Mars rover engineers have built a test bed simulating the condition of the Mars Rover Opportunity, which became stuck in a Martian sand dune on April 26.  The test bed uses a combination of materials including play sand for children's sandboxes, diatomaceous earth for swimming pool filters, and mortar clay powder.
Opportunity dug into soft sand to wheel-hub depth as it drove over a dune about one-third meter (one foot) tall and 2.5 meters (8 feet) wide. "We've climbed over dozens of ripples, but this one is different in that it seems to be a little taller and to have a steeper slope, about 15 degrees on part of its face," said Mark Maimone, a Jet Propulsion Laboratory (JPL) rover mobility engineer. Tests will be conducted using an Earth-bound twin of the rover and the simulated Martian dune to determine how to best maneuver Opportunity out of the predicament before transmitting commands to the actual vehicle.
"We choose to proceed cautiously, so we don't expect to begin actually driving out of the dune before next week, possibly later," said Jim Erickson, rover project manager at JPL. "Both Opportunity and Spirit have already provided many more months of scientific exploration than anyone expected. By taking good care of them, we hope to keep them exploring for more months to come. Tests so far have sustained our optimism about Opportunity's ability to drive out of this dune, but we have more testing ahead to understand how robust that capability is."
Engineers constructed a simulated dune last week using sand that was already at JPL's rover testing facility and put a test rover into a similar dug-in position. The test rover had no difficulty driving away, even when sunk belly-deep. However, that sand offered better traction than the finer, looser material that appears to make up the surface at Opportunity's current position. "We needed to do tests using material more like what Opportunity is in, something that has a fluffier texture and cakes onto the wheels," said JPL rover engineer Rick Welch, who is leading the tests.
Based on images of wheels and wheel tracks from Mars, Dr. Robert Sullivan of Cornell University, a rover science team member, working with engineers in the JPL test bed, helped  match the properties of the test sand as closely as possible with those of the sand beneath Opportunity, "We found that when the wheels dig in, the material we're using does stick to the wheels and fills the gaps between the cleats, but it doesn't stick when you're just driving over it. That's good because it's the same as what we see in the images from Opportunity," Sullivan said. Experiments show that the test rover, after some initial wheel-spinning, can drive out of the more powdery material.  
The team went to several home supply and hardware stores to find enough bags and boxes of the ingredients to make more than 2 tons of the simulated Mars sand for the more realistic mobility tests, according to JPL rover mobility engineer Jeff Biesiadeckia.

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Mars rover engineers build test sandbox