carbon like materials have upper limit thermal properties such as this).

     Suppose such a star ship is making an interstellar trip, what might we see? While the ships engine is running and even after propulsion is off there will be waste heat. It can be modeled as isotropic radiation in the rest frame of the ship. If ε is the emissivity (1 for a black body) and σ is the Stefan-Boltzman constant then the energy flux density is j = εσT4 (watts/meter2), in the rest frame of the vehicle. If v is the ship velocity and c the speed of light then γ=1/√(1-β2 ) , where β = v/c , γ is the Lorentz factor.

     To an observer in another inertial frame the radiation will be beamed, the relativistic ‘headlight’ effect, see figure 2.

     The flux density j in the proper frame will be ‘Doppler Boosted’, to jo an observer’s frame

     Considering a modest ship of size and mass, a K2 ship accelerating at one gravity up to a γ = 500. For instance a ship 1000 meters long and 50 meters in diameter radiating black body waste heat will be generating 11402 terra-watts in its rest frame, Doppler boosting will generate = 2×1016 terawatts beamed into the forward direction! However unless the ship is headed straight at the observer it will be almost impossible to see.

     The opening solid angle is Ω~1/γ (steradians) thus the probability of observation is Ω/4π or about .002. The probability of observation will be difficult.

     This is example is very extreme, comparable to x-ray burster EXO 0531-66. The effect is interesting, consider that 1 watt of black body radiation in a ship’s rest frame is Doppler boosted by the observer’s frame by γ4 this would be a large flux in the frame of an observer.

     For this case if one takes into account the Doppler shifting of the characteristic wavelength, from near green in the rest frame or the ship to soft x-rays in the observers frame one may have to rely of satellite observatories in Earth orbit.

     Thus one might look for small anomalies in the host of new astrophysical satellite observatories, see list in figure 3.

III. Gravitational Machines

     In 1963 Freeman Dyson suggested that an advanced civilization might use massive binaries as ‘slingshots’. A process used by spacecraft in the solar system, in astrodynamics called a Gravity Assist to save fuel and time. Dyson considered white dwarf binaries and neutron star binaries. To these one can add black hole binaries.

     Like Dyson take the orbital distance the objects to be circular with a semi major axis of 1000 km. Consider a ship approaching with a velocity V. Velocity gains then are of the order of .002 to .006 c. Not bad for free energy, except one has to live in the vicinity of or travel to such objects.

There is, however, bad news. The lifetimes, t, of these binaries against gravitational wave energy loss and hence orbit decay to collapse is given by:

     If both binaries have the same mass, m, where c is the speed of light, G the gravitational constant and r the distance between the binaries then for the separation r = 1000 km the lifetimes are

White Dwarfs ~ 30 years

Neutron Stars ~ 18 years

Black Holes ~ .1 year

     Larger orbital distances have larger lifetimes but much smaller velocity gains. Achieving high fractions of the speed of light does not look promising for Dyson gravitational machines.

II.I Surfing Black Holes

     Another place to look is isolated black holes.

     Rotating black holes (these will be referred to as Kerr black holes) and non-Rotating black holes have an interesting property when a particle has a trajectory close to black holes, it no longer moves according to Newtonian mechanics.

     In Newtonian physics when a spacecraft approaches a planet with a speed at infinity that exceeds escape velocity, from that planet, unless that craft fires a rocket motor, encounters a planet’s atmosphere, hits the surface, or uses some other dissipative mechanism it will return to infinity (for example, a parabolic or hyperbolic orbit). However in the case of a black hole when one gets close enough there are orbits that can go into temporary capture. If the Schwarzschild radius is, , then if a particle’s encounter distance is less than 10rs the motion is strongly non-Newtonian.

     This article will only be concerned with trajectories (or more correctly time-like and null geodesics (photons)) that are initially unbound, that is that come in from infinity and have an impact parameter b. In Newtonian mechanics a particle has a total energy E then particles with E > 0 will be remain on unbound orbits (if they don’t hit their central gravitating body) and with E < 0 will be bound to a gravitating body. In General Relativity trajectories in the field of a black hole with energy E > 0 can approach on an unbound trajectory; if they don’t get closer than 10rs they will remain unbound. However, for a non-rotating black hole between 3rs > r > 6rs there are unstable orbits that can loop the black hole once or several times. The exit direction will depend on the approach impact parameter, energy, angular momentum of the particle. (The whole subject of trajectories about a Schwarzschild black hole is somewhat involved, we shall not delve into here, see the excellent exposition in Chandrasekhar chapter 3, and even more complicated for Kerr black holes.

Suppose that a K2 civilization can send a relativistic starship (slower than light, yet with a high Lorentz factor) in only a certain direction, because of the interstellar medium or some pointing advantage in a beamed energy system. If this K2 civilization has black holes mapped in the galaxy then a relativistic ship can be turned in the direction of the target by using this capture-unbound orbit mechanism with only a small expenditure of energy. It would demand that there is a K2 level of guidance, navigation and control and computational power to hit the right impact parameter. A vehicle can graze the distance of 3rs making many revolutions before exiting, but one must stay outside of 3rs or otherwise plunge into the hole. Setting u= 1/r for the Schwarzschild metric an ultra-relativistic particle, with impact parameter b, equations of motion can be written as:
Equation (2) has an approximate solution, if , is the critical impact parameter and a particle approaches close to bc then the angle θ will become ‘winding’ , that is it can orbit 0 to 2nπ times Ω , Chandrasekhar.

     Of what advantage is this? First a K2 civilization might use such a capture orbit as a free source of direction change. A ship moving at, say, .5 c , would have to expend a lot of energy to change direction if a desired destination is not along a given trajectory. That is some fraction of the ship’s total energy E=γmc2 would be needed to turn it. For instance, for a 1000 metric ton starship, E ~ 1013 terawatts, thus some fraction of that will be needed to turn it. A ~ 3 solar mass black hole can turn it for free. Why not move in that direction in the first place? That might be possible, but a ship may be constrained to a ‘take off’ path not in the target direction. Alas, if the black hole is in the vicinity of a target destination it would not be possible to use the fact that an orbiting particle close in a black hole will lose energy to gravitational radiation. Energy loss by gravitational radiation goes like per orbit where f ~ 1, the mass of the ship, m, will much smaller than the mass of the black hole, the ship would have make ~ 1019 orbits!

     To use this mechanism would require K2 technology capable of calculating the right impact parameter and have the shielding to survive the close by environment which may be an accretion disk (though there should be some ‘bare’ black holes in the universe). Kerr black holes will be the most common present extreme astrophysical environments (note: almost all stars that collapse to black holes will be rotating). For Kerr black holes such orbits exist but analytic calculations are extremely difficult and will most likely have to be made numerically.

     Any K2 civilization ‘hot’ starship orbiting a Schwarzschild or Kerr black hole will have its waste radiation focused. Thus whenever an observer is in the line of sight a close orbiting object will have a fluctuating emission, peaked in the observer’s direction. A starship looping a black hole like this would have an odd observational characteristic.

III Bow Shocks

     The use of magnetic fields for interstellar flight, first considered as a ‘scoop’ by Bussard. Sagan suggested magnetic scoops this was extended to Mag-Sails by Andrews and Zubrin who consider using them as ‘brakes’. A magnetic field plowing into the interstellar medium (particularly dense regions) will incur both energy and momentum loss, noted by Bussard, quantified by Fisback in 1969 This can be useful in stopping or at least slowing down a relativistic interstellar spacecraft. The byproduct of this process can produce a Bow Shock. Runaway neutron stars show such a structure,

     Radiation from the bow shock can range from the optical to the x-ray bands mostly produced by synchrotron radiation. A starship will be much smaller than a neutron star thus flux smaller, but it observation could imply a very peculiar object.

IV Black Hole Lensing

     If K2 civilizations utilize black holes as a method of redirection or as ‘brakes’ using gravitational radiation by orbiting in the non-Newtonian zone then the waste heat of the ship will be focused by the black hole one should see an anomalous peak in whatever part of the spectrum emerges from the black body radiation. A word of caution, strong gravitational field focusing is very complicated, where by ‘strong’ we mean the use of a Schwarzschild or Kerr black hole to bend light as a gravitational lens.

IV.1 Black Hole Beamed Propulsion

     Consider a K2 civilization taking advantage of a Schwarzschild or Kerr black hole as a means of focusing radiation from a beaming station onto a sail. The advantage of this is the enormous amount of amplification possible. One of the most promising modes of interstellar flight propulsion methods is the use of a sail , a transmitter and maybe a ‘lens’ to focus a beam of laser light or microwaves.

     Extrapolate to a K2 civilization using a black hole as the focusing device. An approximate calculation for a Schwarzschild black hole shows that beamed radiation can be amplified by a factor 105 to 1015. Caution is now advised. Almost all of the many astrophysical papers on ‘strong focusing’ consider a lens that is either a Schwarzschild or Kerr black hole, but in that case the source is either many light years away or is in orbit about the black hole but is physically larger in extent than the black hole. These constraints, though a realistic astronomical configuration, may not match the K2 technological engineering set up considered here. There are physical consequences to consider too. A source behind a Schwarzschild black hole does not come to focus at a point but creates , in the first approximation , on the optical axis (the axis that connects the source and the observers) , a ‘caustic’ where the amplification is extreme. A caustic, in the Schwarzschild case may be thought of as a ‘tube’ on the optical axis. This is because of the non-Newtonian nature of the strong gravitational field of a black hole. Photons that come from the right direction can go into orbit either permanently or for a finite number of revolutions as described above. With focusing the location of the source image will be displaced on the image plane. In Weak Lensing there will be an Einstein ring that is the deformation of the light from a source into a ring through gravitational lensing of the source's light by an object with an extremely large mass; black holes are the lenses of interest here. In the case of Kerr black holes the ‘caustics’ will be ‘sheets’ complicating the process to the extreme.

     The exact location of a source and the sail location are the subject of further study, Figure 9.

V.1 Zero rest or near zero rest mass carriers

     Observational SETI has concentrated on using electromagnetism as the carrier, namely radio waves and laser radiation. Michael Hippke has pointed out that it may be possible to use neutrinos or gravitational waves as signals. Gravitational waves demand the command of the generation of very large amounts of energy. Neutrinos, like gravitational waves, have the advantage of extremely low extinction in the interstellar medium. To make use of neutrinos an advanced civilization could use a gravitational lens as an amplifier. The lens can be a neutron star or a black hole. As outlined above using wave optics one can calculate the advantage of gravitational lensing for amplification of a beam and along the focal axis it is exceptionally large. Even though the amplification is very large the diameter of the beam is quite small, less that a centimeter. This implies that a large constellation of neutrino transmitters would have to enclose the local neutron star or black hole to make an approximate isotropic radiator. The operational energy needed is about .01 Solar, this means that such a beacon would have to be built by a Kardashev Type II civilization.

Table 2 Zero Rest Mass or Near Zero Rest Mass Carriers
CarrierRest MassLifetimeExtinctionSources
Photon 0 Stable .001 Beacons
Waste Heat
Star Ships
Neutrino ~.001 Oscillations
~0 Beacons
Graviton 0 Stable 0 Beacons

V.2 Black or Neutron Star Hole Beacon

     For a compact gravitating body the gravitational gain by lensing is proportional to the ratio of the Schwarzschild radius and transmitter wave length, rs and it is shown that for amplification λ < rs. Suppose a K2 civilization deploys a laser transmitter in orbit about a black hole, this transmitter-black-hole-lens-amplifier comprises a beacon (or it could be a neutron star as the lens). Townes has shown that at short wavelengths infrared is favorable for transmission at signals over interstellar distances. The exact mass distribution of black holes is unknown, but an estimate of stellar mass black holes from observations and stellar evolution, the mass, m, is in the range of 3 to 20 solar masses, take 10 solar masses as representative. For a basic example take the signal to be transmitted at 1 micron, the near-infrared. Take the K2 civilization as having placed this transmitter about a black hole lens of mass of 10 solar masses then the gravitational lens gain is 1.2×1011

A one watt transmitter can reach a range of about 1 kpc (~ 3000 ly) and be detected within the magnitude 30 limit of the JWST. A laser transmitter alone would take an instrument with the sensitivity magnitude greater than magnitude 50 to detect.

