How does the International Space Station dodge space junk?

In early February, a Russian news service reported that the International Space Station would have to shift its orbit to dodge debris from a Chinese anti-satellite weapons test. The report was wrong, as it turned out. NASA monitored the debris cloud carefully, and finally decided it wouldn't need to move the station.

The incident does raise a question, however: How does the ISS change its orbit, and how quickly could it do so in an emergency?

The station is equipped with a set of 220-pound gyroscopes—stainless steel flywheels that rotate 6,600 times per minute. At least two of these are needed to produce the torque that keeps the station holding the proper attitude without having to waste propellant.

But this kind of tweaking isn't enough to push the ISS into a different orbit. For that, good old-fashioned thrust is needed.

The station has a couple of options for boosting its orbit. Every so often, while unmanned Russian Progress supply ships are docked to the station, their thrusters are fired. To move the ISS safely, Progress' eight engines pulse in a pattern that pushes their thrust evenly through the station's center of gravity.

There are times, though, when no Progress is attached. In that case, thrusters on the Russian Zvezda service module, one of the first large pieces attached to the facility in 2000, could be used. The Zvezda engines haven't been fired in six years, but a test is expected by year's end, says Jack Bacon, a systems integration engineer on the ISS program. Redundancy is part of the ISS modus operandi. "We always have a plan for a reboost," he says. "We never leave ourselves exposed."

Orbit boosts are routine before any ship docks with the station, although they're not always trouble-free. Last December, for example, the thrusters on a Progress fired for 1,364 seconds to raise the station's orbit by five miles before space shuttle Discovery arrived. It was the second attempt—the first try was aborted after the Progress' engine computer malfunctioned after moving ISS just a single mile.

Aside from these regular orbit changes to meet incoming spacecraft, Bacon says that over the course of its life, the ISS will have to be moved several times to counter the effects of atmospheric drag on the huge solar panels (yes, there's a tiny amount of air even at that high altitude).

But what about bona fide emergencies? How quickly could the station dodge a piece of space junk?

Bacon, who has been part of the ISS program since he joined NASA 16 years ago, says that even with advanced warning, engineers wait until "the last minute" to fire thrusters to avoid a potential threat. The last minute means 1.5 orbits, or about 135 minutes, from the predicted point of collision. That gives the ISS two chances to move over or under any incoming danger. Waiting also allows the Space Control Squadron at Cheyenne Mountain in Colorado, which tracks debris in orbit, to refine its predictions about a possible collision. Changing orbits is very disruptive to the work done on the station, and "nine times out of ten you don't have to," Bacon says. "You don't want to maneuver if you don't have to. It's a big deal to fire the engines."

And there are no guarantees—Bacon notes that Cheyenne Mountain can track only objects the size of baseballs and larger. The ISS is designed to survive collisions with objects the size of peas and smaller. That leaves plenty of space junk large enough to cause damage and too small to spot. So even with the ability to move out of the way, luck will continue to play a part in keeping the station safe.