The Hubble Space Telescope (HST) is a
telescope in orbit around the Earth, named after astronomer
Edwin Hubble. Its position outside the Earth's atmosphere
provides significant advantages over ground-based telescopes —
images are not blurred by the atmosphere, there is no background
from light scattered by the air, and the Hubble can observe
ultra-violet light that is normally absorbed by the ozone layer
in observations made from Earth. Since its launch in 1990, it
has become one of the most important instruments in the history
of astronomy. With it, astronomers have made many observations
leading to breakthroughs in astrophysics. Hubble's Ultra Deep
Field is the most sensitive astronomical optical image ever
taken.
From its original conception in 1946 until its launch, the project to build a
space telescope was beset by delays and budget problems. Immediately after its
1990 launch, it was found that the main mirror suffered from spherical
aberration due to faulty quality control during its manufacturing, which
severely compromised 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.[1]
Hubble is a collaboration between NASA and the European Space Agency. The Hubble
is the only telescope ever designed to be serviced from 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.
Since SM3B, the Hubble has lost use of two major science instruments and is
operating with viewing restrictions because of rate-sensing gyroscope failures.
There are six gyroscopes onboard Hubble and three are normally used for
observing. However, after further failures, and in order to conserve lifetime, a
decision was taken in August 2005 to switch off one of the functioning
gyroscopes and operate Hubble using only 2 gyros in combination with the Fine
Guidance Sensors. This mode retains the excellent image quality of Hubble, and
provides a redundancy should it be needed. Further redundancy is available now
that an operational mode requiring only one gyro has now been developed and
tested. Six new gyroscopes are planned to be installed in SM4.
The two instruments that have failed are the Space Telescope Imaging
Spectrograph (STIS) which stopped working in August 2004 and the Advanced Camera
for Surveys (ACS) which ceased operations in January 2007 (operations were later
restored for its little used far-ultraviolet mode). Currently (mid-2007) Hubble
observations are being taken with the Wide Field and Planetary Camera 2 (WFPC2)
and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Astrometry
is being carried out with the Fine Guidance Sensor (FGS). Without a reboost to
increase the diameter of its orbit, drag will cause Hubble to re-enter the
Earth's atmosphere sometime after 2010.
Following the 2003 Columbia Space Shuttle disaster, the fifth servicing
mission (SM4), initially planned for 2004, was canceled on safety grounds. NASA
determined that a manned mission would be too dangerous, due to a lack of access
to the International Space Station (ISS), which can serve as a safe haven for an
astronaut crew. The Shuttle cannot travel between the Hubble and ISS orbits. The
organization later reconsidered this position, and, on October 31, 2006, NASA
administrator Mike Griffin gave the green light for a final Hubble servicing
mission to be flown by Atlantis. The mission is now planned for September
2008.[2][3] As a safety precaution, NASA will have the orbiter Discovery
standing by at Launch Complex 39B to provide rescue in the event of an
emergency. The planned repairs to the Hubble will allow the telescope to
function until at least 2013, when its successor, the James Webb Space Telescope
(JWST), is due to be launched. The JWST will be far superior to Hubble for many
astronomical research programs, but will only observe in infrared, so that it
will not replace Hubble's ability to observe in the visible and ultraviolet
parts of the spectrum.
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