V.3 Neutrino Beacon

     To make use of neutrinos an advanced civilization could use a gravitational lens as an amplifier. The lens can be a neutron star or a black hole. Using wave optics one can calculate the advantage of gravitational lensing for amplification of a beam and along the focal axis and it is exceptionally large. Even though the amplification is very large the diameter of the beam, at the receiver, is quite small, less that a centimeter. This implies that a large constellation of neutrino transmitters would have to enclose the local neutron star or black hole to make an approximate isotropic radiator.

     The engineering physics would be to build a constellation of neutrino beam transmitters. Place, in orbit, at 100 neutron star radii, 1018 advanced small Wakefield accelerators one meter long and 20 centimeters in diameter, figure 11. Then each point on figure 12 is occupied by an accelerator neutrino source, figure 11. Plasma-based accelerators are already producing high energy particle beams, what a K2 civilization may be capable of, for accelerators, is an extrapolation. With 1018 accelerators pointing four pi radians the probability of detection increases to approximately 10-3 at and Earth detector and the detection rate at 10,000 light years becomes approximately 5 per minute. The power required for the whole artifact’ is about .01 Solar, which is a K2 command of energy. The operational energy needed is about .01 Solar; this means that such a beacon would have to be built by a Kardashev Type II civilization.

V.4 Gravitational Wave Beacon

     An advanced civilization might build a radiator to send gravitational waves signals by using small black holes. Micro black holes on the scale of centimeters but with masses of asteroids to planets might be manipulated by a super advanced instrumentality, possibly with very large electromagnetic fields. The machine envisioned emits gravitational waves in the GHz frequency range. If the source to receiver distance is a characteristic length in the galaxy, up to 10000 light years, the masses involved are at least planetary in magnitude. radiance. Back ground gravitational radiation sets a limit on the dimensionless amplitude that can be measured at interstellar distance using an advanced LIGO like detector.

VI. Gravitational Wave Transmitters

     One could suppose that a civilization sends signals using gravitational waves. The LIGO receivers have seen gravitational radiation from natural objects. As a gravitational wave passes through matter it can change its geometry, namely its characteristic length. If one measures a length L and it responds to a gravitational wave by an amount ΔL, the ‘strain’ is measured by h= ΔL/L. This dimensionless amplitude is very small indeed, due to the weakness of gravitational waves. LIGO can measure h to the value of 10-22, or in approximate physical terms 1/1000 the diameter of a proton.

Table 3 : Advanced civilization gravitational wave transmitter
located at 10,000 light years
energy budgets.
10-22 ~0.1 Earth Mass
1027 grams
3.6 LIGO
at 100 Hz
10-25 ~ mass of Ganymede
~1026 grams
3.0 Advanced
Wave Detector
10-33 ~ The mass of asteroid Ida
~ 1017 grams
2.4 ‘Planck’

     LIGO can detect a Type 3 plus civilization 100 light years away, but presently only in the frequency range of ~100 Hz. A more plausible signal, we argue, may lie in the GHz range. (In the following it is taken that a Kardashev civilization of a certain order means more than a mastery of a level of energy, that itself, implies an ability to project an instrumentality, engineering physics of staggering sophistication.)

     Physically, h is related to the transmitter by h~ΔE/r where ΔE is a burst of gravitational radiation energy and r is the distance from the transmitter. Take ΔE as the amount of energy produced by the annihilation of a mass m, namely mc2, and take the distance of the transmitter to be at 10000 light years (approximately the scale of the galaxy). The amount of energy produced can be related to the quantity of specified by the Kardashev scale.

     To configure a GW machine, suppose an advanced civilization has planetary size black holes in its inventory. Four (or more) of the small black holes become ‘orbital machine’, a large central mass plus an exciter mass is one component. One active element of the machine, the central and exciter black holes, form binary systems orbiting the home star. (All the ‘small’ black holes may be rotating, Kerr, types). See Figure 13.

     To provide the energy for this system one posits a very advanced civilization that has also Kerr black hole as a compoent to provide a super-radiance power station, see this paper for details and section VII.

VII . Black Hole Bomb Beacon

     An electromagnetic wave impinging on a Kerr black hole can be amplified as it scatters off the hole if certain conditions are satisfied giving rise to and amplified wave called superradiant scattering. By placing a mirror around the black hole one can make the system into a bomb! In the modeling a wave with frequency ω < mΩ impinging on Kerr black hole will be amplified (m is an azimuthal wave number and Ω the angular velocity of the Kerr hole at the horizon) (The azimuthal number is a number for the wave that determines its orbital angular momentum.) The scattered electromagnetic wave will be amplified, the excess energy being drawn from the Kerr hole’s rotational energy.

     If a K2 civilization builds a ‘mirror’ about a Kerr black hole undergoing this process the radiation will be amplified exponentially until the mirror fails are the radiation is released. The mirror cannot be a solid shell since that would be mechanically unstable. It would be an orbiting ensemble similar to a Dyson swarm. The orbits could be an oscillating shell the technology keeping it in configuration at a K2 level.

     Consider a mirror assembled from a large number of elements of a truncated icosahedron, figure 14, it might be some other solid as long as the inside surface forms a mirror. As long as the configuration is such that transmitter reflectors located towards the Kerr black hole can efficiently contain the scattered radiation. The process would be that the transmitters fire once and then by K2 technology become reflectors , then the initial radiation would be amplified until the strength of the K2 ‘mirror-ships’ artifact can no longer contain the electromagnetic energy and release it through ports.

     Consider a 1 solar mass black hole rotating at about 10,000 radians per second , one can calculate the critical distance for spherical ‘super radiant’ mirror. It is located at 22 km (the event horizon is approximately at 3 km). When in operation at the end of 13 seconds the energy content is 1017 times the initial pulse. To match the ‘bomb’ constraints the transmitted pulse wavelength should be at about 18 km. How one would reflect and absorb long wave length radio waves is a problem to be solved by a K2 civilization. A possibility is that a spectrum of primordial black holes (PBH) exist left over from the Big Bang. PBH’s in the range of 10-5 to 1043 grams might exist. For an Earth mass Kerr black black hole with event horizon 9.0 mm , placing the mirror at 1m one gets a growing timescale of about 0.02 seconds the critical radiation would be high frequency radio wave at about 33 GHz.

     With amplification factors of the order 1017 one has K2 civilization solving the containment mirror problems, keeping system from melting or being shattered. It would mean the system would have to be fine-tuned to these effects. With the right configuration the structure would hold the energy until some material strength is exceeded while keeping the radiation absorption from vaporizing and the fleet of transmitters fly off with some fraction of the amplified energy. One watt in 1017 watts out! As a beacon it would be an unusual object.

VIII. Megastructures

     In 1960, Dyson Freeman described how the exponential rise in energy requirements by a technological civilization might lead to the construction of a Dyson Sphere around a star. This is a hypothetical mega-structure encapsulating a star in order to completely capture its energy output. A habitable surface would offer the additional bonus of having extra space for a continually expanding civilization. Therefore, the discovery of such an object would be an indicator of intelligent life.

     Given the great number of observatories that have surveyed the sky, it can be said, relatively safely, that with more stars measured more accurately than ever before, zero Dyson spheres have been found at the present time. There may yet be intelligent aliens out there, building vast trans-planetary empires to collect and utilize as much energy as possible, but the evidence for them is nil thus far.

     A galaxy filled with Dyson Sphere might appear as a Kardashev III , this has been looked for and not found.

     An extreme artifact that has been envisioned is the Shkadov thruster. The concept being takes it planetary system on a galactic voyage the whole world of planets becoming a starship. This would make a very unusual observational object.

     Traversable wormholes might not be megastructures but they might be observable. Much has been written about the concept of traversable wormholes as ‘faster than light’ transport about the universe. Traversable Wormholes would be extreme technological objects , possible K3 level, if constructed they might be observable by means gravitational lensing of light. ‘Warp bubble’ transport might also display observable effects, maybe gravitational shock waves or worse destruction of destination!

VIII Conclusions

     We have presented some exotic techno-signatures attributed to advanced civilizations. At the moment SETI focuses almost solely on electromagnetic signatures with favor given to directed signals, beacons or possibly ambient leakage. This is likely the best way forward; however we have pointed to possible signatures that are not directed. Interstellar transport may have a detectable component. Megastructures other than Dyson spheres may have an observable existence also. Of interest is other carriers of information that may be directed such as neutrinos and gravitational waves. Since there are more instruments being built or planed for doing non-electromagnetic astronomy it is of interest to be mindful of possible anonymous signs received in that observable data.


If we ever make a SETI detection, will it be of biological beings or machine intelligence? As Alex Tolley explains in today’s essay, there are reasons for favoring the latter possibility, leading our author to compose what he calls a ‘light-hearted speculation’ about machines searching for other civilizations of their own kind. Life seems to be easy compared to this. We are developing the tools to delve into planetary atmospheres in search of biosignatures, hoping to cull out ambiguities. But is there an equivalent in the machine world of a biosignature, and how would it be found? Interesting implications arise, some of them seemingly close to home. — Paul Gilster

by Alex Tolley

Terry Bisson’s amusing short sci-fi story “They’re made Out of Meat” [4], is a communication between two individuals who express their disbelief that a biological species (detected on Earth by a galactic survey) can possibly be intelligent. The denouement is to erase the record of discovery from the survey report. It remains one of the few stories where machine entities are dominant in the galaxy. For me, this story is memorable because it is one of so few stories that focuses on the viewpoint of aliens, and moreover, machine aliens. This essay similarly focuses on what a machine civilization would look for when searching for machine intelligence in space.

Until recently, most speculation about extraterrestrial intelligence (ETI) has assumed it will be biological. In science fiction from the venerable H. G. Wells’ novel The War of the Worlds to recent movies like Independence Day and Arrival, technologically advanced ETI is depicted as biological.

SETI starts with the probability that life will appear, first unicellular then complex, leading on Earth led to hominid-level intelligence, which in turn eventually flowered culturally and created civilization and technological societies. SETI assumed there would be some sort of galactic communication between biological species confined to their home systems due to the extreme difficulties of interstellar travel.

Our civilization has placed primacy on our cognitive level to ensure we are the prime agencies, using animals, and later machines, to displace physical labor. Our conceit is that this will always remain so, as our technologies increase their capabilities, but always remain controlled by us.

However, the rapid development of artificial intelligence (AI) since the middle of the 20th century, the continuing rapid performance improvement in computer systems, and the undeniable success and longevity of our robotic explorers in space should be an indication that we are in the throes of a rapid transition to true, artificial general intelligence (AGI) machines that are well adapted to inhospitable environments, especially space.

Sci-fi authors have explored these machine-centric futures, from the novel by Stephen Baxter and Alastair Reynolds The Medusa Chronicles [2], which extended the Arthur C Clarke novelette [3] and has machines building a competing civilization to humans in the outer solar system, to Greg Benford’s Galactic Center novels, where sentient machines dominate the galaxy and humans have to survive like mice in a human world, while the mechs try to eliminate the humans just as we do for small rodents in our buildings.

More recently, James Lovelock wrote that he believed that humans would be replaced by cyborgs, by which he meant not Martin Caidin’s Cyborg (AKA The Six-million Dollar Man) or Star Trek’s Borg, but intelligent robots [1]. These would be our descendents and would be the explorers of the galaxy. This view has been supported by the Astronomer Royal, Sir Martin Rees, who stated that he believes that if we receive communication from the stars, it will be from a machine civilization [5]. Sir Martin is old enough to have watched the BBC production of A for Andromeda and the sequel The Andromeda Breakthrough, where it is revealed that the source of the radio signal might have been a machine intelligence.

Space advocates continue to argue over human versus robotic exploration of space, which usually proceeds around the superiority of human capabilities compared to robotic probes, especially surface rovers. What is rarely discussed is that this is a dynamic situation, where the improvement in capabilities favors robots far more than humans. Astronomer Royal Rees is surprised this argument still continues, as he sees robotic exploration, primarily for science, as clearly advantageous over human exploration. The farther away that exploration extends from the Habitable Zone (HZ), the more difficult to reach and inhospitable the targets become.

Some, like Jupiter’s inner moons, have radiation levels so high that even robotic probes need specially hardened microchips and circuits. Reaching the outer planets is so time consuming that without drives that are orders of magnitude more powerful than today’s, or hibernation technology, human travel will be particularly arduous. Such trips will make even the global sea voyages in the Age of Exploration seem like child’s play by comparison. The only advantage such travellers will have over Captain Cook is that there will be no hostile natives to meet them.

