XMM is launched on top of a Ariane 5 rocket. It will be the first commercial launch of the Ariane 5 rocket. Weighing 3.8 metric tons at launch, XMM is truly a satellite tailored for Ariane 5's heavy-lift launch capabilities.
XMM is an observatory designed to operate in the soft X-rays portion of the electromagnetic spectrum. With a large collecting-area telescope, its scientific instruments will be able to make observations of X-ray sources with an unprecedented sensitivity.
The XMM spacecraft will be placed in a highly eccentric orbit, which provides for long viewing times.

The countdown rehearsal for Flight 119/Ariane 504 was completed on 20 november 1999, providing a complete run-through of the final launch procedure to a point just prior to main engine ignition.

The photo at right shows the Ariane 5 after its final positioning at the ELA-3 Launch Zone. A launch technician at the base of the orange-colored bogie (to the left of ESA logo on the launch table) is verifying that the fixation device is properly installed to lock the table in place.

Overview photo at right provides an excellent panorama of the ELA-3 Launch Zone. The Ariane 5 is in the middle, partially masked by the tower with wind baffles. To the left and right are four pylons that provide lightning protection for the launcher and the ELA-3 facility.

The two large flame ducts that funnel the Ariane 5 solid booster stages' flames/exhaust after ignition and during liftoff are visible at left (behind the tree) and at right (partially hidden by the trees and plants).

This view downward from one of the Final Assembly Building's workstands shows the completed Ariane 5 as it awaits its transfer to the Launch Zone. The main cryogenic stage dominates the left and center of the photo, while one of the solid boosters is at right.