The International Space Station, now in its 14th year, is by far the largest structure ever placed in orbit. Like any satellite boosted to orbital velocity, it circles the planet endlessly (at five miles per second), with almost no need for additional propulsion.
But for a spacecraft as big as a football field, the story is more complicated. The station orbits about 250 miles above Earth, and though the atmosphere at that altitude is wispy, it still exerts drag—enough to slow the ISS and cause it to lose altitude. 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. Even the pull of gravity varies from one end of the massive structure to the other.
As a result, the ISS needs to be reboosted at regular intervals, and its heading and alignment need to be adjusted constantly.
The station's attitude is corrected continuously by four identical Control Moment Gyros mounted near the station's center of gravity, on the backbone-like truss. Each CMG has a four-foot-diameter stainless steel flywheel that spins at 6,600 revolutions per minute. Onboard software changes the spin axis of the wheels so that, moving in unison, their combined angular momentum cancels, or absorbs, the torque that gravity or other disturbances impart on the station. Each CMG produces up to 190 foot-pounds of torque.
Here, astronaut Dave Williams handles one of the CMGs during a 2007 spacewalk.