NASA readies 'sandbox' to plot Mars rover's escape

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NASA plans to fill a 'sandbox' with simulated Martian soil this week to test escape maneuvers for the Mars rover Spirit, which has been stuck in a sand trap called "Troy" since early May. When the rover first became stuck, its wheels slipped so much in the fine, flour-like soil that Spirit moved just centimeters despite the fact that its wheels had rotated enough to move it about 10 metres away. Mission managers then stopped trying to drive the rover, whose wheels had already become buried halfway into the loose soil.

NASA plans to fill a 'sandbox' with simulated Martian soil this week to test escape maneuvers for the Mars rover Spirit, which has been stuck in a sand trap called "Troy" since early May.

When the rover first became stuck, its wheels slipped so much in the fine, flour-like soil that Spirit moved just centimeters despite the fact that its wheels had rotated enough to move it about 10 metres away. Mission managers then stopped trying to drive the rover, whose wheels had already become buried halfway into the loose soil.

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NASA now hopes to begin testing possible escape strategies this week, using a mock-up of the sand trap.

To simulate the conditions at Spirit's site, the rover team will use a cement mixer to combine several tonnes of clay and diatomaceous earth, a type of chalk-like rock made of the fossilised remains of diatoms, algae with cell walls made of silica. The team may also add sand to the mix.

A similar cocktail was used to test escape strategies for Spirit's twin, Opportunity, which was trapped for about five weeks in 2005 on a 30-centimetre-high ripple of soil, nicknamed "Purgatory Dune".

Once the simulated Martian soil is ready, the mixture will be shovelled into a test bed at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. It will then be landscaped to match the tilted terrain at Spirit's site, which is largely responsible for the fact that the rover is currently tilted by about 14 ° to the left.
Driving techniques

Engineers will then embed a model rover in the simulated Martian soil and used it to test escape manoeuvres. "What we want to do is run through the whole spectrum of possibilities in the test bed to see which one would work," says John Callas, the rover project manager at JPL.

Some of the team's worst fears seem to have abated since May, when mission members worried the rover might be so deeply embedded that its belly might actually be resting on rocks it had spotted earlier from a distance.

But images taken using the microscopic imager on the end of Spirit's robotic arm in early June revealed a single, pointy object on the ground beneath the rover's belly. Further analysis suggests the rock-like object is just barely touching the underside of the rover but is not bearing any weight.

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