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Rosetta/Philae mission (2014)

First momentum wheel to guide a lander to land on a comet’s surface, Rosetta/Philae mission (2014)

SSTL adapted the design of our small reaction wheel to perform as a momentum wheel  for the Rosetta mission.  A momentum wheel spins at a constant speed to 'lock' the spacecraft to point in one direction which would ensure that the feet on the Philae lander always pointed towards the comet’s surface after it was released from the Rosetta mothership.
 
Our momentum wheel design had to take account of the slightly different environment and requirements of the Rosetta mission.  One of the first decisions we made was to use a solid lubricant to 'oil' the bearings, rather than the traditional liquid lubricant. This choice was based on the fact that Philae would be spending ten years dormant on the mothership as it made its journey towards the comet and we couldn’t run the risk of an oil lubricant evaporating over the years.
 
It obviously worked as Philae stayed the right way up during its initial landing and the subsequent 'bounces'!

Philae touchdown credit ESA ATG medialab

Philae flywheel housing and PCB