Hubble Space Telescope Servicing Mission 3A fact sheet

The Hubble team believes they understand the cause of the failures, 
although they cannot be certain until the gyros are returned from space and 
taken apart. Based on nearly one and a half years of intensive chemical, 
mechanical and electrical investigations, the team believes that the thin 
wires are being corroded by the fluid in which they are immersed and 
ultimately this corrosion causes them to break. The fluid is very thick 
(about the thickness of 10W-30 motor oil), and in order to force this fluid 
into its float cavity, pressured air was used. The team believes that 
eventually, oxygen in the air interacted with the fluid to create a small 
amount of corrosive material and the wires were partially eaten away. 
Sometimes the wires were strong enough to carry electricity and some-times 
they were not and they broke. Pressurized nitrogen is now used instead of 
pressurized air. Using pressurized nitrogen eliminates the introduction of 
oxygen into this fluid.