By Helen Briggs
BBC News Online
The British spacecraft Beagle 2 is about to begin the final leg of its journey to the surface of Mars.
Beagle is about to descend to the Red Planet
On Friday, it will be released from its "mothership", Mars Express, to travel the last three million kilometres to a rocky plain on the Red Planet.
Beagle 2 is expected to land on Mars on Christmas Day, when it will search for signs of life, past or present.
At the same time, Mars Express, Europe's first solo mission to another world, will go into orbit around Mars.
It will use a suite of scientific instruments to seek out water, ice and key chemicals under the Martian surface.
Friday's events are crucial to the success of the whole mission.
"When everything is go, the lander has to be pointed in the right direction", said Roland Trautner, Beagle 2 project manager at the European Space Agency (Esa), "and then at the right time it has to be ejected at a certain speed so it arrives at the right point in the Martian atmosphere."
'Hands of the Gods'
Engineers at the European Space Operations Centre in Darmstadt, Germany, have been carrying out final checks on both spacecraft ahead of the manoeuvre.
Mars Express is being eased into the correct position to send Beagle on the way to its destination.
A team at LogicaCMG in Surrey built the software for cruise, descent and landing
Beagle 2 is attached to Mars Express by what is known as a Spin-Up and Ejection Mechanism (Suem).
When the command is sent to the main spacecraft to release Beagle 2, a pyrotechnic device will fire and trigger a compressed spring.
Beagle 2 will spin away on a path that should pop it into the top of the Martian atmosphere.
The lander has no propulsion system of its own for the last leg of the journey. It is "in the hands of Mr Newton" as a US space agency (Nasa) astronaut once put it.
"To use a topical analogy, it's like throwing a rugby ball," said Beagle's creator, Professor Colin Pillinger. "You're in the hands of the Gods."
If the ejection procedure fails, Mars Express will be crippled. Beagle 2 will have to ride piggyback for the rest of the mission, preventing the main spacecraft from getting into the desired orbit for experiments.
Steve Burnage, head of technology at the UK defence firm Insys, which made the Suem, said the ejection system had been tested many times.
Airbags will cushion the landing (Image: All rights reserved Beagle 2)
"We can't do any more than we have done," he said. "We can't see anything going wrong with it."
Mission controllers will find out within a few hours whether ejection has been successful.
The stereo cameras on Mars Express will take an image of Beagle 2 as it glides away into the distance.
It will follow close behind Beagle 2 for the next six days. The lander should reach the edge of the Martian atmosphere in the early hours of Christmas Day.
The next few minutes are the most dangerous of all. Beagle 2 will plunge towards the crater of Isidis Planitia slowed by a heat-resistant shield and parachutes, and cushioned by airbags.
Meanwhile, Mars Express will fire its rockets to blast itself into orbit around the Red Planet.
Any Mars mission is a risky business. Since the 1960s, more than 30 Russian and US spacecraft have set off to explore Mars.
About two-thirds of these have failed, either on launch, landing or during the cruise to Mars.
Beagle only has enough battery power to survive a day
Only three missions have landed successfully to send back significant amounts of data: America's Viking 1 and Viking 2, which landed in 1976, and its Mars Pathfinder, which explored the surface in 1997.
Beagle 2 is using a simplified version of the Mars Pathfinder landing technology. It has parachutes and airbags but no rockets.
"Beagle has been designed and built very quickly on a very limited budget," said Jonathan Gebbie of LogicaCMG, the company that developed the software for the Beagle 2 landing.
"There are many things that could go wrong. I think the best estimates are that it has a better than evens chance of landing successfully but more than that I'm not sure we can really say."