International Ultraviolet Explorer (IUE or Explorer 57, formerly SAS-D) was the first space observatory primarily designed to take ultraviolet (UV) electromagnetic spectrum. The satellite was a collaborative project between NASA, the United Kingdom's Science and Engineering Research Council (SERC, formerly UKSRC) and the European Space Agency (ESA), formerly European Space Research Organisation (ESRO). The mission was first proposed in early 1964, by a group of scientists in the United Kingdom, and was launched on 26 January 1978, 17:36:00 UTC aboard a NASA Thor-Delta 2914 launch vehicle. The mission lifetime was initially set for 3 years, but in the end, it lasted 18 years, with the satellite being shut down in 1996. The switch-off occurred for financial reasons, while the telescope was still functioning at near original efficiency.

It was the first space observatory to be operated in real-time by astronomers who visited the ground stations in the United States and Spain. Astronomers made over 104,000 observations using the IUE, of objects ranging from Solar System bodies to distant quasars. Among the significant scientific results from IUE data were the first large-scale studies of stellar winds, accurate measurements of the way interstellar dust absorbs light, and measurements of the supernova SN 1987A which showed that it defied stellar evolution theories as they then stood. When the mission ended, it was considered the most successful astronomical satellite ever.

The human eye can perceive light with wavelengths between roughly 350 (violet) and 700 (red) nanometres. Ultraviolet light has wavelengths between roughly 10 nm and 350 nm. UV light can be harmful to human beings and is strongly absorbed by the ozone layer. This makes it impossible to observe UV emission from astronomical objects from the ground. Many types of objects emit copious quantities of UV radiation, though: the hottest and most massive stars in the universe can have surface temperatures high enough that the vast majority of their light is emitted in the UV. Active Galactic Nuclei, accretion disks, and supernovae all emit UV radiation strongly, and many chemical elements have strong absorption lines in the UV so that UV absorption by the interstellar medium provides a powerful tool for studying its composition.

Ultraviolet astronomy was impossible before the Space Age, and some of the first space telescopes were UV telescopes designed to observe this previously inaccessible region of the electromagnetic spectrum. One particular success was the second Orbiting Astronomical Observatory (OAO-2), which had a number of 20 cm (7.9 in) UV telescopes on board. It was launched in 1968 and took the first UV observations of 1200 objects, mostly stars. The success of OAO-2 motivated astronomers to consider larger missions.

The orbiting ultraviolet satellite which ultimately became the IUE mission was first proposed in 1964 by British astronomer Robert Wilson. The European Space Research Organisation (ESRO) was planning a Large Astronomical Satellite (LAS), and had sought proposals from the astronomical community for its aims and design. Wilson headed a British team which proposed an ultraviolet spectrograph, and their design was recommended for acceptance in 1966.

However, management problems and cost overruns led to the cancellation of the LAS program in 1968. Wilson's team scaled down their plans and submitted a more modest proposal to ESRO, but this was not selected as the Cosmic Ray satellite was given precedence. Rather than give up on the idea of an orbiting UV telescope, they instead sent their plans to NASA astronomer Leo Goldberg, and in 1973 the plans were approved. The proposed telescope was renamed the International Ultraviolet Explorer.

The telescope was designed from the start to be operated in real-time, rather than by remote control. This required that it would be launched into a geosynchronous orbit – that is, one with a period equal to one sidereal day of 23 h 56 m. A satellite in such an orbit remains visible from a given point on the Earth's surface for many hours at a time, and can thus transmit to a single ground station for a long period of time. Most space observatories in Earth orbit, such as the Hubble Space Telescope, are in a low Earth orbit in which they spend most of their time operating autonomously because only a small fraction of the Earth's surface can see them at a given time. Hubble, for example, orbits the Earth at an altitude of approximately 600 km (370 mi), while a geosynchronous orbit has an average altitude of 36,000 km (22,000 mi).

As well as allowing continuous communications with ground stations, a geosynchronous orbit also allows a larger portion of the sky to be viewed continuously. Because the distance from Earth is greater, the Earth occupies a much smaller portion of the sky as seen from the satellite than it does from low Earth orbit.