Rate Sensor Units (Gyroscopes)

The gyroscopes are part of Hubble's pointing system. They provide a frame of reference for Hubble to determine where it is pointing and how that pointing changes as the telescope moves across the sky. They report any small movements of the spacecraft to Hubble's pointing and Control System computers. The computers then command the spinning reaction wheels to keep the spacecraft stable or moving at the desired rate. The gyroscopes work by comparing Hubble's motion relative to the axes of the spinning masses inside the gyroscopes. In the absence of external torques, these axes remain stable relative to the fixed stars in the sky. By keeping Hubble fixed relative to these axes, Hubble stays stable relative to the stars.

Hubble uses three Fine Guidance Sensors for steady pointing and measurement. Astronauts will exchange one of these optical sensors with a refurbished unit that has enhanced, on-orbit alignment capability. The new unit is a refurbished version of the one that was returned from Servicing Mission 3A in 1999.

Each battery consists of 23 cells in series along with heaters, heater controllers, pressure measurement transducers and electronics, and temperature-measuring devices and their associated electronics. Three batteries are packaged into a module measuring roughly 36 by 36 by 10 in. (90 x 90 x 25 cm) and weighing about 475 lb (214 kg). Each module is equipped with two large yellow handles that astronauts use to maneuver the module in and out of the Telescope in space.