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Last Updated: Wednesday, 24 December, 2003, 12:15 GMT
Arrival at Mars: The Beagle landing
By Helen Briggs
BBC News Online

The British spacecraft Beagle 2 is heading for Isidis Planitia, a large flat plain where water may once have existed and where traces of life, past or present, could have been preserved.

The lowlands, some 1,200 kilometres across, line the floor of a vast basin which was probably carved more than four billion years ago when an asteroid or comet crashed into Mars.

Topography map of Mars. The lowlands are blue; the highlands red and orange. (Image: MOLA science team.)

The basin straddles the boundary between the planet's vast, smooth northern lowlands and the rugged, heavily cratered highlands that rise to the south.

On Christmas Day, an object will fall out of the sky, hurtling towards the barren, rocky landscape.

Imagine the scene. It is early afternoon on Mars and sunlight filters through the dusty butterscotch sky.

The desert-like scenery is littered with craters, boulders and wind-blown sand dunes.

Suddenly, an object appears in the sky illuminated by a fiery tail of flames.

A sonic boom echoes around the valley. As the probe looms ever closer, two grey parachutes spring open, the metal heat shield falls away and giant airbags inflate to cushion the landing.

The cone-shaped probe hits the ground on its three airbags, making great leaps over the soil, flinging up clouds of dust.

It rolls to a halt, the airbags sever, and a silver cone-shaped object drops to the ground.

The outer casing springs open like a clam shell, exposing solar panels and a robotic arm packed with scientific instruments.

'Extraordinary sight'

"It will be an absolutely flabbergasting sight," says John Bridges, a scientist at the UK's Open University in Milton Keynes who helped select the landing zone.

Beagle 2 descent (Image: All rights reserved Beagle 2)
Airbags will cushion the landing (Image: All rights reserved Beagle 2)
"To see this parachute lander coming down, then these great inflation bags suddenly expanding around the lander, then it's going to hit the surface of Mars and then it's going to bounce up and go on great vast bounces for hundreds of metres, keep bouncing for several kilometres, maybe 10 big bounces in all.

"It will be an unprecedented, extraordinary sight.

"One day, I hope someone will be able to film it as it happens on Mars but at the moment we can only really imagine it."

The landing site was selected on the basis of practical considerations besides the Holy Grail of finding evidence of water and possibly life on Mars.

Rock garden

Beagle's engineers believe the basin is smooth enough for a soft landing while there are plenty of rocks around to satisfy data-hungry scientists.

Isidis Planitia is at a relatively low elevation. This provides sufficient depth of atmosphere for the parachutes on Beagle 2 to kick in and slow its descent.

The weather conditions also play a role in the landing. Spring has arrived in the northern hemisphere, providing sunlight to power Beagle's solar panels.

Temperatures on the cold, dry planet fall to below Arctic extremes at night but it should stay warm enough for Beagle to survive, given the amount of insulation packed around it.

The centre of the desired landing site is just north of the equator, 10.6 degrees North, 273 degrees West, to be precise.

Beagle 2 should land somewhere within an area 114 kilometres long and 46 kilometres wide, roughly twice the area within London's orbital motorway, the M25.

The exact spot depends on factors like wind speed during touchdown, what happens to Beagle 2 during its descent and the crucial moment of release.

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