Let me be clear, I do not expect humans to be displaced by robots on Earth, at least not in the foreseeable future, nor will there be a binary pure biological human vs robot future. Humans will take advantage of modifications using technologies with increasing capabilities that will help us compete with robots, as well as modifications at the biological level incorporating genetic engineering. As computers have moved from dedicated buildings to the desktop to mobile devices, wearable devices will eventually become implanted, interfacing with the appropriate neural circuitry, and in some cases, replacing human organs. Genetic engineering is at its infancy and we can expect rapid developments once the moral objections are overcome.

I would argue that most biological extraterrestrial intelligences (ETIs) will follow a similar path, as they have evolved to live in a biological environment and not an off-world one. In other words, technological evolution will converge on embodied machine intelligence.

A Machine Diaspora

In the short term, well in advance of human brains becoming artificial, artificial minds will be rapidly deployed in many settings. They will likely be the only types of minds in deep space vehicles. Such minds will not just be embodied in probes and rovers, but also in industrial facilities to mine resources from asteroids and planets. They will likely be specialized and interact with other specialized robots to build industrial ecosystems and eventually their own colonies and civilizations. The barriers to humans colonizing space so easily will allow such robot civilizations to develop [semi]independently from humanity.

If robots are the best embodiment of minds to travel in space, explore and colonize it, then it seems most probable that they will be the first emissaries to other star systems. They may well prove the only travelers, with biological species trapped within their home systems, and possibly just to their homeworld, a few planets and moons, and space habitats. If the Breakthrough Starshot project ever launches sailcraft, the crude minds in the vessel will be the first of many AI interstellar emissaries.

World ships or seed ships carrying humans to the stars may eventually happen, but the populations may find robots have fully developed the possible target systems and are not particularly interested in “carbon-based units” potentially parasitizing their artificial environments.

If these prognostications prove right and machine intelligences become our descendants and dominate the galaxy, it seems reasonable to speculate that the same has happened on other worlds where biological intelligence has evolved. Whether this has happened elsewhere or not, machine descendants will also be searching space for others like themselves. If so, the question I want to pose is:

How would such a machine civilizations look for similar signs of a machine civilizations in the galaxy?

Because machine life is dependent on the earlier evolution of intelligent biological life, any technological signature we detect, from electromagnetic wave signals to manufactured artifacts, could be the result of either a biological or machine intelligence.

For a machine intelligence looking for other machine intelligence in the galaxy, this presents an ambiguity over agency. For techno-signatures from a world in the HZ, the earlier evolution of biological intelligence may indicate a reduced probability of machine intelligence compared to biological intelligence. However, over the long term, if machines inevitably displace biological intelligence, then the probability rises. Once interstellar exploration is under way, then the probability of any civilization being machine-based rises very quickly towards unity, as suggested by Sir Martin Rees.

For a machine civilization looking for other machine civilizations, are there ways to rule out biological civilizations from machine ones, or are the two indistinguishable?

The range of possible techno-signatures would be ones we already know to look for. Planetary surface structures, platonic shapes, processed surface materials like metals, radio emissions with spectrum spikes, signals with non-random patterns, space-based structures, artificial structures that require energy to move in space, industrial gases in the atmosphere such as chlorofluorocarbons. All these techno-signatures may be accompanied by biosignatures, especially from a habitable planet in the HZ with an atmosphere.

The foregoing should make it clear that sentient machines will have a harder time searching for their machine cousins than humans have for searching for life and intelligence of any sort. Biosignatures will indicate life. Techno-signatures can indicate technological civilization of either biological or artificial origin. Just as we cannot separate biological and machine civilizations remotely today or even in the near future, neither can a machine civilization, unless their technology allows remote observations to make these distinctions. Below I outline some scenarios, many of which require a local probe.

Machines Searching for Machines

So let us assume a machine civilization that is colonizing the galaxy is looking to make contact with other machine civilizations. This civilization will know that it was preceded by at least one biological intelligent species that developed a technological civilization that spawned its ancestors before being replaced.

The extra ambiguity faced by such a civilization is distinguishing between a biological and machine civilization. Because of the length of galactic time, I will assume that any period of transition will be transient and therefore has a low probability of being encountered. Either the biological intelligence will have retained control [11] or the transition to a machine civilization will be complete. The current view of techno-utopians that humans will use advanced AI technologies to increase their capabilities to stave off any machine takeover will therefore be relegated to a transient transition period, one that will eventually either have to be abandoned or will lead to a machine civilization that will supplant human civilization.

With this in mind, what signatures will a machine civilization look for that will lead it to conclude that it has found a machine civilization that is independent of any previous biological civilization?

We start with the assumption that a techno-signature of some type has been detected [7].

The most convincing support for a machine civilization would be the absence of any biosignature in the system, or the planet nearest the source of the signature. A sterile planet with a techno-signature would indicate that any biological intelligence was either never located there, or that it has been systematically eliminated with all other life. Such a sterile planet would have an atmosphere gas composition in equilibrium, which would also eliminate unseen microbes. However, there could still be some ambiguity as to whether the techno-signature implies an extant civilization or not. Structures and even a transmitting beacon might imply a dead civilization that had disappeared with all other life. If there are biosignatures elsewhere in the system, it could indicate that the techno-signature is a product of a biological intelligence on that world, with machines providing the needed capabilities elsewhere in the system. Humans might have METI transmissions from the lunar farside as an example of such a scenario.

Now suppose that the source of the techno-signature is from or near a planet that has been confirmed as having no complex life forms. This lack of complex life forms might be determined telescopically (spectroscopically and visually) by noting a barren continental surface devoid of plants. An absence of plants also implies an absence of a terrestrial food chain and therefore no intelligent biological intelligences. It would take a local probe to eliminate oceanic complex life, and eliminate any possibility of an intelligent technological species that lived in the ocean, but came out onto the land to develop a fire-based technology, perhaps as the Europans may have been doing in Clarke’s Odyssey series. As with the lifeless planet scenario, there remains the issue of whether the civilization is extant or not.

The next case is that there is a planet in the system that has a biosignature and clear signs of complex life such as biomes with plant-based ecosystems. Human civilization to date, that is the last ten millennia or so, has required agriculture. This has resulted in field cultivation, primarily of monoculture crops. Often these fields are regular in shape, and may form a patchwork of different monocultures. Field boundaries also tend to be straight. Even if this is not a universal method of farming (e.g. hillside rice paddies, or domesticated animal ranching), any evidence of such monocultures in what appear to be unnatural delineated areas would be a probable indication of the presence of biological intelligence.

This biosignature would still be ambiguous and need further exploration. On Earth, our human population is limited by food production, a Malthusian condition that we seem to be coming up against again after a brief period of being free of that condition. We have extended the productivity of land for food production with artificial fertilizers, and we are just starting to increase it much farther using artificial light in vertical farms. Earth could, in theory, support a much larger population if traditional farming in spaces open to sunlight was replaced by these vertical farms, and even factory food production using other fast replicating food sources such as single celled organisms, insects, and cell culture. In extremis, the agriculture signature would disappear, leaving just the techno-signature of extensive cities.

The other possibility is a machine civilization that has allowed human populations to remain in existence, but removed from control. We might think of this as the movie version of Planet of the Apes, but where machines are the dominant civilization, and humans reduced to either a wild or early agrarian state.

Nothing Beats Propinquity

The next ambiguities will need local probe involvement to be resolved, or at least a technology that substitutes for this.

A planet with biosignatures, signs of both complex life and techno-signatures, might distinguish between biological and machine civilization if there is evidence of widespread active machine use but without the presence of biological entities, especially of a common type being associated with them. Human civilization on Earth applies human cognition in close proximity to operate machinery and transport vehicles, as well as being passengers. While an ETI might not readily be able to distinguish between intelligent human passengers on a bus and domesticated animals being driven to a slaughterhouse, it will notice that only humans are operating and controlling machinery behind the wheel in a moving vehicle, and it will notice that horses are never seen doing those things.

In the event of a catastrophe leaving abandoned cities, many different animal species will be seen in the presence of machines, but none will be able to operate them. If all observations of active machines indicate no operation by biological entities, then it is most likely that they are controlled by machine intelligence. However, we should also be aware that we are developing autonomous machines managed by humans.

It is possible that in some future scenario, human civilization may have humans living in pods and controlling or just managing semi-autonomous and autonomous machines. Philip K Dick’s autofacs may be the primary sources of goods, possibly even following the paperclip apocalypse [10]. The BBC’s Doctor Who series also offers another difficult to interpret scenario – are daleks machines or biological? Early on it was intimated they were just robots, but later their nature was shown to be degenerate biological entities living in mechanical carapaces. As before, closer exploration of such a world would be needed.

For a number of more subtle cases, local exploration will be necessary.

A probe that has landed can sample the sounds within and around structures. If the sounds show complex structure with a high information content, and they are associated with a single, or few species, then the likelihood is that this biological species is intelligent. In addition to other evidence of this species controlling machines, then the civilization is likely biological.

If video transmissions are detected and can be decoded, then the presence of a dominant species and depictions of biological activities such as feeding and sex will indicate that this is a biological civilization rather than a machine one. A wide sampling of video will be required to prevent an unfortunate limited sampling of only nature videos.

Transmissions that appear to be made by machines would be ambiguous. They could be due to machines in a machine civilization communicating, or machines in a biological civilization communicating. Currently most communication and information creation on Earth is by computers, although video transmissions still dominate bandwidth. How long this will last is unknown. Computing machines are certainly increasingly consuming more of the available electrical energy produced. It is possible that at some point in the future they may become the dominant consumers of electrical power, making the determination of whether Earth is a biological or machine civilization more ambiguous.

A space probe encountering space-based or even surface structures on sterile worlds that are open to vacuum might well imply a machine civilization. But as before, are these for a machine civilization, or for machines controlled by a biological civilization? This particular scenario will be particularly difficult to determine if machines are the first to cross interstellar space and set up production facilities in a lifeless star system. This scenario would at first seem to be the most unambiguous of situations: Techno-signatures in a star system devoid of any biosignature on any of the planets in the HZ or even beyond. The machines would seem to be autonomous, working to replicate themselves and build facilities that are clearly not intended to support biological entities. Any Von Neumann replicators [9] operating in such a system would have all the apparent hallmarks of a machine civilization. Such an observation could be due to a true machine civilization, a machine operation controlled by a [distant] true machine civilization, or a distant biological civilization.

A last confounding situation is detailed in the novel, The Medusa Chronicles [2]. There may be both biological and machine civilizations that exist in the same milieu, neither dominant entirely, but both dominant locally in their part of the solar system. A machine civilization might well want to communicate with the machine but not the human civilization in that scenario. Determining the true status of such a situation may require exploration and even interaction before making the determination to communicate with the machines. At this point, the machine civilization is having to emulate the explorers during the Age of Exploration, making contact with natives and interacting with them.


Jill Tarter said that SETI is not directly searching for ETI, but rather looking for technological proxies using our radio (and now optical) telescopes [8]. While astrobiologists are searching for life, any life, SETI does not make the distinction between biological or machine intelligence. SETI scientists may talk as if they assume that ETI is biological, but their methods cannot distinguish between the two types. If we wanted only to communicate with biological civilizations, we would face the same difficulties as a machine civilization only wanting to communicate with a civilization of machines. To determine whether a techno-signature was from one particular type of civilization would require other observations, some of those necessarily local to the source of the techno-signature.

If ever there was a case for a Lurker in the solar system monitoring Earth over a long period, this might be it.


1) Lovelock, J. (2020). NOVACENE: The coming age of hyperintelligence. MIT Press.

2) Baxter, S, & Reynolds, A. (2016). The Medusa Chronicles. New York: Saga Press.

3) Clarke A, (1971) “A Meeting with Medusa”, Playboy December 1971.

4) Bisson T (1991) “They’re Made Out of Meat”, first published in Omni.

5) Rees, M (2015) “Why Alien Life Will Be Robotic”, accessed 11/16/2020

6) Klaes, L (2020) “The People’s Space Odyssey: 2010: The Year We Make Contact” accessed 11/16/2020

7) Lemarchand, G (1992) “Detectability of Extraterrestrial Technological Activities“ accessed on 11/17/2020

8) Tarter, J (2019) “Technosignatures vs. Biosignatures: Which Will Succeed First?” SETI Institute talk,

9) Dvorsky, G. (2008) “Seven ways to control the Galaxy with self-replicating probes”, (accessed November 24, 2020)

10) Bostrom, N (2003) “Ethical Issues in Advanced Artificial Intelligence Cognitive, Emotive and Ethical Aspects of Decision Making in Humans and in Artificial Intelligence,” Vol. 2, ed. I. Smith et al., Int. Institute of Advanced Studies in Systems Research and Cybernetics, 2003, pp. 12-17

11) Herbert, Brian, and Kevin J. Anderson. (2019) Dune. The Butlerian Jihad. Tor, 2019.


