William sent in a question to the Coalition for Space Exploration asking about NASA's plans to deal with large spare parts for the International Space Station once the Space Shuttle is retired in 2010. He was particulary concerned about the Station's gyroscopes, or CMGs, and if future problems with the devices could send the ISS back to Earth like Skylab.

I talked with NASA spokesman Rob Navias from the Johnson Space Center and learned the agency's intention is that two spare gyros will be brought up to the station and stored at the outpost in case they are needed.

Extra batteries, a Ku-band antenna, an S-band antenna, ammonia tank assemblies a robot arm component, and a pair of high-pressure gas tanks are some of the other bulky spare items intended to be packed on the station before the Space Shuttle's retirement in 2010.

After then it will be several years before NASA can send bigger replacement parts up to the station so the idea is to plan ahead and load up the ISS with supplies. Most of that cargo will be launched on the final two shuttle missions planned by NASA .

William's question specifically referred to the CMGs. Four CMGs are installed outside the station, each spinning 6,600 times per minute. At least two need to be working to do their job, which is to keep the space station pointed in a specific direction as it orbits the Earth.

If the station loses its pointing ability a number of unwanted things could happen to the station, but it won't just suddenly tumble out of orbit and crash to Earth, which was the ultimate fate of SkyLab and the Russian space station Mir.

Fortunately there is a second way to control the station's attitude and that is by using the outpost's thrusters, or steering jets on visiting docked spacecraft. This method is often employed when the CMGs need maintenance, or "desaturation."

The CMGs have been troublesome in the past and two have required replacement by spacewalking astronauts.

Jim Banke
Cape Canaveral, Fla.