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Last Updated: Monday, 22 December, 2003, 17:00 GMT
Beagle probe faces its big challenge
By Dr David Whitehouse
BBC News Online science editor

Never has a spacecraft been built so quickly, on so little money, and been sent on such a long journey fraught with so many dangers.

Beagle chute, Esa
The chute is made from a special nylon to save mass
Beagle 2 has been carried to the vicinity of Mars by the Mars Express mothership, and released successfully to go its own way for the final leg of the journey.

The easy part is over.

Beagle's atmospheric entry, descent and landing on Mars on Christmas Day will be the most worrying six minutes in the history of unmanned space exploration.

It is during that time, after it strikes the upper region of Mars' atmosphere at 20,000 kilometres per hour, that engineers hope that the speed at which it was designed and built, and the technical compromises that were made, will not jeopardise the mission.

But they also know that most probes sent to Mars fail and that the Red Planet is unforgiving.

Engineers have to be good, and Beagle's are. Now it will have to be seen if they are lucky.

Nosecone worries

The first concern is Beagle's nosecone, designed to shield it from the 1,700 deg Celsius temperature of atmospheric entry.

BEAGLE SEPARATION DAY

The nosecone is based on the design being used for the Huygens probe that will be deployed on Saturn's major moon Titan by the Cassini spacecraft next year.

But Huygens will descend into Titan's nitrogen atmosphere; Mars' atmosphere is quite different, being predominantly carbon dioxide.

That matters a lot. The aerodynamics of a spacecraft depend critically on the shape of the nosecone and it is a matter of survival to ensure aerodynamic stability during entry into Mars' atmosphere.

All the other probes that have successfully landed on Mars (Viking 1 and 2 in 1976, and Mars Pathfinder in 1997) have used a blunter nosecone.

Beagle 2 descent, Esa
The shape of the nose could make or break the mission
The more pointed nose on Beagle will call for split-second timing if the probe wants to get down safely.

The effect of the nosecone design is to require the parachutes to open very late in the descent, just 2,600 metres above the surface and 3.75 minutes before touchdown.

Winds on Mars are unpredictable but they must be low while Beagle enters.

Too much wind and Beagle will probably not survive. Its landing site has already been changed once to avoid a region of high winds.

Parachute worries

Then there is the problem of swing. When the parachutes open, the probe will swing from side to side.

Mars Pathfinder did this; it is inevitable. US engineers tried to minimise it by having a long tether to the parachute. Beagle's tether is smaller and if it swings too much it could be lost.

But Beagle's parachutes are a technical marvel. In mid-2002, only 12 months before launch, it was realised that the then current parachute system was too small and that Beagle would strike the Martian surface too fast to survive.

So in just 15 weeks engineers designed a new system, and delivered it eight weeks later.

Beagle bags, Esa
The bags must clear away from Beagle after the touchdown
Airbag worries

Then there are the airbags. If anything goes wrong the engineers suspect it will be them. They failed their first tests and had to be designed and built without a full testing regime.

As soon as they are filled, they will start to leak. They must inflate right on cue, at an altitude of 275 metres above the surface, just 15 seconds before impact, and they must protect Beagle for up to 12 bounces.

Then they must detach and roll away leaving Beagle on the surface. But the potential pitfalls are not over.

Surface worries

When Beagle gets to the surface its power is almost spent and it must immediately open up and expose its solar panels to the sunlight to charge its batteries and run its systems. Too much of a delay and it will die.

Only then will it be able to start its scientific mission to look for life, and even then there is a potential problem looming.

Beagle survives on the energy from its solar panels and has no way to clean them if they get dirty because of, say, a dust storm.

And there are dust storms brewing on Mars. At the moment they are a long way away from Beagle's landing site, but Martian weather is unpredictable.

So good luck Beagle. You will need it.





WATCH AND LISTEN
The BBC's David Shukman
"We are told that Beagle 2 is now on an accurate course for landing"



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