The telescope is safe, though, and awaiting the arrival of the servicing crew. During four days of spacewalks, the crew will perform housekeeping duties on the Hubble replacing several pieces of equipment. Most important is the replacement of all six gyros. Hubble's gyros spin at a constant rate of 19,200 rpm on gas bearings. The wheel is mounted in a sealed cylinder, which floats in a thick fluid.
Electricity is carried to the motor by thin wires approximately the size of a human hair.
The Hubble team thinks perhaps oxygen in the pressurized air used during the assembly process caused the wires to corrode and break, so the new gyros were assembled using nitrogen instead of oxygen. Each gyroscope is packaged in a Rate Sensor assembly. It is these assemblies that the crew will change. The assemblies each weigh 24.3 pounds and are 12.8 by 10.5 by 8.9 inches in size. The crew will also replace a Fine Guidance Sensor and the spacecraft's computer.