By BBC News Online's Damian Carrington in Kourou, French Guiana

The X-ray telescope XMM, due to launch on Ariane 5's first commercial flight on Friday, is the most expensive and complex science satellite the European Space Agency (Esa) has ever built.

It is also the largest X-ray telescope ever built.

At stake on the launch pad will be $690m of costs, 15 years of planning and the hard work of many hundreds of scientists and engineers.

One of those who will be watching with fingers crossed is Dr Alex Short, from Leicester University's Space Research Centre and principal investigator for XMM's key detection system.

He says that the demand from astronomers for observing time, when the satellite begins its watch on the Universe is huge: "It is oversubscribed by three times."

Nasa recently launched another X-ray telescope, Chandra, but Dr Short says that the two are complementary: "Chandra has very high spatial resolution but XMM is far more sensitive and is also good for spectroscopy which reveals which chemical elements are involved in the astronomical events viewed."

X-rays are "the screams of pain when matter is being shredded in black holes", according to an Esa spokesperson, but measuring the rays is difficult.

UK input

The heart of the detection system are the charge coupled devices or CCDs, versions of which are found in video cameras. Over 30 CCDs have been specially manufactured for XMM by UK company EEV, at a cost of tens of thousands of pounds each.

XMM's launch will see the number of CCDs in space increase tenfold, says Tony Abbey, an engineer also from Leicester University. He notes that, "the telescope area of XMM is larger than every other X-ray satellite so far added together.

"The collecting area is the size of a tennis court."

Two of the three instruments on XMM have been created by teams led from the UK, with the other being managed from Holland.

But designing and making Esa's greatest engineering challenge to date has involved universities and companies all over Europe.

Y2K problems

After launch, XMM will remain in safe mode until 4 January, to avoid any possible millennium bug problems, according to Dr Short: "There will be no problem with XMM but if any communications systems fail while the satellite is not in safe mode, there could be a problem."

After a commissioning period, XMM will start examining the Universe in March.

Its unusual, highly elliptical orbit takes it from 7,000 km at its nearest point to Earth to 140,000 km at its furthest. This allows the maximum time away from the glare of the Earth.

The data will be transmitted to stations in Perth, Australia and Villafranca in Spain.

XMM is expected to operate for a minimum of two years, but could last as long as 10, if the hydrazine fuel on board is not exhausted by having to adjust its orbit.