By Jonathan Amos
Science reporter, BBC News
Europe's biggest, most sophisticated spaceship is about to bring its six-month mission to an end by plunging into the Pacific in a ball of flames.
The "Jules Verne" freighter undocked from the Leaders threaten to isolate Fiji ...
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Most of the vehicle is expected to burn up in the atmosphere; only fragments will make it down to the ocean water.
The first of two engine firings to bring the ship down has been completed.
The de-orbit boost lasting six minutes began at 1000 GMT. The second firing will slow the vehicle sufficiently to take it out of the sky. It was timed to begin just before 1300 GMT.
Events are being overseen from the European Space Agency's (Esa) freighter control centre in Toulouse, France.
Mike Steinkopf, the mission director for re-entry, says a "safety zone" has been drawn in the south Pacific some 2,700km long by 200km wide.
"We issue a notification to the air traffic and maritime authorities to make sure there are no planes or boats going through that zone during our re-entry time," he told BBC News.
Astronauts on the overflying International Space Station (ISS) and scientists in two chase planes will take pictures as the disintegrating mass of metal streaks through the morning Pacific darkness.
"Visually, we will see what appears to be a very bright meteor," explained Jason Hatton from the chase team set by Esa and the US space agency (Nasa). "It will start as a point of light with a trail, and then as it comes apart, we will see fragments."
Jules Verne - also known by the generic name Automated Transfer Vehicle (ATV) - cost about 1.3bn euros to develop.
Although Esa has produced many complex scientific satellites, none match the scale of the freighter.
After launch, the space truck can work out where it needs to go in space, and then makes a fully automatic docking once it arrives at its destination.
It was developed as part of Esa's ISS membership agreement, to haul cargo, propellant, water and oxygen to the space station; and also to provide propulsion capacity at the station.
But such has been the performance of Jules Verne that Esa officials and industry chiefs are already talking about upgrading the ship's design - potentially to carry astronauts.
The first step, however, would be to develop technologies that enable the safe return of cargo to Earth.
European space ministers will discuss the issue at their meeting in The Hague in November.
Since its undocking, Jules Verne has been orbiting at about 330km, a slightly lower altitude than the space station.
Taking the freighter out of the sky involves turning it to face the direction of flight so that its rear engines are then facing forward and can be fired to slow the ship's velocity.
The first burn would have put the 13.5-tonne spacecraft on a sharp elliptical orbit.
The second burn - lasting some 15 minutes - should take the ATV on its final trajectory and a steep dive towards the Pacific.
The freighter is expected to be moving at some 7.6km/s as it meets significant atmosphere at 120km. As the plunge continues and temperatures rise, Jules Verne will be torn part.
"We expect the solar panels to break just two-and-a-half-minutes after the entry into the atmosphere; and then we will have fragmentation of the docking adaptor, protective shields and other structural elements," explained Mr Steinkopf.
"Nevertheless, statistically speaking, there will be about 30% of the overall vehicle that may reach the ocean, but only in bits and pieces."
The final moments will be witnessed in the Pacific by two chase planes. The jets, a Gulfstream-V and a DC8, will carry an observation team equipped with a range of spectroscopic imagers and conventional video and stills cameras.
The team wants to establish in detail how the different components of Jules Verne, such as its fuel tanks, come apart.
The information will inform the computer models used by space agencies to plan the safe re-entries of future spacecraft.
"We expect to see signatures associated with the fuel in the wake of the vehicle because you have this trail behind it," Mr Hatton told BBC News from the team's campaign base in Tahiti.
"That's where the spectroscopy comes in. You have a range of different spectrographs all the way from the near-ultraviolet, through the visible to the near-infra-red. In the different colours, we'll probably see what materials are being released from the vehicle, maybe what some of the fragments are - aluminium, for example."
Under the agreement Esa has with its international partners, at least four more ATVs will be flown to the space station in the coming years. The next is due to launch in 2010.
And, ultimately, it is likely that an ATV will be tasked with destroying the space station when the partners have decided the platform is beyond servicing, perhaps towards the end of the next decade.
A freighter will be commanded to drive the whole structure into a similar region of the south Pacific.
Credit: Ciprian Sufitchi (N2YO.com)
Jonathan.Amos-INTERNET@bbc.co.uk
(BBC)
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