How long can satellites remain in space?

In low orbits, the atmosphere does not end, it simply becomes very thin. That residual air brakes the satellite, its orbit decays, and sooner or later it disintegrates upon atmospheric reentry. How quickly this happens depends on altitude, the shape and mass of the body, and solar activity: an active solar cycle heats the upper atmosphere, which expands, and braking increases. That is why the International Space Station needs periodic boosts; without those maneuvers, it would descend on its own.

The higher, the longer: from months to decades and centuries. Above a few hundred kilometers, braking barely suffices to clear an orbit within a reasonable time, and in the geostationary ring a satellite remains practically forever. The oldest proof has been flying over us for decades: Vanguard 1, launched on March 17, 1958, stopped transmitting long ago but is still in orbit, making it the oldest human-made object in space.

Practical lifetime is determined by the fuel for attitude control, the aging of solar cells and batteries, and electronics that gradually fail. Geostationary satellites end up shifted into a graveyard orbit above the ring; low-orbit satellites must reenter the atmosphere in a controlled way. Since too much space debris is accumulating, regulators are tightening deadlines: the US agency FCC set a five-year deadline after end of mission for disposal in low orbits in 2022, much stricter than previous guidelines. Servicing vehicles that have already docked in the geostationary ring are also extending operational life from the outside.