Servicing Mission 1
The first servicing mission, launched in December 1993, provided the first opportunity to conduct planned maintenance on the telescope. In addition, new instruments were installed and the flaw of the optics in Hubble's primary mirror was corrected. The astronauts' task list appears below.

• Change out the Wide Field Planetary Camera. The original instrument was replaced with an updated model called the Wide Field and Planetary Camera 2, or WFPC2. WFPC2 was equipped with built-in corrective optics that compensated for the flaw on the telescope's primary mirror.
  
  
• Remove the High-Speed Photometer and install the Corrective Optics Space Telescope Axial Replacement (COSTAR), which included an assembly of coin-sized mirrors polished to a special prescription that compensate for the flaw on the telescope's primary mirror.
  
  
• Replace the solar array panels, wing-like structures that produce electricity to power the observatory.
  
  
• Install solar array drive electronics, modular hardware that controls the position of the solar arrays.
  
  
• Install the magnetometer, a device that measures the telescope's orientation within Earth's magnetic field to help in attitude control.
  
  
• Install the DF-224 Flight Computer Co-processor, computer memory that improved backup and computational power.

Servicing Mission 2
The second servicing mission, launched in February 1997, greatly improved Hubble's productivity. The installation of new instruments extended Hubble's wavelength range into the near infrared for imaging and spectroscopy, allowing Hubble to probe the most distant reaches of the universe. The replacement of failed or degraded spacecraft components increased the efficiency and performance of the telescope. The astronauts' task list appears below.

• Install the Space Telescope Imaging Spectrograph (STIS), which provides Hubble with unique and powerful spectroscopic capabilities. A spectrograph separates the light gathered by the telescope into its spectral components so that the composition, temperature, motion, and other chemical and physical properties can be analyzed.
  
  
• Install the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which has provided valuable new information on the dusty centers of galaxies and the formation of stars and planets. NICMOS consists of three cameras. It is capable of both infrared imaging and spectroscopic observations of astronomical targets.
  
  
• Install a refurbished Fine Guidance Sensor, which was needed to point the telescope at celestial objects.
  
  
• Install the Optical Control Electronics Enhancement Kit, which provides the electronic pathway for commanding the alignment mechanisms in the refurbished Fine Guidance Sensor.
  
  
• Install a solid state recorder, which stores ten times more data and replaced one of Hubble's three reel-to-reel tape recorders.

Servicing Mission 3A
The third servicing mission, Servicing Mission 3A, was launched in December 1999. NASA decided to split the Servicing Mission 3 into two parts, SM3A and SM3B, after the third of Hubble's six gyroscopes failed. In accordance with NASA's flight rules, a "call-up" mission was quickly approved, and developed and executed in a record seven months.

What was originally conceived as a mission of preventive maintenance turned more urgent on Nov. 13, 1999 when the fourth of six gyros failed, and Hubble temporarily closed its eyes on the universe. The telescope needs a minimum of three gyros to function. The astronauts' task list appears below.

• Install six new gyroscopes, which help keep Hubble steady as it orbits Earth and allow scientists to point the telescope at celestial targets.
  
  
• Add a faster, more powerful, main computer.
  
  
• Install a next-generation solid state data recorder.
  
  
• Add another enhanced Fine Guidance Sensor, which helps in pointing the telescope at objects.
  
  
• Install a new transmitter, new insulation, and six battery voltage/temperature improvement kits.
  
  

Servicing Mission 3B
Servicing Mission 3B was launched on March 1, 2002. The astronauts’ principal task was to install a new science instrument, the first new instrument to be added to Hubble since 1997. Astronauts also installed a new cooling system for the telescope’s infrared camera. The astronauts' task list appears below.

• Install a new science instrument called the Advanced Camera for Surveys (ACS). With its wide field of view, sharp image quality, and enhanced sensitivity, ACS doubled Hubble's field of view and collects data ten times faster than the Wide Field and Planetary Camera 2, the telescope's earlier surveying instrument.
  
  
• Install a new cooling system for the Near Infrared Camera and Multi-Object Spectrometer, or NICMOS, which became inactive in 1999 when it depleted the 230-pound block of nitrogen ice that had cooled it since 1997.
  
  
• Replace Hubble’s four large flexible Solar Array Panels, which collects sunlight and helps power the telescope. The solar panels are smaller and more rigid, producing 30 percent more power.
  
  
• Replace the outdated Power Control Unit, which distributes electricity from the solar arrays and batteries to other parts of the telescope.
  
  
• Replace one of the four Reaction Wheel Assemblies that make up Hubble's Pointing Control System. Flight software commands the reaction wheels to "steer" the telescope by spinning in one direction, which causes Hubble to spin in the other direction.