ESA Science & Technology - INFO 36-1997: IUE's treasure-chest of the ultraviolet Universe

IUE was the most long-lived and (by a wide margin) the most productive satellite so far, in the history of space astronomy. After going into orbit on 26 January 1978, as a NASA-ESA-UK project, IUE was meant to operate for three years. More than eighteen years later, IUE still worked 24 hours a day, harvesting new knowledge for astronomers. The last observations were made from ESA's ground station at Villafranca, Spain, on 26 September 1996, and IUE was switched off four days later.

Since then, team members at Villafranca and at NASAs Goddard Space Flight Center have used modern data-processing and information technology to recycle 100 000 ultraviolet spectra of comets, planets, stars, galaxies and quasars, acquired by IUE during its 18.5 years of operations. As a result, the IUE Final Archive is already available on-line via the Internet to hundreds of users who have registered to work with the data. The last few items (about 2 per cent of the total) will be added before the end of November. Also to be presented at the Sevilla conference is ESA's system called INES ("IUE Newly Extracted Spectra") which offers access, selection and distribution of data products, in a thoroughly user-friendly fashion.

The IUE Final Archive is the third massive compendium made available to the worlds astronomers by ESA in 1997. The Hipparcos and Tycho Catalogues, released earlier in the year, give the positions of stars with unprecedented accuracy, thanks to ESA's Hipparcos satellite.

"Space astronomy has set the example in providing a high standard of data quality and making the data accessible to the scientific community through archives", says Roger Bonnet, ESA's Scientific Director. "Now, ground-based observatories are following suit. The data legacy of IUE will be distributed to he community so that research on IUE data can continue long after the end of IUE's lifetime in space".

Wonders of Ultraviolet Spectroscopy

IUE analysed ultraviolet light, in a wavelength range from 1150 to 3200 angstrom units, which is blotted out by the Earths atmosphere.

Operating far above the atmosphere, IUE generated spectra showing intensities at different wavelengths, coming from the selected objects in the sky. To an astrophysicist, such spectra are much more informative than images, about the mechanisms that produce and dissipate the objects energy. Temperatures, motions, magnetism and chemical composition are all discernable in the ultraviolet spectra.

As a result, astronomers have a far better picture of the hot atmospheres of stars than they did before IUE's launch. Even the Sun, a quiet star of moderate size, possesses a very hot atmosphere emitting ultraviolet light, which is now being monitored non-stop by the ESA-NASA solar spacecraft SOHO. Some other stars, ranging from small white dwarfs to large, massive stars, give off ultraviolet emissions from their very hot surfaces. Hot and fierce winds of gas emitted from stars have a profound effect on the lives and environments of the stars, and on any companions caught up in the winds. IUE unmasked the ultraviolet behaviour of a large menagerie of different star types, and astronomers at the Sevilla meeting will discuss profound revisions in astrophysical ideas resulting from the observations.

Other participants will review IUE's contribution to new knowledge about galaxies. These vast assemblies of stars also reveal violent behaviour in ultraviolet light. In a special campaign, a multinational team used IUE to observe the stormy galaxy NGC 5548 some 60 times in eight months.

As a result, they discovered effects of central outbursts spreading from hot regions at the very core of the galaxy to adjacent cooler regions, in a timescale of weeks. In galaxy NGC 7469, observed simultaneously by IUE and by the X-ray satellite Rossi XTE, the timescale shrank to days.

Quasars are erupting galaxies observable at great distances, and their examination by ultraviolet light, by IUE and more recently by the Hubble Space Telescope, give special clues to the nature of the gas in the almost empty spaces between galaxies, and to the manufacture of the chemical elements within the galaxies. The quasar studies to be reviewed at Sevilla already occupy an important place in the efforts to understand the character and evolution of the Universe at large. The ultraviolet data on element-making suggest that massive stars, far bigger than the Sun, were more numerous when the galaxies were young.

Advantages of a Long Life

The sheer durability of IUE enabled astronomers to revisit many objects over nearly two decades and to see changes occurring with them. The prolonged study of the black hole in 3C390.3 was a case in point.

Another conspicuous example of the advantages of a long life concerns Supernova 1987A. This star was seen exploding in a nearby galaxy, the Large Magellanic Cloud, half-way through IUEs operational life.

IUE was the first space telescope to be turned towards Supernova 1987A. It revealed precisely which star had blown up, identified chemical elements in the debris, and discovered a pre-existing ring of gas and dust surrounding the star. IUE continued observing Supernova 1987A at intervals over nine years, so providing a unique chronicle of the early evolution of a supernova remnant. Lesser stellar outbursts called novae have also provided frequent "targets of opportunity" for IUE.

The return of Halleys Comet in 1985-86 was a long anticipated event, and the ultraviolet observations by IUE measured the rate at which the famous object spewed water vapour into space. But many comets appear unexpectedly, and IUE was able to examine them too, from Comet Seargent in 1978 to Comet Hale-Bopp in 1986. Astronomers have built up a comprehensive picture of comets seen by ultraviolet light at different stages of their evolution, and at different distances from the Sun. As a result, they have a much better understanding of how comets react and change during their rare visits to the vicinity of the Sun and the Earth. IUEs long life also enabled it to observe rare and serendipitous events. The satellite was already more than sixteen years old when Comet Shoemaker-Levy 9 hit Jupiter in July 1994. The event was well anticipated, so IUE was able first to study Jupiter in a normal state, and then to see the changes in the ultraviolet spectra during and after the impacts of the comet fragments.

The Astronomers Favourite Satellite

Operational and scientific statistics about IUE are remarkable. The satellites observations have resulted in more than 3600 scientific papers published by 3000 astronomers from 25 countries. In addition, about 500 doctoral theses using IUE data show the educational value of IUE in universities all over the world. Amateur astronomers have also shown a remarkable degree of interest in IUE results, which provide them with a valuable and important link between their activities and those of professional astronomers.

"Although IUE never had the popular appeal of the Hubble Space Telescope, it was always the professional astronomers favourite satellite," remarks Willem Wamsteker, the Dutch astronomer who is ESA's project manager for IUE. "They could visit Villafranca or Goddard and supervise the operations, just as if they were at an observatory on the ground. Towards the end, they could make their observations remotely, without leaving their institutes. Historians of astronomy may well credit IUE with the big change in professional habits which made space observatories a tool, not just for a few hardware-minded specialists, but for all astronomers."

The meeting in Sevilla will end on 14 November with a review of the prospects for new space missions for ultraviolet astronomy. The Hubble Space Telescope and some Space Shuttle missions offer limited opportunities for ultraviolet spectroscopy. So will ESA's XMM X-ray mission. The only confirmed mission fully dedicated to the ultraviolet region is NASA's FUSE spacecraft, due to be launched in the year 2000, with rather limited scope and duration.

Astronomers in developing countries have been persuaded by experience with IUE that they too can participate in spaceborne observations without emigrating. ESA has helped to foster this ambition. A UN-ESA workshop in Sri Lanka in 1996 recommended the creation of a World Space Observatory, and this will be cited at the Sevilla meeting in the context of possible future facilities for ultraviolet astronomy.

Meanwhile IUE will not be easy to replace. No authorized mission yet in sight will combine a wide spectral range with great flexibility of operation. Current astronomical campaigns, ranging from studies of auroras in planets to the efforts to understand the spectacular yet mysterious gamma-ray bursts far away in the cosmos, already miss the input from IUE. The astronomers sense of loss should be seen positively, as a tribute to IUEs achievements when it dominated ultraviolet space astronomy for nearly two decades.