F
R

2
-
2
0
0
0

ARIANE 5 SUCCESSFULLY LAUNCHES XMM TELESCOPE

The effort was worth it. When Ariane 5 was launched from the European space station in Kourou on 10 December 1999 at 15:32hrs Central European time and successfully put the XMM (X-ray Multi Mirror) in its designated orbit, the last sceptics went still. After a qualification phase of three flights, the first of which in June of 1996 ended with the loss of the rocket and four satellites, this fourth flight (504) went very smoothly.

Seven seconds after firing the Vulcain engine of the main stage the two solid fuel boosters were ignited. After ascending vertically for five seconds, Ariane 5 turned slightly into a north-easterly direction. About two minutes later the boosters burnt out and were jettisoned. After a further minute the pay fairing was jettisoned as well.

Since the rocket's entire flight path pointed towards the sun, it was rolled into a position, which guaranteed that certain sensitive instruments of the XMM Satellite, which weighs almost four tons and is more than ten meters long, were protected from over heating.

Almost ten minutes into the flight the cryogenic main stage was switched off, and a few seconds later the upper stage was fired. Just under 29min after lift-off of the new European heavy launcher Ariane 5 put the XMM into its designated orbit. According to ESA the X-ray telescope was delivered within 1.5km of the target altitudes. The perigee of the first orbit was at 825.6km, the apogee at 113,946km.

Because the satellite was injected into orbit with such a high precision, it will need markedly less fuel to reach its final orbit than was originally anticipated. XMM project manager Robert Lainè was full of optimism after the launch and stated that the XMM might carry out scientific observations up to 20 years instead of the expected ten years if everything continued to go smoothly.

The X-ray observatory is intended to pick up x-rays emitted from spectacular celestial bodies, like exploding stars, pulsars or black holes. In order to do this the satellite is fitted with three Wolter-type telescopes with 174 bundled and gold plated mirrors, which allow a glancing incidence of X-rays. The radiation is led on highly sensitive CCDs (charged coupled devices) to generate pictures. Some of the X-rays are also analysed spectroscopically.

XMM is supposed to reach its operational orbit by the end of 1999. By repeatedly firing its orbital maneuvering engines (four of these correcting manoeuvres are planned), the satellite's perigee is raised to about 7,000km. At its furthest point the X-ray observatory is one third of the distance between Earth and Moon away on its highly elliptical orbit. At 114,000km, its highest point, XMM is at its slowest speed. When passing earth at the closest point of the orbit, its speed in nine times that high.

This orbit has been chosen for two reasons. The XMM instruments only work satisfactorily outside the Van-Allen radiation belt, which surrounds earth at an altitude of about 40,000km. This belt is filled with fast particles and acts as an electromagnetic fog, which can damage the telescope's sensitive imaging sensors.

The highly eccentric orbit allows astronomers a maximum observation time. One orbit of the XMM takes exactly 48 hours. During this time span the satellite is uninterruptedly at the viewers' disposal for 40hrs. Single X-ray sources can be observed for 12hrs at a time. An XMM team member claims that the alignment of XMM can be arranged with as much precision and stability as if someone on earth was observing a melon at a distance of 150km through a (handheld) telescope without experiencing the least bit of jittter.

While XMM passes through the radiation belt, which lasts eight hours, including a two-hour period of radio shadowing behind the Earth, the scientific payload of the satellite is switched off. While this happens, filter wheels protect the sensors from being hit with fast particles.

Following a fixed schedule the satellite is brought into a safe attitude during this phase. The command for this procedure is given at the XMM Mission Control Centre with the ESOC (ESA's Control Centre) in Darmstadt, Germany. For each of these "dormant phases" fixed and easily visible points in the sky are determined, which the satellite's star tracker are supposed to follow autonomously. XMM's attitude control system is designed to run the satellite autonomously for up to 36hrs.

