Hubble scientists contend that Hubble could actually operate with only one gyroscope, however, only at reduced capacity. The latest gyroscope to fail had been in operation for over six years—well over the intended useful life of the devices.

In August 2005, Hubble scientists decided to switch off one of the functioning gyroscopes and, thus, operate the telescope with two rather than three gyroscopes. Along with the two operating gyroscopes, the orientation of Hubble is being helped along with the Fine Guidance Sensors—which is an optical sensor used to provide pointing and stabilization information.

Gyroscopes enable Hubble to stabilize itself. The seventeen-year-old orbiting telescope was originally designed to operate with three gyros, with another three gyros in reserve. Since then, three have failed. Of the two operating gyros, one is fairly new and the other is about four years old. The third one, a backup gyro, is about 6.5 years old.

The shuttle repair mission for Hubble—STS-125 (Hubble Space Telescope servicing mission 4 [HST SM-04]) with space shuttle Atlantis—will also replace batteries on board Hubble, along with installing the Cosmic Origins Spectrometer and the Wide Field Camera 3.

Hubble scientists are also concerned with Hubble’s batteries, which are expected to cease operating in 2010. Any delays in the repair mission could compromise the health of Hubble.

When servicing Hubble, the NASA astronauts will perform five spacewalks, or extravehicular activities (EVAs), in order to perform the necessary repairs and improvements to the orbiting observatory. One team will perform the first, third, and fifth walks, while another team will conduct the second and fourth spacewalks—each about seven hours in duration.

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