Mars rover on its last wheels
NASA’s Spirit rover, part of the Mars Exploration Rover (MER) Mission, is currently spinning its wheels on Mars in an area scientists have named “Troy” after the besieged city in Homer’s Iliad. Spirit landed on Mars on Jan. 4, 2004, three weeks before its twin, Opportunity. Each rover had an anticipated mission span of 90 Martian days, which last the equivalent of 24 hours and 37 minutes, and each was expected to drive about one kilometer total. This January will mark six years on the Martian surface, and the two have driven more than 25 kilometers combined.
“It’s way out of warranty,” said Ray Arvidson, director of the Earth and Planetary Remote Sensing Laboratory here at Washington University. “It’s like an old ’55 Chevy.”
The problems begin
While the mission is unprecedented in the amount of data collected over the length of the mission, Spirit now finds itself in a predicament. Last May, the rover broke through the surface it was driving on, sinking into loose soil at the outer edge of a crater. The rover has not been able to move from its current location.
In addition, researchers found that a large rock was positioned under the rover’s belly, further imperiling its mobility. Buried rocks in the soil could interfere by resisting the rover’s wheel turns to the point that the motor stalls, providing a challenge to the planners.
“Since there are probably buried rocks, it’s so easy to encounter one so there’s so much resisting torque that the motor stalls,” Arvidson said. “We’re concerned there might be these buried objects that could get caught in the wheel.”
The aging rovers are encountering more problems in addition to losing mobility. For one, the right front wheel of Spirit stopped rotating some time ago, so the rover drives in reverse, dragging the nonfunctional wheel behind it.
Arvidson said that the rovers also suffered from bouts of amnesia in which they fail to “wake up” when the sun hits them each morning as they are programmed to do. There have also been problems with storing information in the rovers’ flash memories.
Making the most of it
“In interim, we’ve done more detailed measurements of one place than we’ve ever done before and it turns out to be a fascinating place because there’s a crater that’s 8 meters across and maybe 25 centimeters high, and we wound up right on the side of it,” Arvidson said. “In fact, the edge of the crater goes right down the middle of the rover. The material in the crater is really neat stuff.”
Arvidson said that they have been able to take measurements of the material at several different depths, which had not been done at other locations when the rover was on the move.
“What we’re seeing is the material varies as a function of depth,” Arvidson said.
Arvidson explained that calcium- and iron-containing compounds are differentially distributed in the area based on depth.
“You can’t see those sulfate sands until you turn them up,” Arvidson said.
Students at the University have also been involved in gathering data.
“The advantage to remaining in one place for so long was we could do detailed study with our instruments,” said junior Kirsten Siebach, who has worked on the project while at the University.
Planning an escape
No one can be certain how long the extraction process will take or whether it will be successful. Freeing the rover may take weeks or months, according to online statements by NASA.
To plan an escape route for the rover, tests have been done in a test bed with a duplicate rover at NASA’s Jet Propulsion Lab (JPL) in Pasadena, Calif., and computer-based models of the rover’s predicament have been designed. The test bed can be tilted to the correct angle, and the proper mix of soil can be added.
Graduate student Kim Lichtenberg worked at JPL over the summer trying to find a simulation for a test bed that would best represent the soil around the rover. She brought a geologic point of view to the testing.
“Unfortunately, it turns out that for each of these separate maneuvers we tried in the test bed, there wasn’t one that leapt out at us,” Lichtenberg said. “It was very small, minute differences in how the rover behaved.”
The rover stopped taking measurements in mid-November, and the team started sending commands to the rover in attempts to free it.
Opportunity found itself stuck in a region the team named Purgatory earlier in the mission. In that situation, the solution was to simply back out, but Arvidson said that Spirit’s situation is more complicated.
“This is way more complicated because the vehicle is rolled about 12 degrees, the left wheels are badly embedded, the right rear wheels are embedded, the right front wheel doesn’t work,” Arvidson said. “There’s also a rock that’s pushing up against the underbelly, and there might be buried rocks that could get entangled in the wheel system.”
The team attempted the first extraction drive on Nov. 16 but was unable to make progress. Spirit determined it was tilted farther than parameters for the drive allowed, ending the attempt after less than a second. The team is being careful not to make moves that might put the rover in more danger.
“It’s moving. Every time we command a drive, we move, so it’s not stuck, per se, it’s just mobility impaired,” Arvidson said. “It’s not a situation where we do a command and nothing happens.”
A looming deadline
The mission is up against a deadline with regard to moving the rover. Within about two months, it will be winter in Mars’ southern hemisphere. With each day, less sunlight reaches the rover’s solar panels, giving the rover less power to use. If Spirit cannot generate enough power to heat the important electronics, the rover may not respond when the team tries to communicate to it.
Currently, the rover is tilted to the south 1 degree and needs to be tilted 5 degrees to the north, toward where the sun will be during the Martian winter. There are slopes not far from where the rover is located where it could park for the winter at the necessary angle, but first it needs to regain mobility. In addition, dust accumulates on the solar panels, reducing their efficiency.
“It really depends on how much dust is going to come on the panels. There’s some time pressure to get out of here,” Arvidson said. “On the other hand, you don’t want to rush anything, because we’d be pretty close to turning this into a lander, not a rover.”
Members of the team realize the possibility that Spirit may have reached its final resting place.
“We’re going to try our darnedest to try and get her out, but in the eventuality that she does end up spending the rest of her life in this spot, there are so many scientifically interesting things to study in this area,” Lichtenberg said. “This is really one of the best places we could have picked to get stuck.”