International Space Station

Propulsion

ZvezdaLike the crew that inhabits it, the International Space Station periodically needs a boost, but for different reasons.

Slight though the number of atmospheric molecules is at the Station's orbital altitudes, it's still enough to cause drag on the ISS that pulls it toward Earth. Solar flux -- sunlight and other solar phenomena -- add to the problem.

The boosts (aka "reboosts") keep the Station orbiting at 217 to 285 miles, the altitudes required for the proper operation of everything from labs and living quarters to logistics and launch vehicles.

Control Module Zarya's twin engines provided First Element propulsion for reboosts. When Russia's Service Module was mated with the First Element in 1999 to form the early core of the ISS, its two main engines assumed this duty.

As logistics spacecrafts have begun making their re-supply runs, and ISS assembly progresses, Russia's Progress M1 cargo vehicles have become the primary Station reboost propulsion systems.

Russia has used the single-engine, remotely piloted craft to reboost a number of orbiting vehicles, Mir among them. ISS reboost was planned by the Trajectory Operations Officer at Mission Control Center-Houston and executed by Mission Control Center-Moscow.

The operation of some Station systems and experiments is suspended during reboost, but it is not expected that power reductions will be necessary. Once the Station is maneuvered into a "reboost attitude," propulsion burns are executed for a fixed amount of time without active guidance. Reboosts usually require two propulsion burns.

The ISS is maneuvered back into its normal attitude at the end of the second burn. Then, life can get back to "normal."