International Ultraviolet Explorer

IUE contributed to a number of studies and made discoveries that might not have been possible without the long-term availability of a successfully working satellite. One was the discovery of short-term variations in the auroras in the atmosphere of Jupiter (which were initially discovered by IUE). Since auroras are caused by the interaction between the upper atmosphere of a planet and particles radiated from the sun, and the emission of these particles increases as the sun becomes more active, the long life of IUE allowed unique studies associating Jovian aurora activity with the solar sunspot cycle. IUE was the first instrument to provide a systematic study of the distribution of different species of comets in space. The long life of IUE also enabled the monitoring of variations in the occurrence of different types of comets, the discovery of new material within them, and the classification of comets into groups as a function of age. The behavior and distribution of stellar particle radiation (stellar winds) is now beginning to become more clear, and there is hope of understanding the underlying mechanisms driving the stellar winds because of the observations performed with IUE. IUE spectra combined with optical observations have allowed distances to the Magellanic Clouds, the closest galaxies to the Milky Way, to be determined. Many other studies within and outside our galaxy were also conducted adding significant data to the science of astrophysics. Volumes have already been published on these and many other topics. With the importance of the IUE observations and the concurrent development of the Internet while the data was being received and analyzed, the IUE data archive has become the most heavily used astronomical archive in existence.

Possible future needs were identified during and after the IUE mission, which brought about the concept of creating a World Space Observatory that could provide flexible access to space-based observatories and observation times for astrophysicists world-wide. A working group was formed to further study the associated problems and opportunities.

With all its success, IUE had a few serious problems during its very long mission. All of these came from the fact that five of the six gyroscopes in its attitude control system failed over the years. After the fourth one failed in 1985, IUE continued operations because of the use of its fine sun sensor as a substitute to controlling the attitude of the spacecraft. Even when another gyro was lost in the final year, IUE could still be stabilized in 3-axes, with only one remaining gyroscope, by adding star tracker measurements to other guidance parameters. Until October 1995, IUE was in continuous operation, controlled 16 hours a day from the Goddard Space Flight Center in Greenbelt, Maryland, and eight hours from ESA's Villafranca Satellite Tracking Station (VILSPA) west of Madrid, Spain. After that, ESA took on the role of redesigning control schemes to make it feasible to cover the science operations fully controlled from VILSPA. But then, only 16 hours were used for scientific operations, with eight hours used for spacecraft housekeeping. IUE remained operational until its attitude control fuel was deliberately released into space, its batteries drained and its transmitter turned off on September 30, 1996.