One of the greatest successes ever of any orbiting astronomical instrument lies with the IUE (International Ultraviolet Explorer) observatory. Launched on 26 January 1978 for a nominal three-year mission, the observatory operated for 18.7 years yielding more than 110,000 spectra of nearly 10,000 diverse astronomical targets. IUE operations closed down on 30 September 1996. The International Ultraviolet Explorer satellite was a joint project between NASA, ESA and PPARC (formerly SERC in UK). This was a trilateral project, in which NASA provided the spacecraft, telescope, spectrographs and one ground station, ESA the solar panels and the second ground station, and the UK supplied the four spectrograph detectors. In addition to controlling the satellite, the ground stations acted as typical astronomical observatories, except that the telescope was in a high orbit around the Earth. ESA's IUE Observatory was established in 1977 at the Villafranca del Castillo Satellite Tracking Station (VILSPA), Villanueva de la Cañada, Madrid, Spain. The NASA IUE Observatory was located in the Goddard Space Flight Centre (GSFC), in Greenbelt, MD.
The experiment consisted of a 45-cm diameter f/15 Ritchey-Chrétien Cassegrain telescope with an image quality of two arcsec feeding two alternative spectrometers, one for the range 1150-1980Å (short-wave=SW) and the other for 1800-3350Å (long-wave=LW). Two different dispersions were available, low (R=270 at 1500Å and 400 at 2700Å) and high (R=1.8 10 at 1400Å and 1.3 10 at 2600Å, using an echelle arrangement). The echelle observations used 3 arcsec circular apertures; the low dispersion observations used mainly the oval apertures with dimensions 10"20". Each of the two spectrographs was equipped with two cameras, a prime one (P) and a redundant one (R). The sensitivity of IUE, for low dispersion operations, was as follows:
|Camera (spectral range)||Sensitivity|
|SWP (1150Å-1980Å)||2 10 erg s cm Å|
|LWP (1850Å-3350Å)||1 10 erg s cm Å|
|LWR (1850Å-3350Å)||2 10 erg s cm Å|
In addition, two Fine Error Sensors (FES) were incorporated. These transmitted an optical video image of the sky area around the target reflected off of the focal plane, to the controlling ground station. FES-1 was never used in operations. FES-2 was always used for fine guidance throughout the mission. It has also been important as a photometer to measure the optical brightness of the observed sources. In 1991, scattered light entering the telescope required a revision of the guidance procedures and affected the photometric performance of the FES.
The only serious problems encountered with IUE during its operation stemmed from the failures of five of the six gyroscopes in its attitude control system (in the years 1979, 1982, 1982, 1985, and 1996). When the fourth gyroscope failed, IUE continued operations thanks to an innovative reworking of its attitude control system by using the fine sun sensor as a gyro substitute. Even with another gyroscope lost in the last year of operation, IUE could still be stabilized in three-axes with a single gyroscope, by adding star-tracker measurements. During the unexpectedly long operation period of IUE, its NASA operators at GSFC and ESA personnel at VILSPA devised work-around methods to operate, despite unexpectedly high levels of stray light, when a piece of thermal blanket or reflecting tape fluttered in front of the telescope aperture in 1991.
Until October 1995, IUE was in continuous operation, controlled for 16 hours daily from GSFC and for the remaining eight hours from VILSPA. After that, ESA took on a major role to alleviate financial problems of the partners. The operational schemes were completely redesigned and an innovative control system was implemented. With these innovations, it became feasible to perform science operations controlled only from VILSPA. For practical reasons, only 16 hours were used for scientific operations, while the eight hours in the low-quality (high radiation background) part of the orbit were used for spacecraft housekeeping. In February 1996, ESA decided to discontinue the satellite operations. IUE remained operational until 30 September 1996, when its remaining hydrazine fuel was deliberately vented, its batteries were drained, and its transmitter was turned off.