XMM into operational orbit
12 December 1999Over the weekend, the extreme tension of the launch itself disappeared. The XMM satellite is now in the safe hands of the ESOC control teams. In their Main Control Room, XMM has yet to be chalked up on the record board. It is the 42nd mission to be handled by ESOC since 1968.
Since first acquisition of the spacecraft telemetry, XMM is being gradually manoeuvred to reach its definitive operational orbit. Saturday midday, 24 hours after liftoff, the satellite was at its first apogee, the furthest point from Earth.
The large display screens on the wall indicate the precise distance, 113 850 km. A planisphere indicates where the satellite's position in space relative to Earth and the visibility for each ground station. The display shows that Perth is being used and that the satellite is high in the night sky over Australia.
The atmosphere is calm in the Darmstadt control centre. Some operators appear to be so relaxed that an observer might get the impression that nothing is really happening. But no, with a clockwork rigour following their pre-established flight plan, the ESOC flight control teams are preparing for the second of two perigee boost manoeuvres on this first orbit. The first took place two hours earlier.
The spacecraft is behaving perfectly. As an illustration, during its first 24 hours in space XMM had received some 1240 tele-commands and none had been rejected on-board. Amongst these commands were the activation of the small Visual Monitoring Cameras (VMC). Their pictures created great excitement when they were displayed for the first time after being sent back by the spacecraft.
The perigee boosts are crucial if XMM is to reach its definitive orbit. Each time the operation consists in activating the spacecraft thrusters in order to increase the spacecraft's velocity. This additional speed elongates the orbit so that at its closest point to Earth, XMM will pass at a higher altitude. After launch, XMM had been released by Ariane 5 at 826 km. The first boosts raised the perigee to 4900 km.
Three perigee boosts (the first in two parts) are required to raise XMM's perigee to the nominal 7000 km of its operational orbit. A final manoeuvre will then adjust the apogee to precisely 114 000 km.
Brightly coloured schematics and graphs are displayed on the consoles of the Flight Operations Director, and those of the XMM Project team representatives. On these, one can read the temperature and pressure throughout the propulsion system, for example for each of the thrusters as they are firing and the four hydrazine propellant tanks.
Controllers can also obtain a graphic view of what the spacecraft's star trackers are seeing. For example, on this apogee passage, five stars are represented. It is to these points in space that XMM's Attitude and Orbit Control system has locked-on to maintain the spacecraft attitude.
XMM has eight main thrusters in two redundant sets. For each perigee boost, one set is activated, the other remain as a backup. All four thrusters work at the same time, providing not only the increase in velocity but also maintaining XMM's orientation.
The thruster firings last quite long. 53 and 98 minutes on the first orbit, 40 minutes on the second orbit, 55 minutes for the final manoeuvre. Each time the spacecraft's velocity is increased, for example by 87 m/sec on the longest burn. During these manoeuvres, the greater part of XMM's 530 kg of hydrazine will be consumed. For instance 276kg were used on the first orbit burns.
When XMM is at its furthest distance from Earth, it is travelling at some 670 m/sec, 2400 km/hour. Certainly very fast in human terms, but when it returns at its perigee passage, it whizzes past Earth nine times faster!
By Thursday 16 December, after fine tuning the apogee on orbit 4, the spacecraft will have been placed in its operational orbit. The Launch and Early Operations Phase of the mission will be over. XMM will then have a few days of relative calm whilst the ESOC teams enjoy a well-deserved Xmas and New Year holiday.