Nasa hopes for inch-perfect landing on Mars

Space engineers were yesterday making their last nervous preparations for the landing of their Phoenix probe near the north pole of Mars next Sunday. The spacecraft - designed to look for reservoirs of water and ice in the Martian arctic - has been built using duplicate parts left over from two previous space missions. Both were destroyed as they approached the Red Planet in 1999.

It was later found that one, the Mars Climate Observer, had been lost because technicians had confused inches with centimetres when transmitting instructions to the probe, and so had sent it plummeting into the planet.

Agency staff are working around the clock to ensure there will be no repeat of such embarrassments. Next Sunday Phoenix will enter the Martian atmosphere at 13,000mph and will use a parachute to cut its speed before firing a series of rocket thrusters to bring its velocity down to 5mph. The craft will then settle, on three legs, on the Martian surface. The manoeuvre is highly complex and fraught with danger, Nasa acknowledges.

'We will fire 26 pyrotechnic events in the last 14 minutes of the journey,' said Barry Goldstein, project manager in Nasa's jet propulsion laboratory in Pasadena. 'Each of those has to work perfectly for the mission to come off as we planned.'

These final moments before landing - described by Nasa as 'seven minutes of terror' - will be the most dramatic and nervously anticipated part of the mission. 'The team has done everything possible to ensure a successful landing in a few weeks, but there are always the unknown unknowns,' said Nasa scientist Ed Weiler. 'This is no trip to grandma's for the weekend.'

Unlike recent successful Mars missions, which have set a series of robot vehicles on the Martian surface, Phoenix - which was launched in August 2007 - is a stationary lander equipped with a trench-digging robotic arm. This will be used to scoop up samples of nearby soil and ice. These will then be analysed in a suite of instruments designed to determine if the arctic landing site - a region similar in latitude to central Greenland or northern Alaska on Earth - might have been able to support primitive life in the past.

However, scientists will only have a limited time to make use of Phoenix and its data. About 150 days after landing, the encroaching gloom of winter in the Martian north will mean the probe will no longer be able to charge up its solar batteries, and it will shut down. Over the following months, it will be slowly buried beneath drifts of carbon dioxide frost. However, the craft should have done its job long before that happens, Nasa hopes.