Ultraviolet Astronomy

Astronomers have developed many different kinds of telescopes besides the familiar optical instruments. Radio, infrared, ultraviolet, x-ray, and gamma-ray telescopes all have unique design requirements to maximize their efficiency in the part of the spectrum they are intended to study.

Like gamma-ray and x–ray telescopes, UV telescopes have only been possible in the era of spaceflight, and the longest lived and most important of these so far has been the International Ultraviolet Explorer. Launched in 1978, IUE was designed to observe the UV sky for five years. Instead, the telescope was not shut down until September 30, 1996, and took tens of thousands of spectra of stars, nebulae, and galaxies.

IUE was a joint project of United States and European space agencies, and was operated for 16 hours each day at the Goddard Space Flight Center in Greenbelt, MD, and for eight hours each day at the Villafranca Satellite Tracking Station in Spain. Astronomers around the world used IUE for their research, and it has been one of the most productive missions in the history of spaceflight.

Despite its glowing track record, IUE had some important limitations. Its primary mirror was only 17 in (45 cm) in diameter, and IUE therefore could not observe very faint objects. Also, its instrumentation was developed in the 1970s and was not as technologically advanced as that available in the 1980s and 1990s. For this reason, a new generation of UV observatories was designed and built.

On June 7, 1992, the Extreme Ultraviolet Explorer (EUVE) was launched. This satellite was designed to extend the spectral coverage of IUE, which only went down to 1,100 Å. The EUVE telescope observed at wavelengths as short as 70 Å, and extended the observing capability of space-based observatories throughout the UV.

On June 24, 1999, the Far Ultraviolet Spectroscopic Explorer (FUSE) was launched. This satellite is also designed to look father into the ultraviolet—i.e., to shorter wavelengths—than IUE, observing at wavelengths from 900-1,200 Å. With FUSE, astronomers will explore conditions in the Universe as they existed only shortly after the big bang, in addition to myriad studies of high-energy processes in stars and galaxies.

These observatories—IUE, EUVE, and FUSE—are what NASA calls "Explorer-class" missions. These are smaller, less ambitious and expensive projects, designed to perform a specific task. This is in contrast to the "Great Observatories" such as the Hubble Space Telescope (HST), which includes a UV instrument called the Goddard High Resolution Spectrograph (GHRS). The GHRS can observe the same part of the spectrum as IUE, but the 8.5 ft (2.6 m) mirror of the HST is much larger than the 16 in (45 cm) mirror of IUE, and GHRS can observe much fainter objects than IUE.