By Dr David Whitehouse BBC News Online science editor

The airbags were tested in a huge vacuum chamber

The mission will launch from Baikonur in Kazakhstan.

Beagle was conceived in 1997 by Colin Pillinger of the UK's Open University who gathered together a consortium to design and build Europe's first Martian lander. The goal was to search for life on the Red Planet.

But Beagle came ever so close to being abandoned. A behind-the-scenes documentary to be screened this week on BBC TV tells the story of how the project progressed and of the problems it has had to overcome.

Beagle's airbags, designed to cushion its landing on Mars, proved to be a major headache. At one point, the technical hurdles seemed insurmountable.

Conflicting pressures

The time was late 2000 and Beagle 2 was three weeks behind in a 24-month timetable. Everyone knew the schedule was extremely tight, and that nobody had done anything like Beagle before.

The money had been negotiated, cajoled and borrowed from various sources. The media had been contacted and were doing supportive features. The hunt for sponsorship was also on.

Will Beagle get down safely?

The design and construction of the various scientific instruments that Beagle would use on Mars to look for signs of life was progressing, albeit at differing speeds.

But it all depended upon Beagle's means of reaching the surface of Mars in one piece. The design called for parachutes and airbags.

The parachutes would open first and, just before impact, would jettison; the airbags would then inflate, allowing Beagle 2 to bounce a few times before coming to a, hopefully safe, rest. Or at least that was the plan.

The method of landing dictated the landing site. It had to be below the average Martian elevation so that the atmosphere was thicker, making the parachutes more effective, as well as giving them more time to work.

A mass distribution for Beagle 2 had already been agreed between the scientists and the engineers; so many kilograms for the scientific instruments, so many for the parachutes and airbags. There was no leeway.

Serious situation

Engineers were sent to the US space agency's Plum Brook Station in Ohio to test the airbags.

Plum Brook is a unique facility. It has a huge vacuum chamber. It tested the airbags that enabled the Nasa Mars Pathfinder to successfully bounce on the surface of the Red Planet in 1997.

But Beagle's tests did not go well.

Losing mass: Tough arguments were had

Inside the vacuum chamber, the airbags were inflated prior to their test but, unexpectedly, they burst. It was the only set of intact bags Beagle had.

Back in the UK, Jim Clemmet of Astrium, Beagle's chief engineer, addressed a team meeting to say that they had a serious situation with the airbags - a situation that currently nobody understood.

The implications for the mission were far-reaching.

New airbags and parachutes were needed, made from new materials and to a new design. The really difficult news was that the airbags and parachutes had to be heavier, and that meant losing some mass from the scientific payload.

Challenge met

It was Colin Pilllinger's grim task to break the news to the instrument designers who were already building their sensors. The bottom line was that 3 kg had to go.

The scientists said it could not be done and that the only way out was to lose one complete instrument. But Pillinger, typically, would not have this and sent the team away to try to save more grams. Remarkably they did.

The parachutes finally came through testing

A new parachute design was tested in Arizona and passed. In fact, according to the engineers involved, the new design was the finest they had ever worked on.

But the reworked airbags are still a problem and to some represent a worrying part of the mission.

They must be inflated at just the right time before the landing otherwise they will not have the correct pressure to cushion the probe. There are also concerns about them striking a sharp, jagged rock.

No one knows if they will really work until Beagle 2 actually reaches the surface of Mars.

It will be a tense time, but then Beagle's engineers are used to that.