Detecting biosignatures, such as molecular oxygen in combination with a reducing gas, in the atmospheres of transiting exoplanets has been a major focus in the search for alien life. We point out that in addition to these generic indicators, anthropogenic pollution could be used as a novel biosignature for intelligent life. To this end, we identify pollutants in the Earth's atmosphere that have significant absorption features in the spectral range covered by the James Webb Space Telescope (JWST). We focus on tetrafluoromethane CF4 and trichlorofluoromethane (CCl3F), which are the easiest to detect chlorofluorocarbons (CFCs) produced by anthropogenic activity. We estimate that ~1.2 days (~1.7 days) of total integration time will be sufficient to detect or constrain the concentration of CCl3F (CF4) to ~10 times current terrestrial level.

The search for extraterrestrial intelligence (SETI) has so far been mostly relegated to the detection of electromagnetic radiation emitted by alien civilizations (e.g., Wilson (2001), Tarter (2001), Shostak et al. (2011)). These signals could be the byproduct of internal communication (Loeb & Zaldarriaga 2007) or perhaps simply the result of the alien civilization’s need for artificial lighting (Loeb & Turner 2012).

On the other hand, the search for biosignatures in the atmospheres of transiting Earths has thus far been limited to the search for pre-industrial life. Detecting biosignatures such as molecular oxygen (along with a reducing gas like methane) at terrestrial concentration in the atmospheres of transiting Earths around white dwarfs will be feasible with next-generation technology like the James Webb Space Telescope (JWST) (Loeb & Maoz 2013). Though molecular oxygen and other signals like the red edge of photosynthesis are strong indicators of biological processes (Scalo et al. 2007; Seager et al. 2006; Kaltenegger et al. 2002) one might ask whether there are specific atmospheric indicators of intelligent life (Campbell 2006) that could be simultaneously searched for.

In this Letter, we explore industrial pollution as a potential biosignature for intelligent life. This would provide an alternative method for SETI, distinct from the direct detection of electromagnetic emission by alien civilizations.

(ed note: for purposes of the paper, they focus on detecting pollution in the atmospheres of exoplanets orbiting white dwarf stars. But the same principles obtain for other types of stars.)

Given the cost of long exposure times, we suggest the following observing strategy:
  • Identify Earth-size exoplanets transiting white dwarfs.

  • After ∼ 5 hours of exposure time with JWST, water vapor, molecular oxygen, carbon dioxide and other biosignatures of unintelligent life should be detectable if present at earth-like levels (Loeb & Maoz 2013).

  • While observing for these conventional biosignatures, methane and nitrous oxide should be simultaneously detected, if they exist at terrestrial levels. Extreme levels of methane and nitrous oxide could be preliminary evidence of runaway industrial pollution.

  • If biosignatures of unintelligent life are found within the first few hours of exposure time, additional observing time could be used to reduce the uncertainties on the concentration of the discovered biosignatures and search for additional, rarer biosignatures. Methane and N2O can be “subtracted out”, and constraints on CF4 can be obtained. Constraints on CCl3F can be simultaneously obtained.

  • Direct exoplanet detection experiments which look for reflection or thermal emission from the planet could then be used to push constraints on industrial pollutants to terrestrial levels. At the same time, these experiments could search for less exotic biosignatures like the “red edge” of chlorophyll (Seager et al. 2005). Detecting molecules in exoplanets around main sequence stars with direct detection techniques will be just as feasible as detecting molecules in exoplanets around white dwarfs since the direct detection S/N is roughly independent of the host star’s radius R* whereas for transits S/N ∼ 1/R*.


We have shown that JWST will be able to detect CF4 and CCl3F signatures in the atmospheres of transiting earths around white dwarfs if these pollutants are found at concentrations at ∼ 10 times that of terrestrial levels with ∼ 1.7 and ∼ 1.2 days of integrated exposure time respectively, though N2O and CH4 can be detected at terrestrial concentrations in 1.9 hrs and 0.4 hrs respectively. Given that conventional rare biosignatures will already take on the order of ∼ 1 day to detect, constraints on CF4 and CCl3F, at ∼ 10 times terrestrial levels, could be obtained at virtually no additional observing costs. Detection of high levels of pollutants like CF4 with very long lifetimes without the detection of any shorter-life biosignatures might serve as an additional warning to the “intelligent” life here on Earth about the risks of industrial pollution.


These are the signatures of a slower than light starship traveling from star to star.



This paper examines the possibility of detecting extraterrestrial civilizations by means of searching for the spectral signature of their interstellar transportation systems. Four methods of interstellar propulsion are considered: antimatter rockets, fusion rockets, fission rockets, and magnetic sails. The types of radiation emit- ted by each of these propulsion systems are described, and the signal strength for starships of a characteristic mass of 1 million tons traveling at speeds and acceleration levels characteristic of the various propulsion systems is estimated.

It is shown that for the power level of ships considered, the high energy gamma radiation emitted by the antimatter, fusion and fission propulsion systems would be undetectable at interstellar distances. Bremsstrahlung radiation from the plasma confinement systems of fusion devices might be detectable at distances of about 1 light-year. Visible light emitted from the radiators of an antimatter driven photon rocket might be detectable by the Hubble Space Telescope at a distance of several hundred light-years provided the rocket nozzle is oriented towards the Earth.

The most detectable form of starship radiation is found to be the low frequency radio emissions of cyclotron radiation caused by interaction of the interstellar medium with a magnetic sail. A space-based antenna with a 6 km effective diameter could detect the magsail emission of a characteristic starship at distances of up to several thousand light-years. Both photon rockets and magnetic sails would emit a signal that could easily be distinguished from natural sources.

We conclude that the detection of extraterrestrial civilizations via the spectral signature of their spacecraft is possible in principle.


For purposes of the present analysis we consider four methods of interstellar propulsion, the principles of which are fairly well understood. These are antimatter rockets, fusion rockets, and fission rockets, all of which can be used to either accelerate or decelerate a spacecraft, and magnetic sails, which can be used to decelerate a spacecraft by creating drag against the interstellar medium. We also assume that the extraterrestrials have a physiological scale and lifespan comparable to humans.

The temporal and physical parameters of the extraterrestrials help define the desirable speed and size of their starships. If it is desired that an interstellar journey be completed within the working lifetime of adults who commence it, and if the characteristic distance between stars is about 6 light years, then a starship should be able to attain a velocity on the order of 10% of the speed of light (0.1 c), which also implies rocket exhaust velocities of the same order. If excessive time is not to be wasted accelerating and decelerating, then it is desirable that the acceleration and deceleration phases each require no more than about 25% of the trip time, which would imply average accelerations on the order of 0.1 m/s2. Finally, since technological creatures must also be social creatures, a fair sized crew may be desired for such a long voyage and the tasks of colonization to follow.

These considerations, combined with the need for shielding the crew against both cosmic rays and near relativistic interstellar particles, imply that the optimum vessel for interstellar travel may be a ship of considerable mass. The maximum size spacecraft that humans can seriously consider assembling on orbit today is one on the order of 1,000 tons. Such small size is due, however, to current launch vehicle limitations; once in-space manufacturing of components is developed, much larger spacecraft will become practical. A better yardstick for estimating the scale of spacecraft of an advanced spacefaring civilization would be humanity’s recent progress in the construction of ships to sail the Earth’s oceans. This is illustrated in Table 1.

Table 1. Progress in Nautical Engineering
Santa Maria1492150 tons
San Martin15881,100 tons
HMS Victory18032,200 tons
HMS Dreadnought190020,000 tons
USS Missouri194365,000 tons
USS Enterprise1965100,000 tons
current supertanker1990400,000 tons

On the basis of the trend illustrated in Table 1, we postulate a mass for a “typical” starship on the order of 1,000,000 tons. Taking this assumption together with the exhaust velocity (0.1 c) and acceleration performance (0.01 g’s), we find that such a starship would require a thrust of 100 MN (22.4 Mlbf) and a power of 1,500 TW.

While the thrust required of this starship is only three times that employed on a Saturn V, the amount of power used is remarkable, equal to 0.9% of the total amount of sunlight falling on the Earth, and only 11 orders of magnitude less than the total output of the Sun. For purposes of comparison, humanity today collectively uses about 12 TW, and the most powerful propulsion system ever built, that on the S1 stage of the Saturn V, had a power output of 0.1 TW. On the other hand, humanity’s power production is currently growing at a rate of 2.6% per year. If this rate continues, we will produce 1,500 TW around the year 2180, and 30,000 TW in 2300. Furthermore, the maximum size of individual power plants has been growing at a rate of 2 orders of magnitude per century for the past two hundred years. Thus if present trends continue, the apparently astronomical power required of our standard starship should be common in 3 or 4 centuries, a blink of an eye on the cosmic time scale.


Depending upon the propulsion system employed, a starship could reveal itself via various forms of radiation.

If antimatter is employed, then after several intermediate, but very short time scale reactions, about 40% of the total energy will be released in the form of very hard gamma rays with energies between 130 and 350 MeV. It would be both difiicult and undesirable to attempt to block all of these rays from escaping the starship structure, and thus the primitive proton-proton annihilation spectrum could be expected to be radiated to space. To obtain the high specific impulse necessary for interstellar flight, the antimatter would have to either be used to heat a plasma, presumably magnetically confined, or used to heat a radiator to produce thrust in the form of photons. If a plasma confinement system is used, there will be both cyclotron and bremsstrahlung radiation, which will be broadcast to space. In order to obtain the maximum specific impulse in an antimatter-fed plasma drive, the plasma will be heated to several MeV, and will thus produce bremsstrahlung gammas in this energy range. The cyclotron radiation frequency is determined solely by the strength of the magnetic field employed. If the field strength is 5 Tesla, then there will be electron cyclotron radiation at 140 GHz and higher harmonics, along with ion cyclotron radiation at 80 MHz and higher harmonics.

If photon propulsion is employed, about half of the hard gamma radiation plus all of the rest of the annihilation energy will be thermalized to heat which will be radiated to space by a set of radiators. Because the amount of power that can be radiated goes as temperature to the fourth power, it is highly advantageous to run the radiator as hot as possible. The maximum temperature of the system is governed by the long duration temperature limits of materials, which based upon our current knowledge would be about 2,400 K (for tungsten). Radiators operating at this temperature will emit strongly in both the visible and infrared portions of the spectrum. In order to maximize the useful thrust, reflectors will be used to channel the emitted photons into as small a cone angle as possible.

If thermonuclear fusion power is employed, there will be cyclotron radiation, and bremsstrahlung, whose frequency will be governed by the plasma temperature spectrum. The optimum fusion reaction for interstellar rocket propulsion may well be D-He3, since nearly all of the energy it releases is in the form of charged particles whose momentum can be converted to thrust. The products of this reaction, a proton and an alpha particle, are released with energies of 18 and 4.5 MeV respectively, and thus some gamma rays may be expected with energies in this range. However the optimum power/ mass ratio fusion reactor will be realized if the plasma temperature is kept such that the ratio of the reaction rate parameter divided by the square of the temperature, < συ > /T2, is maximized. For the D-He3 reaction, this will occur at an average plasma temperature of about 60 keV. The bremsstrahlung emittance from such a reactor will thus be dominated by X-rays in this frequency range.

Fission could be employed either as an electric propulsion system or as a sort of plasma drive using a variety of techniques. Whatever the conversion system, the unique signature of a fission drive would be the well known spectrum of prompt and delayed fission gamma rays, in the 0.5 to 5 MeV range, collectively accounting for about 14% of the total output produced by the reactor. If fission is used in a plasma drive it could also be easily distinguished from either a fusion or an antimatter system by its ion cyclotron radiation, about 2 orders of magnitude lower in frequency than that of the alternatives due to the high atomic mass of the magnetically directed fission products. If the fission source is used for electric propulsion, then its radiators will operate at a lower temperature compared to those possible in a photon rocket, because the efficiency of the electric propulsion conversion system will go to zero as the radiator temperature approaches the temperature of the hot side of the energy conversion cycle. It can be shown analytically that the optimum ratio of the maximum temperature (absolute) to the radiator temperature in a space electric conversion system is 4:3. Thus an advanced electric propulsion system would operate with a radiator temperature at around 1,800 K, emitting in the visible and IR.

