Station-keeping

The small, regular burns that stop an orbit drifting into uselessness.

Orbits decay from drag, wander from the pull of the Moon and Sun, and precess because Earth is not a sphere. Keeping a satellite where it is supposed to be means firing thrusters every few weeks, and when the propellant for that runs out the mission ends regardless of how healthy everything else is. It is the usual definition of a spacecraft's lifetime, and refuelling in orbit is attractive precisely because it would sever that link. Geostationary satellites carry the heaviest bill. Holding a slot against the Moon and Sun costs around 50 m/s of velocity change a year, most of it spent fighting a north-south drift, and a satellite that has run its tanks down is boosted a few hundred kilometres higher into a graveyard orbit rather than left in the slot somebody else has been sold.