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.
Watch Your Attitude
As the station orbits, it rolls slowly around its long axis at the rate of four degrees per minute (turning a full 360 degrees over a 90-minute orbit). That keeps communications antennas and Earth-facing windows pointing straight down as Earth curves away underneath, and results in a consistent attitude with respect to the ground.
How does the ISS sense its position? Five times a second, onboard rate gyroscopes monitor how fast the vehicle's position is changing. To determine the station's altitude, position, and velocity, data from GPS receivers on the U.S. side are combined with data from Russian navigation satellites, as well as information from sun, star, and horizon sensors on the Russian segment of the station. All these positioning data are fed to onboard computers, which in turn determine how much correction the CMGs need to make.
On a typical day, drag from the thin atmosphere causes the station to drop about 300 feet in its orbit. If nothing were done, eventually it would dip into even thicker air and start to break apart.
So every three months or so, the station needs a boost. Typically this is done after a cargo ship like the Russian Progress or Europe's Automated Transfer Vehicle (ATV) docks with the ISS. The visiting vehicle, plugged into the Russian Zvezda Service Module, at the station's aft end, burns its engines for one to 20 minutes, increasing the station's orbital speed and therefore its altitude. An acceleration of a few feet per second is all it takes to raise the orbit by a few miles.
Zvezda also has thrusters—and can itself be used for reboost or large maneuvers beyond the capacity of the CMGs. On rare occasions, when some piece of space debris is predicted to come uncomfortably close to the station, the orbit can be raised slightly to reduce the chance of collision.
Avoid the Jitters
On Earth the ISS would weigh 500 tons, but in near-weightlessness, even small forces can push such a massive object out of kilter. Astronauts using exercise equipment or maneuvering the station's robotic arm can disturb the spacecraft enough to require the CMGs to compensate. Certain experiments sensitive to very low levels of gravity might require that these activities be suspended temporarily, for maximum "quiet."
On Earth, long-exposure photographs of the night sky show how our planet rotates on its axis. NASA astronaut Don Pettit, who returned in June from a six month stay aboard the International Space Station, decided to play with this idea from low Earth orbit. Instead of showing Earth's rotation, his photographs show the station's movement on its own axis some 200 miles up. To keep the same side always facing Earth, the station rotates one full turn every orbit, just like the moon.