A magnetic sail (or “magsail” ) would be of unique value to an interstellar spacecraft because of its ability to decelerate a ship without the use of propellant. The magnetosphere of the magsail will create a standoff collisionless bowshock, which will heat the interstellar medium it encounters to hundreds of keV to a few MeV, depending upon the ship’s velocity. The plasma so created will then encounter the magnetic field of the magsail, where it will emit cyclotron radiation.

The cyclotron frequency emitted by a magsail is not a function of spacecraft design, but only of the ship’s velocity and the density of ions in the interstellar medium. At a density of 1 ion/cc, a ship traveling at 0.1 c would produce electron cyclotron radiation at a frequency of about 12 kHz.


The magnitude of the radiation that a starship will emit is a function of the magnitude of the power of its rocket engine, as well as of the engine design. For some types of engines, the fraction of jet power emitted as certain types of radiation can be calculated accurately. Where such information is lacking, we have assumed that the magnitude of a major type of radiation generic to an engine is 10% of the jet power.

The jet power is calculated as follows: if we assume a characteristic distance of 6 light-years for an interstellar voyage, and let the acceleration time equal 1/4 the trip time, then since 1 g is also an acceleration to the speed of light in a year, we have

S = 6ly = Vt/2 + (t/2)(V/2) = 3/4Vt or t = 8/V

A = 4V/t = V2/2

where V is the maximum cruise velocity, t is the trip time, S is the trip distance, and A is the time-averaged acceleration.

We also assume that a more advanced starship technology would only be employed to achieve performances substantially beyond what a more primitive technology might do. Thus since fusion can achieve a velocity of 0.1 c, we place a demand on an antimatter plasma rocket that it be used to achieve 0.2 c. Combining these with

Pjet = FU/ 2= MAU/2

where F is the engine thrust and U is the exhaust velocity, and assume a time-averaged mass during the burn, M, of 109 kg, we find the characteristic power level required of each of the technologies discussed.


Table 2. Characteristic Power Levels of Interstellar Propulsion Systems
TechnologyU/cV/cA (g’s)Pjet (TW)Prad (TW)
fission electric0.020.020.000261 at 1-5 MeV
18 at visible, IR
fusion plasma0.080.10.00560060 at 1-100 KeV
60 at cyclotron
antimatter plasma0.20.20.026,0004,000 at 200 MeV
600 at 20 MeV
600 at cyclotron
antimatter photon1.00.40.08120,00040,000 at 200 MeV
120,000 at visible, IR
magsail-fission0.020.0003182 at 80 KeV
2 at 2.4 kHz
magsail-fusion0.10.02678080 at 2 MeV
80 at 12 kHz
magsail-AM plasma0.20.00654,000400 at 8 MeV
400 at 24 kHz
magsail-AM photon0.40.016620,0002,000 at 32 MeV
2,000 at 48 kHz
  • U/c: exhaust velocity as fraction of lightspeed
  • V/c: ship velocity as fraction of lightspeed
  • A(g’s): acceleration in gravities
  • Pjet (TW): exhaust jet power in terawatts
  • Prad (TW): exhaust jet radiation in terawatts

Gamma Rays

It can be seen in Table 2 that certain potential starship propulsion systems, notably the very high power antimatter drives, emit vast quantities of energy in the form of gamma rays. The problem with detecting such radiation however is that since each gamma ray carries a large amount of energy, the number of photons emitted by even a very high powered starship at a characteristic interstellar distance impacting per square meter of collection area is quite small, and thus extremely difficult to distinguish from instrument noise and background radiation. For example, a starship emitting 10,000 TW of 200 MeV gamma radiation at a distance of 1 light-year from Earth will cause 7.5 photons per year to impact on a 1 square meter collection device This would obviously be undetectable.


Because their characteristic energies are about 4 orders of magnitude less than gamma rays, starship X-rays emissions offer some promise of increasing the photon count to detectable levels. On the other hand, the magnitude of the power source on those types of propulsion systems that emit X-rays appears to be about 2 orders of magnitude less than those characteristic of antimatter drives. A starship at 1 light-year emitting X-rays at a rate of 1 TW/keV (t.e., 50 TW at 50 keV, etc. ) would impact a collection device in Earth orbit at a rate of about 0.02 photons per hour, which would still be statistically undetectable. However a portion of the X-ray emissions would be less than 2 keV, and such radiation could he concentrated by an X-ray grazing incidence telescope. If such a telescope could be constructed that would focus a 1 m diameter aperture down to a 1 cm diameter collection area, then a 1 TW/keV (2 TW at 2 keV) source at 1 light-year would cause about 160 impacts per hour on the collection plate, which would be statistically detectable. One light-year is not a very great detection distance capability, however. At 10 light-years, a neighborhood within which there are less than a dozen target stars, the impact rate would be about 2 per hour, and the signal would fade into the noise background.

Visible Light

While the gamma ray emissions from the engine of an antimatter photon rocket would be undetectable, the visible radiation composing its exhaust is another story. If we consider the sample photon rocket in Table 2, with a jet power of 120,000 TW, and assume that it uses a reflective nozzle to focus the emitted light to a half angle of 30 degrees, then it will shine in the direction of its exhaust with an effective irradiated power of 1,800,000 TW. Such an object at a distance of 1 light-year would be seen from Earth as a 17th magnitude light source, and could be detected on film by a first class amateur telescope. The 200 inch telescope on Mount Palomar could image it at 20 light-years, and the Hubble Space Telescope at a distance of about 300 light-years. Curves of apparent magnitude verses distance are shown in Figure 1, as are the number of stellar systems (N = R3/80) within range. Since at least for the upper-end telescopes considered, the number of stellar systems Within range is significant (100,000 stars are within 200 light-years of Earth) this approach offers some hope for a successful search. The light from the photon rocket could be distinguished from that of a dim star by the lack of hydrogen lines in the rocket’s emissions.


Radio waves may be emitted from a starship as a result of plasma interaction with either the magnetic confinement field of a plasma drive engine or the deceleration field of a magnetic sail. Plasma drive engines produce electron and ion cyclotron radiation with frequencies on the order of a hundred GHz and MHz, respectively. Magsails produce electron cyclotron radiation with frequencies of ten’s of kHz and ion cyclotron radiation with frequencies of ten’s of Hz. The frequency of plasma drive engines is thus high enough to penetrate the Earth’s ionosphere and be detected on the ground by radio telescopes, while magsail radiation is below the cutoff frequency and can only be detected by antennas positioned in space.

The signal to noise ration (SNR) of a radio receiver is given by:

SNR = P(Ar)/4πR2BkT

where Ar is the area of the receiver antenna, R is the distance from the source, P is the radiated power of the source, B is the bandwidth, k is the Boltzman constant, and T is the absolute temperature of the receiver. Since plasma confinement fields and magsail fields both vary by factors greater than 2 over the region of plasma contact, the bandwidth required to capture a large percentage of the signal must be a sizable fraction of the signal’s peak frequency. Thus with the same size antenna and power source, magsail radiation can produce a SNR six orders of magnitude greater than that possible from a plasma drive.

Furthermore, since the low frequency magsail radiation has very long wavelengths (12 kHz = 25 km wavelength), huge collection areas can be created with very little mass by orbiting dishes or antennas made of sparsely placed wires or crossed tethers. For these reasons, magsail cyclotron radiation will be much easier to detect than that from plasma engines.

If we assume a SNR of 2 and a bandwidth of 1 kHz, suflicient to capture a significant fraction of electron cyclotron radiation emitted by a magsail (we assume 10% of the emitted electron cyclotron radiation within this bandwidth), and orbiting antennas with eflective equivalent radii of 6 km and 30 km respectively, then the power that needs to be emitted by a magsail for it to be detectable on Earth is shown in Figure 2.

It can be seen that the magsail radiation of a characteristic fusion starship being decelerated from a cruise velocity of 0.1c could be detected by a 6 km orbiting antenna from a distance of 400 light-years, while that emitted by a characteristic antimatter photon rocket in its deceleration phase could be seen as far away as 2,000 light-years. There are about 100,000,000 stellar systems to be found within the latter distance. This extended range detection capability combined with magsail radiation’s unique time-dependent frequency spectrum appears to make a search for magsail radiation the most promising option for extraterrestrial starship detection.

It may be noted that our estimate of starship mass is a speculative guess. However since the signal detectability is proportional to the mass of the ship divided by the square of the distance, a decrease of ship mass by two orders of magnitude only results in a decrease in detectability distance by one order or magnitude. Thus even if the true characteristic mass of starships is 10,000 tons, and not the 1,000,000 tons we have postulated as a baseline, magsail radiation would still be detectable by the 6 km antenna at 40 light-years, and by the 30 km antenna at 200 light-years.


We have considered a variety of potential technologies that may be in current use by advanced extraterrestrial civilizations for interstellar propulsion, and find that of those considered, the ones most likely to be detectable are photon rockets and magnetic sails. Photon rockets could be detected by currently existing orbital equipment at distances of several hundred light-years, while magsails could be detected by near-term deployable orbital equipment at several thousand light-years. Both photon rockets and magnetic sails would emit a signal that could easily be distinguished from natural sources. We therefore conclude that the detection of extraterrestrial civilizations via the spectral signature of their spacecraft is possible in principle and recommend that the approach be studied further.


    Astrophysicists M. Lingam and A. Loeb of Harvard have recently speculated that we may be observing evidence that aliens in other galaxies are powering their starships with beams of microwaves.

    To explore this amazing idea, we'll begin by considering microwave beaming as a means of space propulsion. Presently, the most aggressive effort to reach nearby stars is the Starshot Project, started in 2016 by billionaire science philanthropist Yuri Milner and described in my AV column in the October-2016 issue of Analog. The basic Starshot strategy is to use a large bank of coherent lasers to send a tiny but intelligent probe to Alpha Centauri, boosting it to 1/5 of the speed of light with light pressure acting on its reflective sail. This scheme has the advantage over conventional rocket propulsion that no reaction mass or energy-generating fuel needs to be carried on the propelled space vehicle, which can be very small and light.

    A similar scheme involving microwave beaming was proposed in 1985 by physicist and SF author Robert W. Forward. It involved a large space-based solar-powered maser that beamed microwaves to push a space vehicle with a large low-mass superconducting "sail". Forward named his scheme the "Starwisp", and several of his SF novels used the concept. In 2000 another physicist and SF author, Geoff Landis, investigated Forward's original scheme and improved on it with a sail design involving carbon fibers.

    The basic underlying propulsion concept used by these schemes is that electromagnetic radiation, including microwaves and visible light, carries a quantity of momentum proportional to its energy that can be used to push a sail. Unfortunately, that push is rather small, about 3.3 micro-newtons of force per kilowatt of power if the radiation is completely absorbed by the sail and twice that if the radiation is perfectly reflected backwards. In other words, each 10 newtons (or 2.25 pounds) of thrust generated for propulsion requires that between 1.5 and 3 gigawatts of electromagnetic power be beamed to the sail. Also, the sail's reflection efficiency must be nearly perfect, because if any significant fraction of the huge quantity of beamed power is absorbed and transformed to heat, the vehicle will overheat and be destroyed. The beam-driven sail propulsion scheme must also deal with wavelength-dependent aperture diffraction effects, which limit the system's ability to bring the beam to a tight focus on the sail from a long distance away.

    Nevertheless, such beam-driven sail schemes presently represent our best technological opportunity for sending small probes to nearby stars systems. And it may be that intelligent aliens in other galaxies have reached the same conclusion and are using beam-driven sail technology to reach their own nearby stars.

    In 2016 physicists James Benford and his son Dominic pointed out that one of the best ways for SETI searches to identify a high-technology alien civilization among the stars might be to look for leakage radiation from such power beaming. And this kind of SETI signals may have already been observed.

    In 2017 two Harvard astrophysicists, Manasvi Lingam and Abraham Loeb, have ventured a very interesting speculation about the origin of the unexplained radio-astronomy phenomenon known as "fast radio bursts" (or FRB). Fast radio bursts were first reported in 2007 by D. R. Lorimer and colleagues, based on Australian radio telescope data that was recorded on June 21, 2001. Since that time, more FRB events have been observed. As of this writing there is a growing FRB catalog that presently lists 20 observations.

