ESA's X-ray observatory, continues its quest for the unknown. This month, after five years of operations, the mission saw the publication of its 1000th scientific paper, corresponding to an equivalent number of results, in top-class scientific journals. This is not the only record-breaking figure for this X-ray 'hunter' mission.

There are several ways to measure the scientific success of a mission. One is certainly to look at the use the scientific community makes of the data obtained by the mission, and at the number, novelty and significance of the results so produced.

From the very beginning of its operations in early 2000, hundreds of researchers all around the world have been subscribing to 'book' observing time with XMM-Newton, eager to obtain data and new clues about the hidden and powerful phenomena taking place in the Universe - black holes, birth and death of stars, active galactic nuclei.

Each of the five calls for observation proposals issued so far by ESA towards the scientific community, resulted in a subscription exceeding by seven times the observing slots available. More than 1600 astronomers, an estimated 20 per cent of the world-wide community, had participated to provide their ideas for using XMM-Newton to target highly energetic, exotic and still mysterious space objects.

Scientific results based on XMM-Newton data are now being published at a steady rate of almost 300 papers per year, comparable to the famous Hubble Space Telescope.

Why this huge interest in XMM-Newton? What gives the mission such a world-class profile?.

The fact is that XMM-Newton's capabilities are unprecedented and unique, with optics that are a masterpiece of engineering. Each of its three X-ray telescopes is made of 58 co-axial cylindrical mirrors, capable of reflecting X-rays coming from numerous cosmic sources onto the spacecraft special detectors. This is enabling astronomers to discover in one day more than any other X-ray mission has discovered over weeks of operations.