Many of the Mars Mission designs have difficulty making enough delta-V to complete the mission, especially if they are chemically powered. A common short-cut is to economize in the return to Terra, the "Earth-Return" segment of the mission.
The expensive option is for the returning spacecraft to brake their velocity and enter Terra orbit. The crew can be taken off the ship by a space shuttle or something, and the ship can be refurbished for re-use.
The short-cut cheapskate option is to not bother braking the spacecraft. Instead as the ship approaches Terra, the crew bail out in a tiny Earth-Return Vehicle (along with any Mars samples they brought) and use free aerobraking to return the crew to Terra. The multi-billon dollar spaceship is abandoned to go zipping off into an eccentric heliocentric junkyard orbit.
In theory a designer could use individual reentry capsules instead of a single Earth-Return vehicle, but they'd have to be really hard up for delta-V to economize that much. Plus most reentry capsules are only rated to aerobrake the relatively modest amounts of Terra orbital velocities, not return-from-Mars velocities.
This hair-raising concept is from Earth Return Aerocapture for the TransHab/Ellipsled Vehicle
Aerocapture is a powerful technique for drastically reducing the required delta V and propellant load for your spacecraft, which is so important for those delta-V challenged chemical rockets. Just send your spacecraft on a hot ride through the planet's atmosphere and you can literally burn off enough delta-V for free. Assuming your heat shield lasts long enough.
However, doing that with a TransHab perched on your spacecraft's nose seems like the height of insanity. Sort of like waving an oxyacetylene torch over a birthday balloon. One large POP! and lots of sad astronaut corpses burning up in reentry.
But the handsome propellant savings from aerocapture are so alluring that some NASA researchers did a study. This would be incredibly useful, especially if you want to reuse the spacecraft for several Mars missions or something. They came up with an aeroshell covering the impact side of the TransHab, dubbed the "Ellipsled" due to its shape.
The Ellipsled has a mass of 3929 kg, while the TransHab in the study was assumed to be 14,522 kg. They also assumed the rest of the vehicle had a mass of 7053 kg, presumably most of the propellant had been already burnt. Total of 25,500 kg. I am unsure from reading the report but I get the impression this represents the TransHab detaching from the rest of the spacecraft, carrying along only the ellipsled and a small rocket engine for change-of-plane maneuvers. Meaning the rest of the spacecraft goes sailing off into the wild black yonder as the TransHab aerocaptures into Terran orbit. The report mentions the TransHab being mated to a new spacecraft for a new Mars mission.
The thing has a lift-to-drag ratio of 0.39 at Mach 24 and above, which is better than the Apollo capsule's 0.3. So it is slightly more maneuverable. The deceleration limit is 5.0 g.
They were aiming for something that could approach Terra at the end of the Mars mission and aerocapture into a 420 kilometer orbit by burning off about 4.3 km/s of velocity. If one wanted to get fancy, it is much easier to brake into a high elliptical parking orbit with periapsis of 407 km and an apoapsis of 120,000 km. Yes it makes it more difficult to refurbuish the TransHab and attach it to a new propulsion bus for a new mission, but it really cuts down on the required trans-Mars injection delta V.