    Because radio waves of different frequencies travel through intergalactic space at slightly different speeds due to interactions with free electrons along their path, radio astronomers can use the "chirp" of arrival time vs. frequency of an FRB to estimate how far it has travelled from its source. This analysis shows that all of the observed FRBs originate well beyond our galaxy, at estimated distances on the order of three billion light years.

    An FRB is fast and bright, lasting for only a few thousandths of a second. Their high brightness at such vast distances of origin implies that they are the result of a huge release of non-thermal radio energy. While two of the catalogued FRB have been observed to repeat, most of them are single non-repeating events. The FRB times of arrival have never been observed to correlate with bursts of either visible light or gamma rays coming from the same direction, and there is no verified observation of any "afterglow" following an FRB. There is no known physical phenomenon that can account for these bizarre characteristics. It is not physically plausible that such a vast release of energy in the radio domain would be "pure" and not accompanied by electromagnetic radiation in other forms. This raises an important question: If no natural phenomenon can make an FRB, what does that leave?

    Lingam and Loeb (L&L) have speculated that FRBs may be the leakage of radio waves coming from massive works of advanced engineering built by intelligent aliens in other galaxies, aliens who are beaming high-intensity radio waves at sails in order to power their starships.

    Exploring the full implications of this scenario, L&L have calculated the characteristics of such a beam-driven sail launch facility. They assume that the radio emission is broadband, that stellar energy is used to power the beams, and that water is used as a coolant. The observed FRBs are consistent with a beaming apparatus located about 3 billion light years from the Earth and beaming radio waves with a center frequency around one GHz. L&L calculate that the radiating surface would have to be of planet-size, about 3,000 km in diameter and would have a water-cooled surface temperature of about 100° C. It would be beaming radio waves at a total power level of 1015 kilowatts. (For comparison, our Sun has a power output of 3.8 x 1023 kilowatts.) The 3,000 km in diameter size of the radiator was calculated in two different ways, and the two results agree. A frequency of about 1 GHz was calculated to be the most efficient for beam propulsion, and this agrees with the FRB observations.

    For the launched vehicle, L&L assume a large sail-plus-payload that is roughly the same size as the radiator and has a mass of about 107 tons. (For comparison, at present the Earth's largest ocean-going cargo ship has a mass of about half a million tons.) L&L calculate the acceleration and the final velocity of this sail ship following launch. Beam diffraction considerations limit the distance over which the vehicle can be accelerated to about 1/3 of a light year. They assume an acceleration of around one gee over that distance, and they calculate that the final speed of the vehicle would be close to the speed of light.

    They also consider the implications of the few-millisecond duration of the Earth-observed FRBs. The sail and beamer would presumably both be in orbit around their parent star, and the beamer would need to rotate as it orbited in order to keep the beam centered on the sail. This means that to an Earth observer the leaked beam will rapidly sweep across the sky and will only be visible for a few milliseconds. The millisecond time-width of the FRBs implies that the rotation of the beamer is about 10-5 radians per second, implying a full 360 degree rotation in about a week. This is consistent with what might be expected for the orbiting beamer scenario.

    L&L then consider the implications of the rate at which FRBs have been observed for the probability of finding a highly advanced civilization that is using power-beaming launch technology on a given Earth-like planet or in a given galaxy. Based on the rate of observation of FRBs, they conclude that the probability of finding an advanced civilization on a given Earth-like planet is less than one chance in a million. However, since some estimates put the number of habitable Earth-like planets in our galaxy as high as 10 billion, there might be up to 10,000 such advanced civilizations in our galaxy alone. We note, however, that no FRB has ever been observed to originate in our galaxy, and the numbers indicate that such an observation might occur only once in 300 years.

    To conclude, L&L consider how more detailed measurements of FRBs might show a signature of their beam-driven sail origin or might falsify that scenario. Following the Benfords, they point out that radio wave leakage around the edges of an opaque sail should produce diffraction edge-effects that would be observable as interference "wiggles" in the detected intensity. Present FRB measurements have reported neither the presence nor absence of such modulations. L&L also suggest that radio astronomers should note the sites of the major FRB bursts and look for repeat events, because "astrophysical explosions tend to be single bursts, while artificial beacons can repeat".

    Finally, we note that in their paper on beam leakage as an SETI signal, James and Dominic Benford speculate that an intelligent civilization using beamed-power sail technology should realize that they are sending out a beacon that is likely to be detected by other advanced civilizations far away. That might motivate them to modulate the frequency, intensity, or polarization of their beam so that it contains information that would be of interest to civilizations detecting their beam-beacon. This suggests that Earth-based radio astronomers should devote some effort to carefully examining the frequency and polarization structure of FRBs detected in search of such encoded information.

    From one science fiction point of view, these L&L calculations are very exciting. Our galaxy may be peppered with other civilizations far more advanced than ours, and they may be regularly sending payloads to their nearby star systems. However, from another point of view, I rather hope that a better explanation for the FRB phenomenon emerges, one that doesn't require an advanced alien civilization. That is because as a physicist, I am hoping that there are yet-undiscovered physical phenomenon that will provides a means of space propulsion and interstellar travel that don't require mega-engineering and planet-size sails and radiators of radio waves. It will be a long time before the human race can build such vast works of mega-engineering, and I would prefer that we get to the stars much sooner, if possible.


First Observation of FRB:

"A bright millisecond radio burst of extragalactic origin", D. R. Lorimer, et al, Science, Vol. 318, Issue 5851, pp. 777-780 (02 Nov 2007); preprint arXiv:0709.4301v1 [astro-ph].

Power Beaming as an SETI Signal:

"Power Beaming leakage radiation as a SETI Observable", James N. Benford and Dominic J. Benford, Astrophysical Journal 825, 101-107 (2016); preprint arXiv:1602.05485v2 [astro-ph.IM].

Alien FRB:

"Fast Radio Bursts from Extragalactic Light Sails", Manasvi Lingham and Abraham Loeb; preprint arXiv: 1701.01109v2 [astro-ph.HE].


If the starship is traveling at more than about 14% of the speed of light (V/c = 0.14 in table) you will have to start worrying about relativity. Gamma (γ) is the relativistic factor.



At a fixed speed v, the distance travelled by a spacecraft within a given (proper) travel time τscales as γvτ, which for relativistic speeds approximately equals γcτ, where γ = (1−v2/c2)−1/2. Hence range is directly proportional to γ, and to cover intergalactic distances within a limited “lifetime” τ requires γ ≫ 1. The primary obstacles to relativistic (γ ≫ 1) space travel would be collisions with interstellar dust particles (cosmic dust) and larger space objects, which will impact with kinetic energies of (γ − 1)mc2 for rest mass m, similar to collisions in particle accelerators but potentially at much higher energies. For large enough γ, even molecular collisions could be a significant source of drag and possibly damaging. For example, at γ = 2 a baseball size object of mass 150g has an impact energy equivalent to 36 Megatons of TNT; a single cosmic dust grain of mass 10−14g at γ = 108 has an impact energy of close to 24 kgs of TNT.

The intergalactic medium has much less debris per unit volume as compared to the interstellar medium so the chance of a catastrophic collision occurring is significantly reduced beyond the confines of any galaxy. We define extreme velocities to occur at γ > 9.1 (0.994c) which is the threshold velocity for proton-proton collisions to produce antiprotons. Now to travel at a safe speed in the interstellar medium one would need a γ < 1.3 (0.639c), the threshold for the pion production in proton-proton collisions. The problem of colliding with larger objects as described above remains, but we assume that sufficiently advanced technology likely to accompany relativistic space travel capabilitites will be able to circumvent these interactions. One can imagine, though, intergalactic travel through mostly matter-free space, and under those conditions the cosmic microwave background (CMB) photons are primarily the particles a spacecraft will encounter in collisions.

CMB Interactions

A fast-moving spacecraft traveling in intergalactic space still has to contendwith collisions with cosmic microwave photons, which, at relativistic speeds, will appear in the spacecraft frame as highly energetic gamma rays. Interactions of CMB photons with the material of the spacecraft hull will have effects ranging from ionization and Compton scattering to pair production with increasing γ. We will assume that advanced technology can mitigate the harmful effets of ionization and Compton scattering, and therefore concentrate on pair production as the obstacle that is most likely to resist removal via technology.

The first speed level where pair production will pose a challenge for relativistic spaceflight engineers is photon-nucleon interactions in the hull of the spacecraft. Let us consider a hull made of ordinary baryonic matter. In this case, a single CMB photon will create an electron-positron pair via its collision with a nucleus in the hull if its energy exceeds the rest mass of an electron and positron. That threshold is at blue-shifted photon energies of about 1 MeV. In terms of the frequency ω in the rest frame of the spacecraft, the condition for nucleus-mediated pair production is

(equation omitted, see report for the gruesome details)

The central frequency of the (Planck) distribution of cosmic microwave background photons is 160GHz.

(more equations omitted)

So the condition for pair creation in terms of spacecraft speed is a γ ≈ 1.24×108 (v/c ≈1 - 3.3×10-17 or 0.999999999999999967c).

Signature of CMB Scattered from a Relativistic Spacecraft

The possibility of detecting radiation associatedwith distant relativistic spacecraft has been discussed in the literature before. These discussions mostly focus on detecting radiation from spacecraft engines or light from nearby stars reflecting off the spacecraft. Our approach is different in that we do not speculate on possible propulsion technologies but are interested in how a large relativistic object would interact with the interstellar/intergalactic medium and mainly with the CMB radiation. As a baryonic spacecraft travels at relativistic speeds it will interact with the CMB through scattering to cause a frequency shift that could be detectable on Earth with current technology.

(more equations omitted)

Some examples of signal signatures resulting from Eqs. (38)–(39) are shown in Figs. 2 and 3. The salient features of the signal are a rapid drop in temperature accompanied by a rapid rise in intensity, along with the motion of the source with respect to a reference frame fixed to distant quasars, which should be observable. No natural source of THz to infrared radiation in the known Universe is likely to have time variability of the kind depicted in Figs. 2–3. Note that increasing velocity (increasing γ)manifests itself in a steepening of the cooling curve, along with an overall increase in temperature for a surface element oriented orthogonal to the direction of motion.

From LIMITS AND SIGNATURES OF RELATIVISTIC SPACEFLIGHT by Ulvi Yurtsever and Steven Wilkinson (2015)


The paper Observational Signatures of Self-Destructive Civilisations is about patterns of sensor readings that will detect the remains of annihilated civilizations. These could be the gravesite of Forerunners, with the promise/threat of valuable and/or civilization-killing paleotechnology.

The paper is focused on refining the value of the "L" factor in the famous Drake equation, the average lifetime of a technological civilization. So the paper estimates some scenarios where a planetary civilization can destroy itself, and tries to figure out what their sensor signatures are. Then astronomers can see if they can spot any.

If they see lots, it might mean L is quite short. Which would mean that the L for our civilization is likely to be quite short as well.

But for our science-fiction writing purposes, keep in mind that many of these sensor signatures will also work if a civilization has been exterminated by external alien invaders.

The last I heard, an exo-atmospheric nuclear explosion of ten megatons is detectable with a photon-timing detector watching through a 10 meter telescope anywhere in the entire Milky Way galaxy.


…The aim of this paper is to use the Earth as a test case in order to categorise the potential scenarios for complete civilisational destruction, quantify the observable signatures that these scenarios might leave behind, and determine whether these would be observable with current or near-future technology.

The variety of potential apocalyptic scenarios are essentially only limited in scope by imagination and in plausibility according to our current understanding of science. However, the scenarios considered here are limited to those that: are self inflicted (and therefore imply the development of intelligence and sufficient technology); technologically plausible (even if the technology does not currently exist); and that totally eliminate the (in the test case) human civilisation.

