International Ultraviolet Explorer

The International Ultraviolet Explorer satellite (IUE) was a joint project of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Planetary Plasma and Atmospheric Research Center (PPARC) in the United Kingdom. NASA provided the spacecraft, telescope, spectrographs, and one ground observatory facility. ESA created the solar panels for powering the craft in orbit, and the second ground observatory site. The PPARC provided the four spectrographic detectors. In addition to controlling the satellite, the ground sites acted as typical astronomical observatories, except that instead of using telescopes at their locations, their direct participation was by a link to a telescope orbiting far out in space. IUE was the longest lasting and most productive orbiting astronomical observatory up to its time. It was also the first orbiting ultraviolet observatory available to general users, and the first orbiting astronomical observatory in high Earth orbit. Because ultraviolet light from space is largely absorbed by our atmosphere, observations by IUE provided a whole new range of information not readily available from the ground. Only a small number of high-altitude observatories on Earth can be used with limited effectiveness for ultraviolet studies.

IUE was launched into geosynchronous orbit on January 26, 1978 and remained there until 1996. During these nearly 19 years of operation, it sent to Earth 104,470 images of 9,600 astronomical objects, ranging from comets in the inner solar system to quasars at the edge of the known universe. IUE was the first scientific satellite that allowed "visiting" astronomers to make real-time observations of ultraviolet spectra with a response time of less than one hour. This provided great flexibility in scheduling observation targets for the satellite. In conjunction with the IUE, simultaneous ground-based observations were performed in wavelengths other than ultraviolet in order to provide measurements of the same objects over a wide range of the electromagnetic spectrum. This provided astrophysicists with a new "multi-wavelength" method of looking at objects. The end result was a vast archive of new and more complete information than ever before made available to the scientific community worldwide.

IUE greatly surpassed its expected lifetime and the original science goals set for the mission. These included:

• Obtaining high-resolution spectra of stars of all types in order to determine their physical characteristics. The IUE extended the range of observations available from ground-based observatories into the ultraviolet region.
• Studying streams of gas in and around binary star systems, which are difficult to observe from the ground or with standard optical telescopes even from space.
• Observing faint stars, galaxies, and quasars at low resolution, and comparing these spectra to high-resolution spectra of the same objects.
• Obtaining ultraviolet spectra of planets and comets, again extending our knowledge by looking at them in new ways. Such spectra help determine the composition of the atmospheres of planets and gas content of comets.
• Making repeated observations of objects with spectra that change over time in order to reveal new information about them. The long duration of IUE allowed several long-term studies to be performed on objects in areas never before possible.
• Studying the changes of observed starlight passing through interstellar dust and gas. This can reveal how much and what type of gas and dust exists between Earth and the objects from which the light originated.

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.