How high up are astronauts?

The International Space Station flies in a nearly circular orbit of about 400 km, inclined 51.6° to the equator, and completes a full loop every 90 minutes: its occupants see sixteen sunrises a day. That altitude is not fixed. Friction with the traces of atmosphere still present up there makes it lose altitude every day, so the station is reboosted using the engines of visiting spacecraft; after each maneuver its altitude rises a few kilometers and drops again over the following months.

China's Tiangong stays within a similar range of low Earth orbit. Below about 300 km the atmosphere slows a craft down too much and orbital life is measured in weeks; above about 1,000 km the inner Van Allen radiation belt begins, a hostile environment for staying months.

The chosen altitude is a compromise between the propellant needed to get there and stay, the radiation protection offered by Earth's magnetic field at that height, and the possibility of a quick return in an emergency. The 51.6° inclination, inherited from launches from Baikonur, also allows overflight of much of the inhabited surface and makes access easier from almost any launch site.

Only twenty-four people have left Earth orbit, all between 1968 and 1972 and all aboard Apollo spacecraft. The distance record belongs to the Apollo 13 crew, who, looping around the far side of the Moon in April 1970 with a damaged spacecraft, traveled just over 400,000 km from Earth; they exceeded the average distance to the Moon because they passed behind it near lunar apogee. No one has broken that record since.