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.

The orbiting ultraviolet satellite which ultimately became the IUE mission was first proposed in 1964 by British astronomer Robert Wilson.

[5] 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.

This meant that the telescope had to be relatively small, with a 45 cm (18 in) primary mirror, and a total weight of 312 kg (688 lb).

ESA provided the solar arrays to power the spacecraft as well as a ground observing facility in Villafranca del Castillo, Spain.

NASA contributed the telescope, spectrograph, and spacecraft as well as launching facilities and a second ground observatory in Greenbelt, Maryland at the Goddard Space Flight Center (GSFC).

According to the agreement setting up the project the observing time would be divided between the contributing agencies with 2/3 to NASA, 1/6 to ESA and 1/6 to the UK's SERC.

The telescope was designed to give high-quality images over a 16 arcminute field of view (about half the apparent diameter of the Sun or Moon).

The primary mirror was made of beryllium, and the secondary of fused silica – materials chosen for their light weight, moderate cost, and optical quality.

The instrumentation on board consisted of the Fine Error Sensors (FES), which were used for pointing and guiding the telescope, a high-resolution and a low-resolution spectrograph, and four detectors.

There were two Fine Error Sensors (FES), and their first purpose was to image the field of view of the telescope in visible light.

This was a caesium-tellurium cathode, which was inert when exposed to visible light, but which gave off electrons when struck by UV photons due to the photoelectric effect.

Firstly, as soon as its instruments were switched on, the IUE observed a small number of high-priority objects, to ensure that some data had been taken in the event of an early failure.

If an astronomer was awarded time, then when their observations were scheduled, they would travel to the ground stations which operated the satellite, so that they could see and evaluate their data as it was taken.

This mode of operation was very different from most space facilities, for which data is taken with no real-time input from the astronomer concerned, and instead resembled the use of ground-based telescopes.

[19] Because of its elliptical orbit, the spacecraft spent part of each day in the Van Allen radiation belts, during which time science observations suffered from higher background noise.

[25] Halley's Comet reached perihelion in 1986, and was observed intensively with the IUE, as well as with a large number of other ground-based and satellite missions.

An example of this kind of system is Sirius, and at visible wavelengths, the main sequence star is far brighter than the white dwarf.

However, in the UV, the white dwarf can be as bright or brighter, as its higher temperature means it emits most of its radiation at these shorter wavelengths.

These stars exist for a few million years, and during this time the stellar wind carries away a significant fraction of their mass and plays a crucial role in determining whether they explode as supernova or not.

Designated SN 1987A, this event was of enormous importance to astronomy, as it was the closest known supernova to Earth, and the first visible to the naked eye, since Kepler's star in 1604 – before the invention of the telescope.

[34] The hot gas, heated by cosmic rays and supernova, extends several thousand light years above and below the plane of the Milky Way.

IUE observations were used to study the black hole at the centre of the galaxy, with its mass being estimated at between 50 and 100 million times that of the Sun.

Before IUE, observations of this so-called Lyman-alpha forest were limited to very distant quasars, for which the redshift caused by the expansion of the universe brought it into optical wavelengths.

[40] This experiment included the ultraviolet spectrograph package carried by the IUE, consisting of two physically distinct echelle-spectrograph/camera units capable of astronomical observations.

As the SEC Vidicons could integrate the signal for up to many hours, data with a signal-to-noise ratio of 50 could be obtained for B0 stars of 9th and 14th magnitudes in the high- and low-resolution modes, respectively.

[21] In 1995, budget concerns at NASA almost led to the termination of the mission, but instead the operations responsibilities were redivided, with ESA taking control for 16 hours a day, and GSFC for the remaining 8 only.

[48] The IUE mission, by virtue of its very long duration and the fact that for most of its lifetime, it provided astronomers only access to UV light, had a major impact on astronomy.

[3] For many years after the end of the mission, its archive was the most heavily used dataset in astronomy, and IUE data has been used in over 250 PhD projects worldwide.