The Hubble Space Telescope (HST; also known colloquially as "the Hubble" or just "Hubble") is a space telescope that was carried into orbit around the Earth by the Space Shuttle Discovery in April 1990. It is named for the American astronomer Edwin Hubble. Hubble's position outside the Earth's atmosphere allows it to take extremely sharp images and, although not the first space telescope to be deployed, Hubble has become one of the most important instruments in the history of astronomy. Hubble's Ultra Deep Field image, for instance, is the most detailed visible-light image of the universe's most distant objects ever made. Many observations made using the telescope have led to breakthroughs in astrophysics.
The construction and launch of the Hubble was beset by delays and budget problems. Then, soon after its 1990 launch, it was found that the main mirror suffered from spherical aberration due to faulty quality control during its manufacturing, severely compromising the telescope's capabilities. However, after a servicing mission in 1993, the telescope was restored to its intended quality and became a vital research tool as well as a public relations boon for astronomy. The HST is part of NASA's Great Observatories series, with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope. Hubble is a collaboration between NASA and the European Space Agency.
The Hubble is the only telescope ever designed to be serviced in space by astronauts. To date, there have been four servicing missions, with a fifth and final mission planned for September 2008. Servicing Mission 1 took place in December 1993 when Hubble's imaging flaw was corrected. Servicing Mission 2 occurred in February 1997 when two new instruments were installed. Servicing Mission 3 was split into two distinct missions: SM3A occurred in December 1999 when urgently needed repairs were made to Hubble; and then SM3B followed in March 2002 when the Advanced Camera for Surveys (ACS) was installed.