The International Space Station orbits about 250 miles above Earth. Although the atmosphere at that altitude is wispy, it still exerts enough drag to slow the ISS and cause it to lose altitude. At the same time, the giant, wing-like solar arrays swivel to track the sun, introducing disturbances to the station's orbit and alignment that build up over time. As a result, the ISS needs to be reboosted at regular intervals, and its heading and alignment need to be adjusted constantly.
Animation: Bill Abreu (0:26