INFO 02-1998: ESA unveils its big XMM spacecraft
10 February 1998Looking like a black pillar more than 10 metres tall, Europe's largest scientific spacecraft now stands in a test bay at ESTEC, ESA's space and technology centre at Noordwijk in the Netherlands where journalists and cameramen have a unique opportunity to view the XMM satellite at close quarters.
XMM, the X-ray Multi-Mirror mission, is due do be lanched in 1999. It is a European conception with innovative telescopes. XMM will revolutionize the study of X-rays coming from the Universe, by harvesting far more X-rays per hour than any previous mission. Its enormous capacity will enable astronomers to analyse many strong sources of cosmic X-rays very quickly, and to discover and characterize many faint sources previously beyond their reach.
As the most popular and competitive branch of space astronomy, X-ray astronomy reveals special places in the Universe where very high temperatures or violent forces generate energetic radiation. These sources include black holes, exploding stars, paris of stars orbiting very close together, and the central region of clusters of galaxies.
XMM's optical monitor, viewing the scenes by visible light, will help in the interpretations. The combination of X-ray telescopes and optical monitoring should be well-suited to tracking down gamma-ray bursters - extraordinary explosions in space that mystify the astronomers.
Full descriptions of the X-ray sources will depend on precise spectral analysis of the relative intensities of X-rays of different energies, including the signatures of identifiable chemical elements. Such spectral analysis is XMM's task, using instruments of the highest quality fed by the remarkable telescopes.
As seen at ESTEC today, the spacecraft stands upside down. Its front end, where the mirror modules of the X-ray telescopes pass through the satellite's service module, is closest to the ground. At the top is the section containing detectors at the focus of the X-ray telescopes. Surmounting the assembly, a pair of cones will carry heat away from the detectors. XMM's appearance is, though, dominated by the long tube that spans the telescope's focal length, and by the black thermal blanket that will protect the spacecraft from unequal heating on the sunny and shaded sides.
A miracle of telescope engineering
"You have to imagine the big tube of XMM filled with focused X-rays en route to the detectors", says Robert Lainé, ESA's project manager for XMM, "That is the whole purpose of the mission, and our chief preoccupation has been with the three multi-mirror modules that accomplish it. Critics thought we were too ambitious, trying to nest 58 precisely formed mirrors together in each module. No one had ever attempted such a feat before. It was not easy, but thanks to excellent innovative work by European industry, XMM's telescopes are even better than we hoped."
X-rays are focused by glancing them off a carefully shaped mirror, like a bucket without a bottom. In a single-mirror telescope, most of the incoming X-rays miss the mirror. To catch more of them, designers nest multiple mirrors inside one another. Before XMM, astronomers had to choose between many mirrors with relatively poor focusing, or a very few mirrors with a sharp focus. With 58 precision-made mirrors in each of its three X-ray telescopes, XMM combines enormous gathering power with accurate focusing.
Carl Zeiss in Germany made shaped and polished mandrels (moulds) for mirrors of 58 different diameters, up to 70 cm for the widest. Media Lario in Italy made the mirrors by electrodeposition of nickel on the mandrels, coated their inner surfaces with gold, and carefully assembled them in their nested configuration, in a framework fabricated by APCO in Switzerland.
The performance of each XMM mirror module has been verified in special facilities of the Centre Spatial de Liège in Belgium and the Max-Planck Institut für extraterrestriche Physik in Germany. The first flight model conformed with the specification, and the second and third were even better.
Some facts about XMM
The total surface area of the extremely thin mirror that gathers X-rays in XMM's three multi-mirror telescopes (taken together) is larger than 200 m2.
Two of the three X-ray telescopes are fitted with reflection grating spectrometers for the most detailed analysis of the X-ray energies.
XMM is designed to fit in the fairing of Europe's new Ariane 5 launcher.
Some 46 companies in 14 European countries (and 1 in the USA) have contributed to XMM's construction under the prime contractorship of Dornier Satellitensysteme. The investigators responsible for the instruments in XMM come from the Netherlands and the UK, with investigators in Belgium, France, Germany, Italy, Switzerland and the USA.
XMM will spend most of its time south of the Earth, travelling quite slowly out to distances of more than 100,000 kilometres, well clear of the Earth's radiation belts.
XMM will be controlled by ESA's satellite operations centre (ESOC) from Darmstadt (Germany) and Villafranca (Spain) via ground stations in Perth (Australia) and Kourou (French Guiana).