The orbit of the up to date biggest satellite, which was commissioned by the European Space Agency ESA, (the responsible industry contractor being the German Dornier Satellitensysteme), has an orbital inclination of 40 degrees. The apogee lies in the southern hemisphere. Two ground stations - one in Kourou and one in Perth, Australia, - guarantee the satellite's contact with the ESOC in Germany. The two ground stations also receive the scientific data, which are then forwarded to the XMM science centre in Villafranca, Spain, in real time.

XMM has sent first pictures of itself to Earth only five hours after the launch. The photos were taken by two micro cameras, which are positioned at the outside of the satellite's instrument platform. The pictures, which were taken along the satellite's longitudinal axis, show that the solar panels have opened as planned. The next planned procedures are the so-called out-gassing (the telescopes' ventilation of possible remaining gases), the unfolding of the sun protection shield and the opening of the three mirror doors.

During the next two months a first calibration of the observatory and its scientific instruments will be carried out. Once this has been completed the performance of the three telescopes and experiments can be verified, after which the satellite's scientific mission can begin. XMM's first spectacular X-ray pictures are expected for the spring of 2000.

The European X-ray Observatory will then add to the observations of the American Chandra telescope, which has been orbiting since the middle of 1999. There is plenty of work for both satellites, for which joint calibration observations are planned in the second half of the year. The two telescopes complement each other well, because XMM has a higher collection capacity and Chandra can recognise finer detail.

XMM and Chandra are today state-of the-art in X-ray telescopy. A markedly improved performance can hardly be achieved with a single telescope. One of the reasons is that with the use of bigger mirrors (for a bigger collective performance) the focal length would have to become bigger. The two new super telescopes with their focal length of about 7.5 meters and a corresponding construction length of more than ten meters have meant a big effort during construction, ground testing and especially during the launches with Ariane 5 and the Space Shuttle respectively.

The next logical step for X-ray astronomy would be to distribute observation over several telescopes. A program thought up by NASA is the so-called Constellation-X. It consists of four identical telescopes, which fly precisely co-ordinated in a formation about 100 meters apart. It is planned to station this telescope formation 2.4 millon km from earth at the so-called "outer Langrangin Point". Here, the satellites' orbit would not be influenced by the Earth's or the Sun's gravitational forces.

Another possibility is being considered in Europe: the so-called XEUS observatory (X-ray Evolving Universe Spectroscopy) could be launched as part of the utili program of the International Space Station. Instead of spreading the collective performance onto several telescopes (c p Constellation-X), XEUS consists of two separate satellites. One of these carries the mirror module (Mirror Spacecraft, MSC) and the second the depicting instruments (Detector Spacecraft, DSC).

Both spacecraft would be launched with an Ariane 5 into a low orbit and would be stationed near the space station. Once positioned both modules would fly in a precise formation and form a telescope with a focal length of up to 50 meters. There is one distinct advantage to this set-up: Both satellites are equipped with a docking mechanism and a complex attitude and manoeuvring system. During the course of the mission they will be able to dock with the space station, allowing to enlarge the effective mirror collection area from 6 square meters up to 30 square meters. (By comparison XMM has an effective mirror collection area of 0.4 square meters.) It is also planned to replace the DSC during an ISS visit with a new satellite. The drawback of a stationing in low orbit is a much reduced observation time because of the short orbits (90 minutes).

The mirror module of the XEUS Observatory does not consist of complete tubular elements like the XMM or Chandra but because of the initial diameter of 4.5 meters (10 meters in the final assembly stage) of five segments. However, the Wolter mirror design is being maintained. As far as the mirror technology is concerned, it is planned to utilise the good experiences gained from building the XMM. ESA has undertaken an internal feasibly study to evaluate the revolutionary mirror technology. This led the European Space Agency to commission Carl Zeiss in Germany to manufacture a mirror mandrel and Media Lario in Italy to build a demonstrator mirror support structure (petal) at a scale of 1:3. These companies were already responsible for the manufacturing of the mirror systems used on the XMM, from which astronomers are expecting first spectacular images in the spring of 2000-

From page 52 of FLUG REVUE 2/2000