Mission Overview
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      The International Ultraviolet Explorer (IUE) was launched
      successfully on 26 January 1978, from Cape Canaveral, Florida. The
      satellite was developed to provide a general facility for observing
      ultraviolet (UV) spectra of astronomical sources over the wavelength
      range from about 1150 A to 3200 A.  The project has been a joint
      undertaking in which the US National Aeronautics and Space
      Administration (NASA) provided the spacecraft, the optical and
      mechanical components of the scientific instrument, the US ground
      observatory and spacecraft control software; the UK Science Research
      Council (SRC) in collaboration with University College London (UCL)
      provided the television cameras used to record the spectroscopic
      data; and the European Space Agency (ESA) provided the solar arrays
      and European ground observatory. The image processing software was
      developed jointly by NASA and the SRC's Appleton Laboratory. The
      satellite has been placed in geosynchronous orbit over the Atlantic
      Ocean and is operated for 16h each day, for NASA sponsored
      observers, from the US ground observatory located at the Goddard
      Space Flight Center (GSFC) near Washington, D.C., and for the
      remaining 8h by ESA, for ESA and UK sponsored observers, from the
      European ground observatory located near Madrid.
 
      A synchronous orbit has other advantages. Because the Earth subtends
      an angle of only 17 degrees the unconstrained area of sky is much
      greater than for low orbits and, as its movement along the ecliptic
      is at the diurnal rate, the occurrence of occultations is
      infrequent. Consequently, over large portions of the sky, neither
      long exposures nor the observation of variable phenomena need be
      periodically interrupted. A penalty is that in synchronous orbit
      observations are normally made in full sunlight so the telescope
      must be baffled to ensure adequate rejection of stray sunlight and
      earthlight.  The offset guiding system can be influenced, however,
      and relatively bright guide stars are needed near the Earth's limb.
 
      The IUE mission was terminated on Sept 30, 1996 at 18:42 UT after a
      life span of 18 years, 8 months, 4 days, 1 hour and 6 min.

DESIGN CONCEPTS
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      It was decided that a high spectral resolution of the order of 0.2 A
      would be required to study the atmospheric characteristics of bright
      stars and planets and that a lower resolution of about 6 A should be
      available to obtain information on faint sources. In addition, two
      other requirements were fundamental in determining the design of the
      scientific instrument and of the spacecraft to support it: first,
      the instrument should be able to record simultaneous data over large
      spectral ranges and, second, the spacecraft should be able to take
      maximum advantage of the observer's real-time judgement concerning
      the quality and content of his data.
 
      The latter requires good communication between the spacecraft and
      the observer so a geosynchronous orbit was chosen. The IUE orbit
      allows continuous real-time control from the NASA control centre and
      at least 10h each day from the ESA control centre. The arrangement
      is fundamentally different from low altitude orbiting observatories
      which have to acquire data while remote from ground control. In the
      case of IUE, control and performance monitoring are exercised
      continuously. The telescope field is displayed to the observer, who
      identifies his target star and directs the course of the observation
      in real-time. The 'IUE observatory', thus comprises the ground
      control centre, where the astronomer views his television monitors,
      and the optical and electronic instrumentation located in
      synchronous orbit.