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In the TV show Last Man On Earth we follow a small group of immune survivors after an apocalyptic plague has essentially wiped out humanity. It is revealed that a single astronaut on the ISS has also survived albeit he is not very hopeful about his future.

Presumably there is nobody left at NASA, ESA, Roscosmos, or any other agency that would be monitoring and managing space activities. So that leads to my question: what options exist for this lone astronaut? Would it be possible to return to Earth? Does the Soyuz capsule need ground support? Can a single astronaut even leave the ISS in Soyuz by themselves? Obviously astronauts need physical therapy and support after returning from flight so that is also an element to be included in your answer.

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    $\begingroup$ Theoretically you could make all calculations necessary for re-entry on ISS. They calculated the return trajectory of Apollo 13 with slide rules. You're most likely lacking the telemetry data on ISS and Soyuz though, so you can't monitor if you're really on the right course, that is however not necessary when you did your calculations right and everything works on the Soyuz capsule. $\endgroup$ – Adwaenyth Dec 10 '15 at 6:50
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    $\begingroup$ That makes sense, and they have plenty of computer power on the ISS so computing trajectories shouldn't be difficult. What I'm more interested in is the execution of the return. $\endgroup$ – Brian Lynch Dec 10 '15 at 6:52
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Yes, Soyuz spacecraft can serve as a lifeboat and return a single crew to Earth. From Anatoly Zak's Russian Space Web page:

In case of emergency on the station, the Soyuz can be sent up unmanned or piloted by a single cosmonaut to serve as a lifeboat; or be used as an unmanned cargo ship to return 250 kilograms from orbit.

If for some reason contact with ground control is lost, vehicle can't use ground-based telemetry for guidance, or its autonomous flight mode failed, it will default to a high g ballistic entry. This means it will likely miss its landing site by quite some margin, but it has happened on several occasions before. Soyuz TMA is quite a robust crew return vehicle. Refer e.g. to James Oberg's Soyuz Landing Historical Reliability Study for more details, it sounds nearly impossible in what all circumstances Soyuz managed to return crew back home alive, though deadly accidents during deorbit burn and landing did, sadly, also happen.

As for post landing support, well, if there isn't anyone left to await the returning single person crew and thus no recovery forces to meet him or her at the launch site, that kind of forfeits any physical therapy and post-landing support, too. That person is forced to manage on their own.

With a bit of luck however, depending on when the emergency happened, that person wouldn't have been on orbit and in microgravity conditions for a very long time. Deorbit burn will have to happen in up to 200 days after Soyuz TMA was launched into orbit, since that's it's maximum on-orbit standby time. A single crew without all the Mission Control Center's flight controllers keeping an eye on all the sensor readings is also unlikely to maintain the station safe for a very long time, so it would have to be evacuated pretty much the same day as the emergency scenario happened. It also depends on each individual how adverse effects of a prolonged stay in a microgravity environment will be, and how fast they acclimate back to Earth gravity conditions.

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    $\begingroup$ So the bottom line is that this it is at least possible for one astronaut to prime the Soyuz, get in and depart alone. Nothing stopping the vehicle from executing an unsupported reentry, and assuming a safe landing on solid ground they astronaut could get out and be on there way (although maybe with a significant struggle to adjust to gravity). $\endgroup$ – Brian Lynch Dec 10 '15 at 6:59
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    $\begingroup$ @BrianLynch That's the gist of it yes. $\endgroup$ – TildalWave Dec 10 '15 at 7:13
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    $\begingroup$ Using spare room for supplies (food, water, repair/construction materials like sheet plastic), and the well-supplied survival kit, pick a landing spot in a safe area preferably near inhabited settlements (HAM Radio!), deorbit and break camp around the capsule, undergoing rehabilitation on their own. Look up Voskhod 2 return... no prolonged microgravity problems but not a neat warm climate either... $\endgroup$ – SF. Dec 10 '15 at 7:50

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