How does the International Space Station stay in orbit so low?

The International Space Station orbits at an altitude of roughly 400 kilometers, where the atmosphere is thin but not absent. It stays up because it travels fast enough that as it falls toward Earth, Earth curves away beneath it, creating an orbit. However, residual atmospheric drag gradually slows the station, so it must be raised occasionally using visiting spacecraft or station thrusters. This process is called reboost. The ISS therefore needs periodic maintenance to keep its orbit stable and safe for crews and experiments.