After being released by the launcher, XMM will be placed in a highly eccentric 48-hour orbit. It will rise to a distance of 114,000 km from the Earth before swooping back to within 7,000 km of our planet. Ariane 5 will carry XMM into orbit This orbit offers the best possible contact with ground-tracking stations and allows the satellite to pass rapidly through the Earth's radiation belts, which could harm its delicate science instruments. Above all, it will offer astronomers the longest-possible observation periods. X-rays from space cannot penetrate the Earth's atmosphere and were first measured in the 1950s, by instruments mounted on sounding rockets.
Many astronomers consider the turn of the millennium a golden age for X-ray astronomy.
XMM is set to fly, Nasa's Chandra Observatory is already in space and Japan is set for a fifth X-ray mission early next year. And Europe is already planning a next generation X-ray astrophysics facility, called Xeus. This will use the International Space Station and will allow, for example, the study of the very first black holes.