The third servicing mission was split into two parts. The split was required to address the urgent need to replace Hubble's failing gyroscopes, which point the telescope at celestial targets.
Servicing Mission 3A took place in December, 1999. The crew replaced a transmitter, all six gyroscopes, and one of three fine guidance sensors, which allow fine pointing and keep Hubble stable during operations.
The astronauts also installed an advanced central computer, a digital data recorder, an electronics enhancement kit, battery improvement kits, and new outer layers of thermal protection.
Servicing Mission 3B took place in February 2002. Astronauts added the Advanced Camera for Surveys, the first new instrument to be installed in Hubble since 1997. ACS doubled Hubble's field of view and collects data 10 times faster than the Wide Field and Planetary Camera 2, which was installed in 1993.
The crew also replaced Hubble's solar array panels with smaller, more effective ones, replaced the Power Control Unit and one of the four reaction wheel assemblies that make up Hubble's pointing control system, and installed a new cooling system for the Near Infrared Camera and Multi-Object Spectrometer. In order to replace the Power Control Unit, the telescope had to be powered down for the first time since its launch in 1990.
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