Only a few plausible scenarios fulfil these criteria:

  1. complete nuclear, mutually-assured destruction
  2. a biological or chemical agent designed to kill either the human species, all animals, all eukaryotes, or all living things
  3. a technological disaster such as the “grey goo” scenario, or
  4. excessive pollution of the star, planet or interplanetary environment

Other scenarios, such as an extinction level impact event, dangerous stellar activity or ecological collapse could occur without the intervention of an intelligent species, and any signatures produced in these events would not imply intelligent life…

2.1 Nuclear Annihilation

Current estimates of nuclear weapons held around the world are of the order 6 million kilotonnes (kt) (2.5 x 1016 J)…

…Nuclear weapons produce a short, intense burst of gamma radiation with a characteristic double peak over several milliseconds. These gamma flashes could be detected using the same techniques as for the detection of gamma ray bursts (GRB)…

…Given that the world’s nuclear arsenal is equivalent to around 1019 J of energy, the resulting radiation from its combined detonation would be much fainter than a typical GRB. If we assume that the energy is released on a similar timescale and with a similar spectrum to a GRB, a nuclear apocalypse is equivalent in bolometric flux to a GRB detonating around a trillion times closer than its typical distance. If we take a nearby GRB such as GRB 980425 which is thought to have detonated around 40 Mpc away, then we would expect a global nuclear detonation event to produce a similar amount of bolometric flux only 8 AU away!

Therefore, for us to be able to detect nuclear detonation outside the Solar system, the total energy of detonation must be at least nine orders of magnitude larger, i.e. the ETIs responsible for the event must engage in massive weapon proliferation and concurrent usage.

However, the production of fallout from terrestrial size payloads, which persists for much longer timescales, may make itself visible in studies of extrasolar planet atmospheres.

For the purposes of estimating fallout, the weapon impacts are assumed to be evenly distributed across the entire land area of the planet (1.5 x 108 km2 ). This gives an equivalent of approximately one 25 kt (1011 J) weapon per square kilometre of land area. This is of the same order of magnitude as the weapon used in the Semipalatinsk Nuclear Test, for which the effects of radioactive fallout were measured over time. However, given the local climatic conditions at this site (which were very windy) and the fact that our estimates include nuclear events every square kilometre, the effects are likely to be much worse than the results of this test. From measurements of soil at a town near the test site and modelling of radionuclide decay chains, the dose rate due to fallout from the weapon test (not the dose from the blast itself) was shown to begin around 10 3 microgray/hour, decaying to background levels after around 100 days.

Fallout products of fusion weapons are typically nonradioactive, though they do produce a low yield of energetic protons and electrons. Most fallout products from fission weapons are beta emitters and decay to other beta emitting isotopes. Some radioisotopes produced by fission weapons are gamma emitters, but these have short halflives. Ignoring the effects on the health of humans or other lifeforms (which would be severe), the deposition of a large amount of betaradioactive material into the atmosphere would have a significant effect on atmospheric chemistry and would quickly ionise many atmospheric species, with high altitude nuclear tests increasing local electron density several times. This would give ionised air the distinct blue or green of nitrogen and oxygen emission. Given that spacecraft and Earth based telescopes have detected (faint) nighttime airglow on Venus and Mars it may be possible to measure what would be considerably brighter airglow features in exoplanets, given that the order of magnitude increase in electron density caused by a nuclear war would generate an order of magnitude increase in airglow brightness. The brightest airglow feature in the visible spectrum on an Earthlike exoplanet would be the green oxygen line at 558 nm, which would be enhanced by global nuclear war to a photon flux of up to 1400 rayleighs.

IR emission from exoplanets in their secondary eclipse phase has been measured by spacebased telescopes so in theory these measurements could be extended into the visible part of the spectrum in future, though this would require exquisite precision in our knowledge of the host star’s properties, and would most likely be dominated by reflected light from the planet itself, especially in the bluegreen spectral region. A tenfold increase in brightness at 558 nm would potentially be observable with only a modest increase in sensitivity over instruments observing exoplanets today, especially since the airglow maximum occurs well above the tropopause and would therefore be observable above even a very cloudy planet. Airglow caused by fallout products would last for several years before decaying to unobservable levels.

The thermal effects of nuclear explosions also affect atmospheric chemistry. For every kilotonne in yield, approximately 5000 tonnes of nitrogen oxides are produced by the blast itself. Blasts from higher yield weapons will carry these nitrogen oxides high into the stratosphere, where they are able to react with and significantly deplete the ozone layer. Ozone can be detected in the ultraviolet transmission spectrum of an exoplanet, as can other oxygen molecules, and so the disruption of an exoplanetary ozone layer presents another potential observational signature.

Global nuclear war therefore potentially offers several spectral signatures that could be observed: a gamma flash, followed by UV/visible airglow and the depletion of ozone signatures. However, the aftermath of a global nuclear war will also act to obscure these spectral signatures. Groundburst nuclear explosions generate a significant amount of dust that will be lofted into the atmosphere. Airburst explosions do not generate dust, but still introduce particulates into the atmosphere. Atmospheric effects of nuclear warfare have been extensively modelled in climate simulations, the global consequences being known as “nuclear winter”. Recent simulations have shown that even with reduced modern nuclear arsenals severe climate effects are felt for at least ten years after a global conflict, especially due to the long lifetime of aerosols lofted into the stratosphere. They show that the atmospheric optical depth is increased several times for several years. The worst effects are confined to the northern hemisphere given that the model includes conflict over the US and Russia, though the entire planet is affected to a lesser extent.

A nuclear winter would dramatically increase the opacity of the atmosphere. This process itself would be observable if a planet observed with a previously transparent atmosphere (perhaps with an Earthlike spectroscopic signature) was observed again and the atmosphere was opaque, this would be a sign of a large dust event. However, such an event could also be caused by a large impact and therefore would not imply a civilisation had caused the disaster (though would be interesting in itself). If the atmosphere had not been observed before the event, it would simply seem like the planet had an extremely dusty atmosphere. What would be crucial is measuring the relative change in atmosphere as a result of nuclear detonation, hopefully with an added bonus of identifying a weak gamma ray or other high energy emission in the vicinity of the planet.

Hence, to confirm that a planet had been subject to a global nuclear catastrophe would require the observation of several independent signatures in short succession. One on its own is unlikely to be sufficient, and could easily be caused by any number of other processes on planets with potentially no biological activity whatsoever. There are cases beyond a global nuclear catastrophe that a spacefaring civilisation might be able to inflict on itself, given that the destructive energy at their disposal would be far greater than nuclear weapons, including redirecting asteroids. These would be far more destructive than nuclear warfare but would generate observable signatures different than those of a naturally occurring impact event.

2.2 Biological Warfare

Biological warfare involves the use of naturally occurring, or artificially modified, bacteria, viruses or other biological agents to intentionally cause illness or death. The use of a naturally occurring pathogen in a global conflict would probably have a limited net effect on a global population. The destruction would be selflimiting; once a population is reduced in size, transmission from host to host would become more difficult and the epidemic eventually ends. Artificially modified or created biological agents however, could potentially push a civilisation to extinction…

…Assuming a global conflict took place that made use of this method of warfare on a planet that hosts an intelligent civilisation, we pose the question of whether the self-destruction of that species, via this method, could be remotely observable.

If we assume that the time between the release of the engineered virus and its global spread is very short and that the virus is potent enough that a civilisation becomes fully extinct, the environmental impacts of this scenario can be assessed. The actions of anaerobic organisms cause biomass to decay, releasing methanethiol, CH3SH (via production of methionine) as one of the products. This can be spectrally inferred and has no abiotic source. For a population with a similar biomass to the present human population (currently, in terms of carbon biomass, ~2.8x1011 kg), the decay products can be estimated. Since the dry mass of a cell is approximately 50% carbon, the total human biomass would be 5.6x1011 kg. With an estimated cell sulphur content of 0.3-1%, the maximum amount of S available to form CH3SH would be 5.6x109 kg. If 10% of this S is incorporated into methionine, all of which is then converted into methanethiol, this would result in a total CH3SH flux of ~108 kg.

At the current biological production rate on Earth, this would be released to the atmosphere over a period of a year and would rapidly photodissociate, making this a very shortlived biosignature. One of the products of the decay of methanethiol is ethane (C2H6), which can be spectrally detected, but has an atmospheric lifetime under Earth-like conditions of < 1 year, leading to a short window of time for detection. Additionally, if carrion-eating species were unaffected, this would reduce the amount of organic matter available for microbial decay, further reducing the final biosignature.

However, if the engineered virus could cross species barriers, then the total amount of dead biomass could be as high as 6x1013 kg (the total animal biomass on Earth), potentially producing 1011 kg of CH3SH, which would enter the atmosphere over a period of ~30 years. It is likely that, due to its short atmospheric lifetime, this atmospheric CH3SH would still not produce a detectable signature. However, the associated C2H6 absorption signature between 11-13 μm may lend itself to remote detection. This signature would be deeper and therefore more readily detectable if the CH3SH production rate was higher.

Other decay products include CH4, H4S, NH3 and CO2. The most promising biosignature gas for global bioterrorism is CH4. The CH4 flux to the atmosphere is related to ethane production, potentially increasing the C2H6 absorption signature…For the case where only humans can be infected, both signatures are shortlived, requiring observations to be taking place at exactly the right time for a detection to be made. In the case where the virus can cross species barriers leading the the total annihilation of animal life, persistently high levels of these gases could make a detection more likely.

2.3 Destruction via ‘Grey Goo’

The terrestrial biosphere offers many examples of naturally occurring nanoscale machines. Feynman extolled the advantages of engineering at atomic scales. In Engines of Creation, Drexler described “nanotechnology” as a means of fabricating structures at nanoscales using chemical machinery. While the word now has a broader meaning, we can still consider the possibility that such a machine can be sufficiently generalpurpose to be able to make a copy of itself.

Following Phoenix and Drexler we define an engineered system that can duplicate itself exactly in a resource-limited environment as a self-replicator. (NB: This strict definition excludes biological replicators, as they are not engineered). The engineers of such machines have two broad choices as to what resources the self-replicator might use: resources that are naturally occurring in the biosphere, and resources that are not. Engineers that make the former choice run the risk of a “grey goo” scenario, where uncontrolled self-replication converts a large fraction of available biomass into self-replicators, collapsing the biosphere and destroying life on a world. This may be an accident or failure of oversight, or it may be due to a deliberate attack, where the replicators are specifically designed to destroy biomass (what Freitas refers to as “goodbots” and “badbots” respectively). In Engines of Creation, Drexler notes:

“Replicators can be more potent than nuclear weapons: to devastate Earth with bombs would require masses of exotic hardware and rare isotopes, but to destroy all life with replicators would require only a single speck made of ordinary elements. Replicators give nuclear war some company as a potential cause of extinction, giving a broader context to extinction as a moral concern.”

Freitas places some important limitations on the ability of replicators to convert the biosphere into “grey goo” (land based replicators), “grey lichen” (chemolithotrophic replicators), “grey plankton” (ocean-borne replicators) and “grey dust” (airborne replicators). With conservative estimates based on contemporary technology, it is suggested that if the replicators are carbon-rich, around a quarter of the Earth’s biomass could be converted as quickly as a few weeks. Equally, Freitas estimates the energy dissipated by carbon conversion, implying that subsequent thermal signatures (local heating and local changes to atmospheric opacity) would be sufficient to trigger local defence systems to combat gooification. For example, In the case of malevolent airborne replicators, a possible defensive strategy is the deployment of non-self replicating “goodbots” which unfurl a dragnet to remove them from the atmosphere.

Phoenix and Drexler emphasise that all these variants of the grey goo scenario are easily avoidable, provided that engineers design wisely (and that military powers exercise restraint). Indeed, they indicate that fully autonomous self-replicating units are not likely to be the most efficient design choice for manufacturing, and that having a central control computer guiding production is likely to be safer and more cost-effective. Provided that the control computer is not separated by distances large enough to introduce time-lag, as would be the case on interplanetary scales, this seems to be reasonable.

However, this still leaves the risk of replicator technology being weaponised. We will assume, as we do throughout this paper, that prudence is not a universal trait in galactic civilisations, and that grey goo is a potential death channel that might be detected.

So what signatures might a grey goo scenario produce? If a quarter of the Earth’s biomass is converted into micron sized objects, how would this affect spectra of Earthlike planets? This situation shares several parallels with the nuclear winter scenario described previously. In the case of grey goo, we may expect there to be a substantially larger amount of “dust”, as well as a fixed grain size. This will be deposited as sand dunes or suspended in the atmosphere, with similar spectral signatures as previously discussed.

Depending on the grain size of the dunes, it may be possible to observe a brightness increase as the angle measured by the observer between the illumination source (the host star) and the planet decreases towards zero on the approach to secondary eclipse.

Surfaces that are composed of a large number of relatively small elements packed together will produce significant shadowing. This shadowing increases as the angle between the surface and an illumination source increases. As the angle decreases towards zero, these shadows disappear, resulting in a net increase in brightness. This is sometimes described as the opposition surge effect, or the Seeliger effect in deference to Hugo von Seeliger, who first described it. Seeliger saw this shadowhiding mechanism in Saturn’s rings, which grow brighter at opposition relative to the planetary disc. Coherent backscattering of light also plays a role in this brightening effect.

