Servicing.

When Hubble was created the scientist designed the telescope to be modified through the years so to could keep up with current technologies, this enabled Hubble to evolve and become more powerful, these are the modifications Hubble recieved:

Servicing mission 001.

The telescope had always been designed so that it could be regularly serviced, but after the problems with the mirror came to light, the first servicing mission assumed a much greater importance, as the astronauts would have to carry out extensive work on the telescope to install the corrective optics. The seven astronauts selected for the mission were trained intensively in the use of the hundred or more specialized tools that would be needed. The Space Shuttle Endeavour mission STS-61 took place in December 1993, and involved installation of several instruments and other equipment over 10 days.

Most importantly, the High Speed Photometer was replaced with the COSTAR corrective optics package, and WFPC was replaced with the Wide Field and Planetary Camera 2 (WFPC2) with its internal optical correction system. In addition, the solar arrays and their drive electronics were replaced, as well as four of the gyroscopes used in the telescope pointing system, two electrical control units and other electrical components, and two magnetometers. The onboard computers were upgraded, and finally, the telescope's orbit was boosted, to compensate for the orbital decay from 3 years of drag in the tenuous upper atmosphere.

On January 13, 1994, NASA declared the mission a complete success and showed the first of many much sharper images. At the time, the mission had been one of the most complex ever undertaken, involving five lengthy periods of extra-vehicular activity, and its resounding success was an enormous boon for NASA, as well as for the astronomers who now had a fully capable space telescope.

Servicing mission 002.

Servicing Mission 2, flown by Discovery (STS-82) in February 1997, replaced the GHRS and the FOS with the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired thermal insulation and again boosted Hubble's orbit. NICMOS contained a heat sink of solid nitrogen to reduce the thermal noise from the instrument, but shortly after it was installed, an unexpected thermal expansion resulted in part of the heat sink coming into contact with an optical baffle. This led to an increased warming rate for the instrument and reduced its original expected lifetime of 4.5 years to about 2 years.

Servicing mission 003a.

Servicing Mission 3A flown by Discovery (STS-103), took place in December 1999, and was a split-off from Servicing Mission 3 after three of the six onboard gyroscopes had failed. (A fourth failed a few weeks before the mission, rendering the telescope incapable of performing scientific observations.) The mission replaced all six gyroscopes, replaced a Fine Guidance Sensor and the computer, installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging, and replaced thermal insulation blankets. The new computer is 20 times faster, with six times more memory, than the DF-224 it replaced. It increases throughput by moving some computing tasks from the ground to the spacecraft, and saves money by allowing the use of modern programming languages.

Servicing mission 003b.

Servicing Mission 3B flown by Columbia (STS-109) in March 2002 saw the installation of a new instrument, with the FOC (the last original instrument) being replaced by the Advanced Camera for Surveys (ACS). This meant that COSTAR was no longer required, since all new instruments had correction for the main mirror aberration built in. The mission also revived NICMOS and replaced the solar arrays for the second time, providing 30 percent more power.

Servicing mission 004.

Servicing Mission 4 (SM4), which took place in May 2009, was the last scheduled shuttle mission (STS-125) for the Hubble Space Telescope. The mission was delayed to replace the Science Instrument Command and Data Handling (SI C&DH). SM4, with a replacement SI C&DH unit, was launched aboard Space Shuttle Atlantis. SM4 installed two new instruments, Wide Field Camera 3 (WFC3), and the Cosmic Origins Spectrograph (COS), repaired the Advanced Camera for Surveys (ACS) and the Space Telescope Imaging Spectrograph (STIS), and replaced other components. These efforts are expected to keep the telescope fully functioning at least into 2014, and perhaps longer. SM4 also installed the Soft Capture and Rendezvous System, which will enable the future rendezvous, capture, and safe disposal of Hubble by either a crewed or robotic mission.