Looking 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 to be launched 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, pairs of stars orbiting
very close together, and the central regions 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 telescopes' 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 Laine ESA's project manager for
XMM. "That's 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 wasn't 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 high accuracy structure
fabricated by APCO in Switzerland.
The performance of each XMM mirror module has been verified in special
facilities of the Centre Spatiale de Liege in Belgium and the
Max-Planck-Institut fur extraterrestriche Physik in Germany. The first
flight model conformed with the specification, and the second and third
were even better.
XMM Description
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
120 meter squared.
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. 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).
