Position control in satellites

The alignment of satellites in space is usually done through the use of reaction wheels or thrusters. They allow satellites, such as space telescopes to align in a certain direction, to perform sky observations.

A reaction wheel is used for the attitude control of satellites to position them precisely in space. In general they consist of a motor, a gyrating mass and drive electronics. The motor sets the gyrating mass into rotation and creates a moment of torque on the satellite, which turns it into the desired direction. Though the law of conservation of angular momentum is used and in doing so the total spin of the satellite always remains constant. Figuratively speaking, by a change of the satellite position the angular momentum always gets exchanged between the body of the satellite and its reaction wheel.

Performance limits of current systems

Conventional reaction wheels use lubricating bearings for to bear the gyrating mass. Due to the high rotational speeds of up to 10.000 revolutions per minute, the lubricating bearings wear out within a few years because of the occurring friction. Their average life time is about five years, therefore in most cases satellites are equipped with redundant reaction wheels.

If the position control system of a satellite gets inoperative like in the case of the Hubble space telescope in 2009 it is not able to make maneuvers anymore and orbits the Earth as space junk. This damage can only be resolved through a very expensive and complex space shuttle service mission, where only the start costs more than a billion dollars.