NASA plans to fill a 鈥榮andbox鈥� with simulated Martian soil this week to test escape manoeuvres for the Mars rover Spirit, which has been stuck in a sand trap called 鈥淭roy鈥� since early May.
When the rover first became stuck, its wheels slipped so much in the fine, flour-like soil that Spirit moved just centimetres 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 now hopes to begin testing possible escape strategies this week, using a mock-up of the sand trap.
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To simulate the conditions at Spirit鈥檚 site, the rover team will use a cement mixer to combine several tonnes of clay and , 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鈥檚 twin, Opportunity, which was trapped for about five weeks in 2005 on a 30-centimetre-high ripple of soil, nicknamed 鈥淧urgatory Dune鈥�.
Once the simulated Martian soil is ready, the mixture will be shovelled into a test bed at NASA鈥檚 Jet Propulsion Laboratory (JPL) in Pasadena, California. It will then be landscaped to match the tilted terrain at Spirit鈥檚 site, which is largely responsible for the fact that the rover is currently tilted by about 14 掳 to the left.
Driving techniques
Engineers will then embed a model rover in the simulated Martian soil and used it to test escape manoeuvres. 鈥淲hat we want to do is run through the whole spectrum of possibilities in the test bed to see which one would work,鈥� says John Callas, the rover project manager at JPL.
The driving will likely consist of variations on two basic techniques: driving Spirit straight out of the sand pit and turning its wheels so that the rover moves sideways downslope, Callas says. If all goes well, NASA may begin commanding Spirit to move by mid-July.
Some of the team鈥檚 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.
That could have meant there was not enough weight pressing down on the wheels to give them the traction needed for an escape.
Barely touching
But images taken using the microscopic imager on the end of Spirit鈥檚 robotic arm in early June revealed a single, pointy object on the ground beneath the rover鈥檚 belly. Further analysis suggests the rock-like object is just barely touching the underside of the rover but is not bearing any weight.
The rock also appears to be resting on top of the Martian soil, suggesting Spirit might be able to press it further into the ground if it does come into contact with the rock. 鈥淲hen we do start to move [Spirit], it鈥檚 likely that rock won鈥檛 be a hazard or obstruction. Right now it鈥檚 a concern, but it鈥檚 not considered a serious concern,鈥� Callas told 麻豆传媒.
Another worry was Spirit鈥檚 left-middle wheel, which seemed to have been jammed by another rock. But the wheel now seems to be rotating freely, suggesting the obstruction is gone. Callas says the team will proceed cautiously in case further churning of the soil causes the suspected rock to fall back in.
