in the spacecraft making one complete revolution of the Earth approximately every 48 hours.

Immediately after launch, XMM began its Launch and Early Orbit phase of operations. On 17 and 18 December 1999, the X-ray modules and Optical Monitor doors were opened, respectively. Instrument activation started on 4 January 2000, and the Instrument Commissioning phase began on 16 January. The Optical Monitor (OM) attained first light on 5 January, the two European