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
===============
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.