Mega-rover ready to hunt for life signs on Mars

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Plutonium powered and as big as an SUV, NASA's Curiosity rover is crammed with souped-up sensors and a new sky-crane landing system

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THE flying-saucer-shaped probe hurtles through space, firing thrusters and flinging off weights to point its heat shield forwards for the scorching trip through Mars's atmosphere. Once it has slowed, the heat shield drops away and, 10 kilometres above the surface, a parachute billows out. More weights are jettisoned, pointing the craft's radar-tipped belly towards the fast-approaching ground.

The probe cuts loose its parachute and upper shell, then fires thrusters towards the ground, slowing its speed further to 3 kilometres per hour. Hovering 20 metres above the Martian surface, the probe starts to unspool an SUV-sized rover from its belly. Dangling on cables like a giant spider, the payload is gently lowered by a "sky crane" to the surface. Curiosity has landed.

So begins a slick animation of NASA's newest rover, which is due to blast off on 25 November. The manoeuvres are so precise in their choreography and timing that their successful implementation seems improbable - especially given that Mars is the Bermuda Triangle of the solar system, dooming to failure two-thirds of the missions that attempt to visit it. "Everything has to behave according to plan," admits mission leader John Grotzinger of the California Institute of Technology in Pasadena.

The stakes could barely be higher. The $2.5 billion rover will be the most ambitious and expensive mission ever sent to Mars, carrying state-of-the-art tools that will reveal whether its landing site was ever habitable and search for signs of life preserved in its rocks. The first rover to be powered by the radioactive decay of plutonium rather than sunlight, Curiosity will be able to work around-the-clock and through the Martian winter.

"It's going to be a huge step forward," says Steve Squyres at Cornell University in Ithaca, New York, lead scientist for the Mars Exploration Rovers Spirit and Opportunity, which landed on different sides of the planet in 2004.

But first Curiosity has to touch down safely. "Anytime you land on Mars, it's a slightly scary thing," says Squyres. Both Spirit and Opportunity were swaddled in airbags and bounced to a landing, but Curiosity weighs five times as much and would simply punch through airbags as if they weren't there. "We pushed the airbag technology about as far as you could push it," says Squyres. "The sky crane system is a good engineering solution to the problem."

Balletic precision

Developing the gargantuan mission, which is also known as Mars Science Laboratory, has not always been smooth. Engineering troubles - including problems with motor-driven gears called actuators - forced a two-year launch delay and added millions of dollars to its already outsized budget. "This has been a trial," admits Jack Mustard of Brown University in Providence, Rhode Island. "But if NASA can pull this off, you've got a demonstrated landing system for large masses, which will be important for the next phase of landing on Mars - bringing samples back."

The ballet of moves that precedes the sky crane's deployment is also an advance. Tilting the probe by jettisoning weights gives it much greater aerodynamic control, enabling the target landing site to be an ellipse just 20 kilometres long - one-seventh of that needed for Spirit and Opportunity. "When you have a larger ellipse, you rule out all the most interesting places," says Grotzinger. That's because larger footprints are more likely to include steep slopes or fields of boulders - terrain too dangerous to risk landing on. "This is the first time in the history of the exploration of Mars where we have been unencumbered by engineering constraints to really debate landing site options," he says.

After years of consideration, the winning site finally emerged in July: the 150-kilometre-wide Gale crater near the Martian equator, whose floor plunges 5 kilometres below the surrounding surface. "It's a pretty deep hole in the ground," says Ralph Milliken at the University of Notre Dame in Indiana. Bizarrely, a mound of rocks within it rises up about as high as the crater walls.

How did such a giant structure form? Geologists are divided, although they are certain water was involved in some way. That's because rocks at the bottom of the mountain are made of layers of clays and sulphate salts, both of which need water to form. Orbiting spacecraft have dated the formation of these rocks to about 3.5 billion years ago.

"If any place had a lake on Mars, Gale would," says David Blake of NASA's Ames Research Center in Moffett Field, California. Rain, snow or rising groundwater might have pooled in the crater, and the clays and sulphates might have been left behind when the water evaporated.

With Curiosity, NASA is going beyond its previous aim to "follow the water". Barely a week seems to go by without instruments on the Mars Reconnaissance Orbiter and Mars Express probe beaming back evidence for past or even present water. No one doubts any more that liquid water once featured on the Red Planet.

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