SSTL have delivered two flight standard momentum wheels to DLR in Germany, one of which will fly on the Rosetta spacecraft that is due to be launched in January 2003, the other as a flight spare.

The Rosetta spacecraft will rendezvous with the comet “46 P/Wirtanen” on 29th November 2011 as it makes one of its periodic visits to the solar system.

The Rosetta Lander module, for which DLR is prime contractor, will detach from the main spacecraft and attempt a soft landing on the comet. SSTL’s momentum wheel will stabilise the module during the descent and landing phases.

The SSTL momentum wheel is unique in that it provides 5.2 Newton meter second (Nms) of momentum with a power dissipation of only 6 watts. By using a dry lubricated bearing the wheel is able to operate in vacuum at lower frictional loss, thereby reducing the energy required to drive the mechanism. This was a crucial factor in the selection of the SSTL wheel – which will have to survive a cruise time of almost 9 years through space for use in the final, but critical, 20 hours of approach and landing on the comet.

Details of the Rosetta Mission are available at

Picture of momentum wheel available electronically at:

Notes for Editors

A momentum wheel is a fast rotating disc. Normally the speed of rotation is a few thousand revolutions per minute – similar to a gyroscope. The wheel is mounted inside the satellite so that the spin direction is inertially fixed, giving the satellite an inherent gyroscopic rigidity in this direction. This means that external disturbances will find it difficult to change the satellite’s attitude away from this pointing direction, in the same way that it is difficult to push over a spinning top (gyroscope).

By changing the momentum wheel’s speed over a limited range (less than 10% of the nominal speed) the satellite can easily be rotated around the spin direction, thus rotational momentum can be exchanged between the wheel disc and the satellite’s body. Therefore, the momentum wheel will keep the 3-dimensional attitude of the satellite fixed in the spin direction, whilst rotating the satellite freely around the spin direction.

Miss Audrey Nice
Surrey Satellite Technology Ltd
01483 879278