Nasa prepares for $2.5bn gamble on Mars landing

'Sky crane' will lower robotic probe on to Red Planet in latest attempt to uncover signs of life

On 6 August at precisely 13 seconds after 6.30 in the morning London time, Nasa scientists should receive a message from Mars telling them whether their $2.5 billion (£1.67bn) gamble on three nylon lines has paid off.

Click here to see the 'Mission to Mars: six minutes to land on the red planet' graphic

This will be the moment when, for the first time, tethers will be used to gently lower a six-wheeled robotic rover the size of a Mini Cooper down to the surface of another planet from a spacecraft hovering precariously overhead.

Once Nasa's Curiosity rover touches the surface of the Red Planet six months from today, its 25 foot-long umbilical cord will be cut from its carrier ship, which will then use its bank of retro-rockets to crash-land well away from the site where the rover will begin its mission to search for the chemical signatures of Martian life.

"We will be very nervous. Landing on another planet is not a walk in the park. It's very challenging and there have been mixed successes and failures in the past," said Charles Elachi, director of Nasa's Jet Propulsion Laboratory in Pasadena, California, where the probe was designed and built.

Several previous space missions to Mars have ended in failure during the approach and landing phase. Britain's Beagle 2 probe disappeared during its airbag-cushioned landing in 2003.

"The reason we are nervous is that it's about 3 tonnes of mass coming in at a speed of almost 12,000mph and we have to land softly in less than six minutes," Dr Elachi said yesterday in London.

"We usually call it the six minutes of terror. When you are coming in at 12,000mph with that kind of mass it's equivalent in terms of energy to 25 high-speed trains going at full speed," he said.

"That's the amount of energy we have to dissipate in those six minutes so that we can land softly on the surface. In addition, the accuracy we have to point to be at the right angle is equivalent to me being in Los Angeles and hitting a golf ball to land in a hole on St Andrew's golf course in Scotland," he added.

Much of the immense kinetic energy of the spacecraft will be soaked up by the probe's heat shield as it enters the Martian atmosphere. This will reduce its speed tenfold to allow parachutes to slow it down still further, Dr Elachi said.

But the critical moments will come towards the end of the six minutes when the carrier craft nears its final approach to the Martian surface. It is then that the retro-rockets must fire precisely to allow the tethers to be released from the carrier's "sky crane".

"Retro rockets will slow us down further and when we are hovering about 30 feet above the surface, the sky-crane will lower the rover down. It's a little bit different from before when we used airbags, because this rover is so much bigger that airbags will not be practicable," Dr Elachi said.

The Curiosity probe will land at the foot of a geologically layered mountain inside a crater called Gale. The site was picked for its accessibility and for the fact that it is near an "alluvial fan" of sediments that could have been formed by running water at a time when life on Mars may have existed.

Asked whether Curiosity could finally answer the question about whether there was life on Mars, Dr Elachi replied: "Yeah, it could happen, if it's there."