European XMM will be friendly rival to Chandra
BY PETER BOND
FOR SPACEFLIGHT NOW
Posted: Dec. 9, 1999
The European Space Agency's X-ray Multi-Mirror (XMM) satellite is currently scheduled for launch from Kourou space center in French Guiana on Friday. The 3.8 tonne XMM is the largest scientific satellite ever built and launched by Europe. It will also be the first commercial payload to be carried aloft by Europe's heavy-lift Ariane 5 rocket.
XMM is designed to explore the X-ray sky, which is invisible to ground-based telescopes, with unprecedented sensitivity. Instead of the 120,000 X-ray objects visible to its predecessor, ROSAT, XMM will detect more than a million celestial X-ray sources. Its three main scientific instruments will enable astronomers to investigate the hottest objects and most violent events in the Universe.
To do this, the giant observatory is equipped with three X-ray telescopes, each composed of 58 wafer-thin conical mirrors which nest inside each other like Russian dolls. With a total mirror surface area of 120 sq. metres - almost as large as a tennis court - XMM has the unparalleled ability to capture some 60 percent of all the incoming X-rays.
Since X-rays pass straight through ordinary mirrors, special gold-plated mirrors have been developed to deflect the cosmic X-ray photons at a shallow angle, like stones skipping across a pond. The 174 mirrors are so smooth that, if they were enlarged to the size of the Atlantic Ocean, the largest ripple visible would be no more than 1 mm high. This incredible precision means that, when the X-rays skim off the mirrors, their direction of travel is altered just enough so that they can be focused on the scientific instruments at the far end of a 10-metre-long tube.
All three of the scientific instruments on board XMM have UK involvement. At the prime focus of each of the telescopes are three European Photon Imaging Cameras (EPIC) which have been built under the leadership of Martin Turner from the University of Leicester. With the aid of highly advanced silicon chips developed by Leicester and UK company EEV Ltd., together with coolers built by Birmingham University and an attitude control system built by Matra Marconi Space, the cameras can seek out extremely weak X-ray sources and detect rapid variations in X-ray intensity.
There are also two Reflection Grating Spectrometers which analyse about half of the incoming X-rays from two mirror modules. These 'fan out' the various wavelengths (much like a prism with visible light) to enable astronomers to recognise the presence of individual elements such as oxygen and iron. One of the Co-Investigators on RGS is Graziella Branduardi-Raymont from MSSL.
"RGS will enable XMM to do long-distance chemistry," said Dr. Turner. "It will study the creation and redistribution of chemical elements in the Universe."
The third main payload is an optical monitor (OM), a 30 cm (12 inch) Cassegrain telescope which will provide images of the sky at visible and ultraviolet wavelengths. Designed and built by a consortium led by MSSL, this instrument will be a vital aid in locating and identifying new X-ray sources.
"Despite its small size, its location above the atmosphere means that the OM will be able to give 0.7 arcsecond resolution," said Keith Mason, principal investigator for the instrument. "For the first time we will be able to obtain simultaneous observations of the same area of sky at wavelengths ranging from X-ray to visible."
XMM is also equipped with a particle detector known as the EPIC Radiation Monitor System. ESA scientists believe that their satellite will be less vulnerable than NASA's Chandra to detector degradation caused by protons trapped in Earth's radiation belts, but they would like to monitor levels of radiation which can affect the sensitive CCDs of the main payload.
Fortunately, XMM will spend very little time inside the Earth's radiation belts. Ariane 5 will insert the observatory into an elliptical orbit which is inclined at 40 degrees to the equator and ranges between 7,000 km and 114,000 km above the planet. The instruments will be shut down for eight hours during its passage through the radiation belts, but, for the remaining 40 hours of each orbit XMM will be free to conduct its groundbreaking observations of the X-ray sky.
The avalanche of science data returned by the observatory will be transmitted to the operations centres in Darmstadt, Germany, and VILSPA near Madrid, before being passed to the XMM Science Survey Centre at Leicester University. There, the raw data will be processed into calibrated images and spectra for distribution to the XMM observers and for archiving.
"For once, NASA went for delicacy, while Europe aimed for power," he said. "Its large mirrors make XMM five or six times more sensitive than Chandra. However, Chandra's smaller mirrors have a sharper view."
The ESA satellite also has the edge in cost.
"The cost of XMM is about half that of Chandra," he explained. XMM will cost Europe about £500 million to the end of its life. The UK share is £75 million, paid by PPARC, mostly over the project's six year construction period. In return, UK astronomers will receive half of the guaranteed observing time."
However, collaboration rather than competition will be the objective as the two pioneering observatories open up a new golden age of high energy astrophysics.
"There is a co-operative sharing agreement between ESA and NASA on the use of XMM and Chandra, because they do complementary things," said Murdin. "NASA has also been very helpful in providing data on the reasons for the degradation of the detectors on Chandra."
If all goes well, for the next 8-9 years, XMM should send back a stream of data which will revolutionise astronomy. Dr. Murdin summarised the contribution XMM is expected to make.
"XMM will use the Universe as nature's laboratory of phenomena impossible to create on Earth in black holes and elsewhere. It will detect millions of X-ray 'stars'. Because it sees such faint ones, it will see far back in time by studying the most distant objects in the Universe. Because it sees so many X-ray stars, it will find the rare types - nature's freaks or its brief experiments. XMM will map the effects of the dark matter that dominates the Universe by studying its mysterious effects."
Flight data file
Vehicle: Ariane 504
Payload: X-ray Multi-Mirror satellite
Launch date: Dec. 10, 1999
Launch window: 1432-1508 GMT (0932-1008 EST)
Launch site: ELA-3, Kourou, French Guina
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