How does the International Space Station dodge space junk?
The 200-ton orbiting behemoth can get out of harm's way, but not very quickly.
- By Joe Pappalardo
- AirSpaceMag.com, March 01, 2007
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).