For NASA's Curiosity Mars rover to arrive undamaged on the surface of the Red Planet, a lot of things will have to go right.

So far, the scorecard for missions toMarsreads attempts 40, successes 14.

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Not so good.

Well over 60 percent of Earth missions toMarshave failed, ever since the pioneering efforts of the former Soviet Union in the 1960s and including Britain's high-profile Beagle 2 space probe.

As NASA's latest mission toMarsheads closer to the Red Planet, the head of NASA'sMarsExploration Program, Doug McCuistion, acknowledged Tuesday that many things could still go wrong before its scheduled Aug. 6 landing date.

The one thing that worries him most is if the spacecraft's heat shield will detach as planned when the U.S.MarsScience Laboratory mission sets down a large, mobile laboratory onMars the roverCuriosity.

"If you look at the scorecard, Earth is doing less than 50 percent; less than 50 percent of Earth's missions toMarshave been successful," McCuistion, a former U.S. fighter pilot, said at the Farnborough Airshow south of London.

In the seven minutes before its planned touchdown, the U.S. spacecraft has a number of tasks it has to complete forCuriosity to make a safe landing. First it must get rid of the heat shield and avoid a subsequent collision with it. Then it has to slow its descent to the Red Planet with the aid of a massive parachute as well as use rockets mounted around the rim of an upper stage. In the final seconds, the upper stage of the spacecraft acts as a sky crane, lowering the upright rover on a tether to the surface.

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