Robot Spins Wheels to Save Stuck Rover on Mars
Mars Exploration Rover team members prepare a testing setup for a subsequent experiment after an experiment driving the rover in a crablike motion, with all four corner wheels angled to the right. Clockwise from top: Scott Maxwell, Pauline Hwang, Kim Lichtenberg.
Credit: NASA/JPL-Caltech

NASA?s Earthbound test rover is doggedly spinning its wheels forward and back in the effort to find a way to free its sister robot Spirit from a sandy quagmire on Mars.

Spirit has been stuck in Martian dirt up to its hubcaps since May 6, when it became mired in a dirt patch (now called "Troy") while driving backward.

Because they don't want to damage Spirit while trying out ways to get the rover out of its sand trap, mission engineers are using a replica model here on Earth.

At the end of June, the test rover was set up in a plywood rig in a dirt pit at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The rig is filled with a dirt concoction mixed to mimic the properties of the sand in which Spirit is stuck. It is also tilted at a 10-degree angle - the same angle of the slope that Spirit is stuck on.

Engineers also placed a rock underneath the test rover's belly because images taken last month by the microscopic imager at the end of Spirit's robotic arm. After analyzing the image, mission managers determined that a dark blob in the middle was a rock positioned underneath the rover.

Plotting an escape on Mars

Mission engineers finally began testing out possible maneuvers on July 6 with the simplest maneuver on their list of options: driving forward with all five operable wheels. (Spirit has only five working wheels after its right front wheel went dead three years ago.)

In the first set of tests, the wheels turned enough to cover tens of meters, or yards, if there had been no slippage. The test rover moved slightly forward and sideways downslope.

On July 8, after refreshing the sandbox setup, engineers tried out straight-backward driving.

Next came testing out a series of crablike moves, in which all four steerable wheels are turned to the same side angle, then rotated the wheels either forward or backward.

By July 10, the team had tested crabwalk patterns driving forward in the test sandbox with the wheels turned at 60 degrees to the right and 20 degrees to the right. The angle of motion was upslope.

These latest tests completed four out of 11 maneuvers that the team has on its current testing list.

Next, engineers plan to test backward (downslope) crabbing with wheels turned 60 degrees to the right.

More tests ahead

So far the tests are going well, but what exactly the next steps will be it's too soon to say, said Steve Squyres, lead scientist for the Mars Exploration Rover Project, in an e-mail.

Weeks of further testing and analysis are expected before engineers identify the best moves to command Spirit to perform.

Meanwhile, on Mars, Spirit isn't just biding its time waiting to be freed. Mission scientists are taking the opportunity to have Spirit examine the environment surrounding it.

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