astronomical research satellite built in the 1970s as a cooperative project of the U.S. National Aeronautics and Space Administration (NASA), the Science and Engineering Research Council of the United Kingdom, and the European Space Agency (ESA). Launched in January 1978, the IUE functioned until it was shut down on September 30, 1996, and was one of the most productive astronomical instruments in history.

The cylindrical satellite measured 4.2 metres (13.8 feet) in length and weighed 644 kg (1,420 pounds) on launch. In addition to telemetry units, a computer, batteries, and a pair of solar panels for power, the IUE carried a 45-cm (17.7-inch) reflecting telescope equipped with two spectrographs linked to television cameras. The spectrographs covered a range of ultraviolet wavelengths that are prevented from reaching the ground by the Earth's atmosphere.

The IUE observed space from geosynchronous orbit and was operated from ground stations at ESA's Villafranca del Castillo Satellite Tracking Station near Madrid (8 hours per day) and NASA's Goddard Space Flight Center in Maryland (16 hours per day). A primary goal in designing the IUE was to provide a satellite observatory that astronomers could use in the same manner as they might use a modern ground-based telescope—i.e., by directing observations in real time and inspecting data as it was collected. Among its strengths was the ability to respond quickly to transient targets such as comets and supernovas. Astronomers accessed the satellite through a competitive peer-review process that encouraged collaboration and time-sharing. Over its operational lifetime, thousands of astronomers used the IUE, publishing more than 2,500 scientific papers and collecting more than 100,000 spectroscopic images.

IUE observations contributed to knowledge of planetary atmospheres, the composition of comets, magnetic fields surrounding stars, stellar winds and other aspects of the stellar environment, the composition of planetary nebulae, the existence of hot galactic halos, physical conditions in the nuclei of active galaxies, and the nature of supernovas—most notably the rapidly changing ultraviolet spectrum of Supernova 1987A, which became the focus of the IUE's attention only hours after its discovery in the Large Magellanic Cloud in February 1987.

The IUE operated for almost 19 years, far beyond its expected three-year lifetime. Although it suffered minor mechanical and optical problems, it remained fully operational because project engineers were able to adapt its control systems to function under reduced capabilities. By the time it ceased scientific operations, an entire generation of astronomers had enjoyed access to the ultraviolet sky. All of the data collected by the IUE is preserved for use through archives established by the participating agencies.