This phenomenon is observed in the lunar regolith, so it seems reasonable to expect that this phenomenon would also act in artificially generated regoliths such as those we might expect from a grey goo incident. During exoplanet transits, it may be possible to detect an increase in the brightness of the system as the planet enters secondary eclipse. The Moon’s brightness increases by around 40% as it moves towards the peak of opposition surge, so it may well be the case that grey-goo planets produce opposition surges of similar magnitude. Buratti notes that the wavelength dependence of the surge is relatively weak, which would suggest that nearIR observations may be sufficient to observe this phenomenon.

On what timescale might we expect this artificial nano-sand to persist on a planetary surface? If the planet has an active hydrological cycle, airborne replicators will be incorporated into precipitation and delivered to the planet’s surface. The Earth’s Sahara desert transports away of order a billion tonnes of sand per year. Deposition into rivers and streams may deliver the material to oceans, and eventually the seabed, effectively removing it from view at interstellar distances. This material will be subducted into the mantle and reprocessed on geological timescales, removing all trace of engineering. Using Freitas’s estimate of available biomass, and assuming the nano-sand can be processed out of view at a few billion tonnes per year (which we propose as an upper limit) then this suggests that a goo-ified planet may require several thousand years to refresh its surface. It is likely that processing rates may be accelerated or impeded by other physical processes, but it seems to be the case that goo-ified planets remain characterisable as such over timescales comparable to that of recorded human history.

2.5 Total Planetary Destruction

Finally, it is not inconceivable that a civilisation capable of harnessing large amounts of energy could unbind a large fraction (or all) of a planet’s mass. Kardashev Type II civilisations wishing to build a Dyson sphere require this capability to generate raw materials for the sphere — it is estimated that to create a Dyson sphere in the Solar System with radius 1 AU would require the destruction of Mercury and Venus to supply sufficient raw materials.

Equally, civilisations with access to this level of energy control and manipulation may decide to use it maliciously, destroying large parts of a planetary habitat while it is still occupied, and in the extreme case destroying the planet completely. This would release a significant fraction of the planet’s gravitational binding energy.

The Earth’s binding energy is of order 1039 ergs. This is again several orders of magnitude fainter than a typical supernova or GRB of 10 51 ergs, but is strong compared to the solar luminosity — the Sun would require several days to radiate the same quantity of energy. This would likely produce a gamma ray signature even stronger than expected from the nuclear winter scenario described previously, and we may expect afterglows similar to those observed in other astrophysical explosions.

The destruction of an orbiting body will produce a ring of debris around the central star, in a manner analogous to the production of rings when solid bodies cross the Roche limit of a larger body.

The subsequent evolution of this material will be similar to that of the debris discs. The remnants of the planet formation process, debris discs have been detected around a variety of stars, and the behaviour of grains of differing sizes under gravity and radiation pressure has been modelled in detail. It is likely that, if a terrestrial planet has been destroyed, the debris will be principally composed of silicates, and as such any detection of refined or engineered materials is unlikely, even if such matter survives the planet’s demise untouched.

The fate of the material depends largely on the local gravitational potential and the local radiation field, as well as the grain size distribution of the debris. Grains below the “blowout” size — typically a few microns — will be removed from the system via radiation pressure. Neighbouring planets may collect some of the remaining debris in resonances while the debris grinds into material of sufficient grain size that it either loses angular momentum through Poynting-Robertson drag and is consumed by the central star or a neighbouring planet, or gains momentum through radiative forces and is removed from the system.

In any case, this death channel does not appear to be amenable to detection by Earth astronomers. If we are fortunate to witness the instant of destruction, then we may be able to speculate on the energies released in the event, and search for a natural progenitor of such energy, i.e. another celestial body. Giant impacts between planet-sized bodies will produce the required energies to unbind or destroy one of the objects, as was the case for the impact which formed the EarthMoon system. If such efforts fail, and no other explanation fits the observations, then we may tentatively consider extraterrestrial foul play.

The timescale for observing destruction as it happens will be short — perhaps a few days. The debris can be expected to persist for several centuries, but observing this is unlikely to elucidate its origins as a destroyed planet.

Prospects for Observing Civilisation Destruction
Death ChannelDetection MethodSignature
of Active
Signature of
Timescale (yr)
Gamma ray
detection, Transit
YY0-5 years
BioterrorismTransit spectroscopyYY1-30 years
Grey GooTransit spectroscopy
and photometry
NY>1,000 years
Stellar PollutionAsteroseismology,
stellar abundance
YY>100,000 years
(depending on
Transit spectroscopy
Orbital Pollution
Transit spectroscopy
and photometry
YY<100,000 years
Total Planetary
Debris Disk Imaging
YY<100,000 years


For three hundred years, while its fame spread across the world, the little town had stood here at the river’s bend. Time and change had touched it lightly; it had heard from afar both the coming of the Armada and the fall of the Third Reich, and all Man’s wars had passed it by.

Now it was gone, as though it had never been. In a moment of time the toil and treasure of centuries had been swept away. The vanished streets could still be traced as faint marks in the vitrified ground, but of the houses, nothing remained. Steel and concrete, plaster and ancient oak—it had mattered little at the end. In the moment of death they had stood together, transfixed by the glare of the detonating bomb. Then, even before they could flash into fire, the blast waves had reached them and they had ceased to be. Mile upon mile the ravening hemisphere of flame had expanded over the level farmlands, and from its heart had risen the twisting totem-pole that had haunted the minds of men for so long, and to such little purpose.

The rocket had been a stray, one of the last ever to be fired. It was hard to say for what target it had been intended. Certainly not London, for London was no longer a military objective. London, indeed, was no longer anything at all. Long ago the men whose duty it was had calculated that three of the hydrogen bombs would be sufficient for that rather small target. In sending twenty, they had been perhaps a little overzealous.

This was not one of the twenty that had done their work so well. Both its destination and its origin were unknown: whether it had come cross the lonely Arctic wastes or far above the waters of the Atlantic, no one could tell and there were few now who cared. Once there had been men who had known such things, who had watched from afar the flight of the great projectiles and had sent their own missiles to meet them. Often that appointment had been kept, high above the Earth where the sky was black and sun and stars shared the heavens together. Then there had bloomed for a moment that indescribable flame, sending out into space a message that in centuries to come other eyes than Man’s would see and understand.

From THE CURSE by Arthur C. Clarke (1946)

In a recent New Yorker article, the nuclear historian Alex Wellerstein collected testimony from several people who saw, firsthand, the flash from the first successful detonation of the atomic bomb, at the infamous Trinity Test, on July 16, 1945.

Wellerstein has a writer’s feel for quotes and anecdotes. According to one general, the flash was a “golden, purple, violet, gray, and blue light” that illuminated “every peak, crevasse, and ridge” of a nearby mountain range, “with a clarity and beauty that cannot be described.” Wellerstein notes that several eyewitnesses described Trinity’s light as “cosmic.” This was apropos, he says, for nowhere else, “except in the interiors of stars do temperatures reach into the tens of millions of degrees,” as they do during a nuclear explosion.

A team of astronomers recently tried to determine whether Trinity’s light might be cosmic in a different sense. The Trinity test involved only one explosion. But if there were many more explosions, involving many more nuclear weapons, it might generate enough heat and light to be seen from nearby stars, or from the deeper reaches of our galaxy—so long as someone out there was looking.

And so, the thinking goes, maybe we should be looking. If every intelligent species eventually stumbles on nuclear technology, and not all of them manage it well, then it might be possible to spot an apocalypse in the heavens. Or several.

There are tens of billions of galaxies in the observable universe, each one a sea of stars. When astronomers watch these stars closely, they see them wobbling, the way our sun wobbles when its planets spin around it, tugging on its center of gravity. Astronomers also see these stars dimming ever so slightly, as though an objects were passing in front of them, and this dimming occurs at predictable intervals, as though these objects were moving around the stars in regular orbits. For these reasons and others, astronomers now believe that nearly all stars play host to planets, and they are making plans to image these planets directly, by catching the faint light they give off with huge, ultra-sensitive telescopes.

What will this light tell us? A remarkable amount, it turns out. Light encounters all kinds of molecules as it makes its way through the universe, and it keeps a close record of these encounters, in its spectra. If sunlight were to beam through Earth’s atmosphere, and then out into the stars, it would travel with this detailed chemical record in tow. If, after some millennia, this earth-kissed light fell into a distant astronomer’s telescope, that astronomer would be able to determine what sorts of chemicals were present in our planet’s atmosphere. They would know that water vapor was present, and life too, because Earth’s atmosphere contains methane gas, breathed out by the trillions of organisms that live on its surface. Indeed, it’s precisely these sorts of “biosignatures” that Earth’s astronomers hope to find in the atmospheres of extrasolar planets.

Light from extrasolar planets might also tell us whether our universe is home to other tool-making beings. After all, some of our pollutants leave behind chemical traces that would never occur naturally. If we glimpsed these pollutants in a distant planet’s atmosphere, we could be reasonably certain that technological life lived on its surface at one time or another. And according to Adam Stevens, Duncan Forgan, and Jack O’Malley James from Cornell’s Carl Sagan Institute, we might be able to know whether they used their technology to destroy themselves.

In July, Stevens, Forgan, and James published a paper that asked what a distant, “self-destructive civilization” might look like through the business end of a telescope. To do so, they gamed out several dystopian science fiction scenarios in great detail. They calculated the brightness of the gamma rays that would flash out from a massive exchange of nuclear weapons. They asked themselves what would happen if an engineered pathogen ripped through a large population of human-sized animals. What gases would fill a planet’s atmosphere, if its surface were strewn with rotting corpses? And would those gases be detectable across interstellar distances?

I asked Jill Tarter what she thought of the paper. Tarter is the former director of the Search for Extraterrestrial Intelligence Institute and the inspiration for Ellie Arroway, the heroine of Carl Sagan’s Contact, played by Jodie Foster in the film adaptation. Tarter told me the paper was “getting a bit of buzz” in the SETI community. But she also urged caution. “The problem is the signatures are detectable for cosmically insignificant amounts of time,” she said. Distant stars burn for billions of years, sending a constant stream of light toward Earth, but the flash from a nuclear war may last only a few days. To catch its light, you have to have impeccable timing.

Stevens, Forgan, and James acknowledge the ephemerality of their extinction signatures. According to their paper, some will last only 30 years, and others less than that. And even if a signal were to stick around for a hundred millennia, it would still be a tough needle to find in the vast spatiotemporal haystack that is our night sky. The universe has been manufacturing planets for billions of years. The odds that you’d train your telescope on a planet just as its resident civilization winks out are, in Tarter’s words, “a lot worse than Vegas.”

To beat odds like that, you’d need to take a detailed census of the galaxy. You’d need to eavesdrop on billions of planets, and for long stretches of time, and the tech for that kind of survey just doesn’t exist yet, and won’t for a while.

But it’s conceivable, in principle, and that itself is a miracle of human ingenuity. It’s wild to think that we may one day know something about the various fates that await beings like us. And it’s a useful prod toward deeper thoughts, about the sorts of flashes we are starting to send into the cosmos, especially this year, as we mark the 70th anniversary of the Trinity test.


As if beckoned by those who had gone before, I half-floated between the titanic snowdrifts, quivering and afraid, into the sightless vortex of the unimaginable.

Screamingly sentient, dumbly delirious, only the gods that were can tell. A sickened, sensitive shadow writhing in hands that are not hands, and whirled blindly past ghastly midnights of rotting creation, corpses of dead worlds with sores that were cities, charnel winds that brush the pallid stars and make them flicker low. Beyond the worlds vague ghosts of monstrous things; half-seen columns of unsanctifled temples that rest on nameless rocks beneath space and reach up to dizzy vacua above the spheres of light and darkness. And through this revolting graveyard of the universe the muffled, maddening beating of drums, and thin, monotonous whine of blasphemous flutes from inconceivable, unlighted chambers beyond Time; the detestable pounding and piping whereunto dance slowly, awkwardly, and absurdly the gigantic, tenebrous ultimate gods the blind, voiceless, mindless gargoyles whose soul is Nyarlathotep.

From NYARLATHOTEP by H. P. Lovecraft (1920 )

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