XMM Space Telescope, International

The project centres on the development and launch into space of the XMM-Newton, an X-ray telescope. The XMM (X-ray Multi Mirror) will explore the universe for X-rays emitted by spectacular celestial sources such as exploding stars and pulsars, and astronomers hope that it will provide conclusive evidence of the existence of black holes.

DEVELOPING THE SATELLITE

The project sponsor was the European Space Agency (ESA). The prime industrial contractor for the spacecraft is Dornier Satellitensysteme GmbH (DSS, Friedrichshafen, Germany) a member of DaimlerChrysler Aerospace AG (Dasa, Munich). Under DSS's industrial lead, over 35 companies were involved in building the space which required an investment of $200 million (€230 million).

The project was underway in 1996. To achieve the goal of complete satellite development within a period of only 38 months, the construction of two development models and immediate manufacturing of the protoflight model was carefully planned. Within the first twelve months, satellite development with all its subsystems had been completed, allowing development models in the Integration Centre of Dornier Satellitensysteme GmbH to begin in April 1997.

MOVING INTO ORBIT

The launch vehicle was the Ariane 5, which injected the spacecraft into a transfer orbit. After just 29 minutes, XMM was released from the upper stage of the vehicle at a height of 2,350km. After a further minute, the communications signal of XMM was received. The XMM ground crew then sent the signal to unfold the craft's solar array with a 16.1m span and deploy the sunshield.

After a few corrective manoeuvres over the course of 24 hours, the craft was then sent into its final high eccentric orbit (7,000km perigee / 114,000km apogee). On its 48-hour orbit, it rises to nearly one third of the distance to the Moon. At this maximum distance (the apogee), the satellite travels slowly. But at its closest point (the perigee) it passes 7,000km above the Earth and nine times faster.

While in orbit, the telescope tube was emptied of any residual gases ('outgassing'), the sunshield was deployed, and finally the doors of the mirror modules were opened. The telescope is almost 11m in length. It has a diameter of 4m, and a total mass of nearly 4t. It is the largest satellite ever built under ESA's science programme.

DECODING X-RAYS

Similar to the colour of visible light which provides important information on the temperature, composition and dynamics of cosmic objects, X-rays offer a wealth of chemical and physical data over a much wider energy range. XMM has a far better resolution than previous telescopes, which will substantially advance the 'decoding' of X-ray sources. In a single day, XMM will detect more X-rays than the first US X-ray mission did in three years 25 years ago.

XMM-Newton carries three very advanced X-ray telescopes, each of which contain 58 high precision concentric mirrors delicately nested to offer the largest collecting area possible to catch the elusive X-rays. The barrel-shaped mirror consists of two segments, with the first segment formed according to a paraboloid and the second to a hyperboloid mirror.

TESTING PROCEDURES

During its design stages, experts made sure that the XMM spacecraft incorporated a satellite bus as a separate module, so that it can also be used for further European research satellites.

DSS submitted the satellite test and analysis results to its customer, ESA. The tests included a space simulation test (at various temperatures and under vacuum conditions), vibration tests (to simulate the ascent loads during launch) and extensive functional checks were performed.

A thermally and mechanically identical flight model was used for testing the integration procedure and proving the performance data under all load conditions of the satellite during launch and in space. For this to be performed, the spacecraft was transported to ESA's test centre in Noordwijk, Holland in September 1997, as this is the only space simulation facility of such unique dimensions found in Europe.

REPLICATING SPACE

During thermal vacuum tests, the equipment was placed in a vacuum chamber with a sun simulator. This was used to reproduce the intensity of sunlight, which recreated the environment of space with its extreme variations in temperature.

Vibration tests checked the strength of the outer casing and instruments on board. The spacecraft was progressively shaken at different strengths on a 'shaker', which created conditions up to 25% more severe than those expected at lift-off. The spacecraft resonance was also measured to evaluate how its different parts would react to set frequencies, including those that it would probably encounter at launch.

The complete spacecraft was submitted in a reverberating chamber to very intense noise similar to which it would encounter during launch.

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