Mission Overview
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      The Hubble Space Telescope is a cooperative program of the European
      Space Agency (ESA) and the National Aeronautics and Space
      Administration (NASA) to operate a long-lived space-based
      observatory for the benefit of the international astronomical
      community. HST is an observatory first dreamt of in the 1940s,
      designed and built in the 1970s and 80s, and operational only in the
      1990s. Since its preliminary inception, HST was designed to be a
      different type of mission for NASA -- a permanent space-based
      observatory. To accomplish this goal and protect the spacecraft
      against instrument and equipment failures, NASA had always planned
      on regular servicing missions. Hubble has special grapple fixtures,
      76 handholds, and stabilized in all three axes. HST is a 2.4-meter
      reflecting telescope which was deployed in low-Earth orbit (600
      kilometers) by the crew of the space shuttle Discovery (STS-31) on
      25 April 1990.

      Responsibility for conducting and coordinating the science
      operations of the Hubble Space Telescope rests with the Space
      Telescope Science Institute (STScI) on the Johns Hopkins University
      Homewood Campus in Baltimore, Maryland. STScI is operated for NASA
      by the Association of University for Research in Astronomy,
      Incorporated (AURA).

      HST's current complement of science instruments include two cameras,
      two spectrographs, and fine guidance sensors (primarily used for
      astrometric observations). Because of HST's location above the
      Earth's atmosphere, these science instruments can produce high
      resolution images of astronomical objects. Ground-based telescopes
      can seldom provide resolution better than 1.0 arc-seconds, except
      momentarily under the very best observing conditions. HST's
      resolution is about 10 times better, or 0.1 arc-seconds.

      When originally planned in 1979, the Large Space Telescope program
      called for return to Earth, refurbishment, and relaunch every 5
      years, with on-orbit servicing every 2.5 years. Hardware lifetime
      and reliability requirements were based on that 2.5-year interval
      between servicing missions. In 1985, contamination and structural
      loading concerns associated with return to Earth aboard the shuttle
      eliminated the concept of ground return from the program. NASA
      decided that on-orbit servicing might be adequate to maintain HST
      for its 15-year design life. A three year cycle of on-orbit
      servicing was adopted. The first HST servicing mission in December
      1993 was an enormous success. Future servicing missions are
      tentatively planned for March 1997, mid-1999, and mid-2002.
      Contingency flights could still be added to the shuttle manifest to
      perform specific tasks that cannot wait for the next regularly
      scheduled servicing mission (and/or required tasks that were not
      completed on a given servicing mission).

      The years since the launch of HST in 1990 have been momentous, with
      the discovery of spherical aberration and the search for a practical
      solution. The STS-61 (Endeavour) mission of December 1993 fully
      obviated the effects of spherical aberration and fully restored the
      functionality of HST.