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Stuck at Troy

Test maneuvers for a stuck Mars rover are on hold for evaluation, set to begin again next week.

Earthbound tests for the Mars rover Spirit, which has been stuck in a soft, fluffy dust trap since early May, have been suspended for a few days while NASA engineers modify their soil simulant and evaluate what they’ve learned from their experiments so far.

Mars rover scientists and engineers are testing
extrication strategies in a simulated Martian “dustbin”
with a copy of the rover Spirit.
Credit: NASA/JPL-Caltech

When Spirit’s wheels dug into the soil of a Martian area named Troy, NASA’s rover teams at the Jet Propulsion Laboratory (JPL) in Pasadena, CA, built a test bed to try out different maneuvers with a Spirit clone, rather than wearing out the real rover’s aged parts on Mars. The engineers mixed 5,400 pounds of clay and diatomaceous earth (chalklike, crumbly rock), and then poured the mixture into a “sandbox” built at a 10-degree tilt to simulate the slope on which Spirit currently perches. They raked the soil and shaped it to match the Mars landscape; then they drove their test rover in, watched its wheels cake up and sink, and began their tests–a series of maneuvers including sideways crab movements and forward and backward driving to see how Spirit might extricate herself from her predicament.

But because the conditions on Earth are different from those on Mars, the team is reevaluating the soil simulant and planning to add a new material to the right side of the rover. “We’re getting a little more traction on the right side with Spirit, so we want to replicate that,” said John Callas, NASA’s project manager for the Mars rovers Spirit and Opportunity, in a webcast yesterday. The team is using a heavier rover copy than Spirit–which weighs 150 pounds on Mars–to test worst-case scenarios and to make sure the test rover is properly embedded in the soil simulant.

Rover driver Ashley Stroupe said in yesterday’s webcast that the test rover moves a little bit–a few centimeters–with each maneuver, which means there’s a good chance Spirit could get out once the drivers start working with her again on Mars.

It’s not the first time a rover has gotten stuck, but Callas said it’s the “worst embedding event either rover has seen on Mars.” Spirit’s twin, Opportunity, was stuck for over a month in 2005 under similar conditions, with fluffy soil, wheels buried deep, and very little traction. But Opportunity was going straight uphill when she got stuck in what rover teams called the “Purgatory” sand ripple, so in the end, her escape meant simply rolling backwards down the hill, through her tracks. Spirit, however, is lodged at an angle and tilted to the side, which makes extricating herself a bigger challenge.

She’s also not working on all thrusters: the front right driving wheel has been broken for three years, so she’s been dragging it behind her, driving backwards with five good wheels since then. To make matters worse, she appears to have a rock stuck underneath her. In a first for the rover, Spirit used her own camera to look under her belly, a maneuver the JPL team tried out on Opportunity first, to make sure it would be worthwhile. It was. Though the camera at the end of the rover’s robotic arm is a microscope, making the photographs out of focus, the photos are clear enough to see a rock touching Spirit’s belly. That could be bad if Spirit’s weight is resting on the rock, Callas said, because that gives each wheel even less traction in the floury soil.

Suggestions from the peanut gallery during yesterday’s webcast were numerous. They included using the robotic arm to lift the rover (no go: the arm pushes with only 14 pounds of pressure, which wouldn’t hold Spirit’s weight) and sending Opportunity to the rescue (oops–she’s on the other side of the planet).

The good news is that Spirit’s energy is “the highest it’s ever been,” Callas said. Wind has been blowing her solar panels clean of the dust that caked onto them and left the rover performing at 25 percent of her capacity during the most recent Martian winter. Now the panels are almost completely clear. That’s a plus for the extraction process, Callas said, when Spirit could be spinning her wheels for hours trying to get out. The rover team expects to resume testing next week and to start working with Spirit again in early to mid-August. Meanwhile, Spirit continues using her scientific instruments to study her location at Troy.

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