A.J.S. RaylJun 15, 2004

Mars Exploration Rovers Update: Rovers Go Up Hills, Down a Crater, and May be Showing Signs of Age

The twin rovers Spirit and Opportunity continue their trek across the varied surface of the Red Planet, climbing hills and descending into a crater. After a two-month journey of over 3 kilometers through rocky terrain, Spirit has now begun climbing the Columbia Hills, which were seen on the horizon in the early panoramas taken from the landing site. The rover is expected to spend much of its remaining life climbing the hills and analyzing their geological make-up. Meanwhile, on the other side of the planet, Opportunity is carefully descending into the stadium-sized depression dubbed “Endurance Crater” by the MER team. The rocky formations revealed on the slopes of the crater promise to provide some of the richest sources for studying the geological history of Mars.

As Spirit was approaching the hills, mission controllers noticed what might be early signs of aging for the resilient rover. The electric motor powering its right front wheel began drawing a growing amount of current, reaching as much as three times the current required by the other wheels. According to Mission Manager Mark Adler of the Jet Propulsion Laboratory in Pasadena, it appears that the wheel is stuck, and moving it requires an ever-increasing effort from the motor. The risk is not only that the power demands of the wheel will be a drain on Spirit ’s limited energy resources, but also that motor will heat to such a degree that it will become inoperable. Adler estimates that if used continuously, the motor may survive a drive of a few hundred meters, but no more.

Mission controllers are not sure what caused the problem. It is possible, said Adler, that “some junk” got into the wheel’s gear mechanism, preventing it from rotating smoothly. Another possibility, he speculated, was that lubricant has congealed into a buttery texture, preventing it from being evenly spread among the gear’s moving parts. If that is the case, Adler suggested, heating the wheel using heaters installed for this purpose, may return the lubricant to its proper liquid state, resolving the problem. But at this point Adler and his colleagues simply don’t know the source of the problem.

If all efforts fail to resolve the problem, controllers plan to use the right front wheel sparingly in the future. For much of the time, Adler explained, Spirit will use only five wheels to get around, leaving the front right wheel to simply roll along. This in itself should not pose a major problem for Spirit , since each of the wheels operates independently, and they are designed to compensate for each other while climbing over rocks or maneuvering in a rough terrain. The main difficulty, Adler warned, may be in steering the rover with precision, since the drag created by the front wheel that is simply along for the ride is likely to affect the rover’s course. Nevertheless, this may be the only way to preserve the stricken motor for those occasions when the use of all six wheels is indispensable, as when climbing steep slopes or traversing rough terrains.

Mission controllers are concerned that Spirit ’s difficulties are not an isolated case but may be a sign of things to come. It may well be, said Adler, that the problem with the front wheel is age-related. “In that case,” he explained, “we should expect to see the same problem appear in the other wheels of the two rovers.” So far this has not been the case, but the JPL team is keeping a close eye on all twelve wheels that are now roaming the surface of Mars.

As Spirit began climbing the Columbia Hills, it immediately encountered some highly unusual-looking rocks along its way. “Some of the Rocks,” said Larry Soderblom, a rover science team member from the U.S. Geological Survey in Flagstaff, Arizona, “appeared to be disintegrating. They have an odd kind of rotting appearance, with soft interiors and resistant rinds or hulls” In some cases, the rocks seem to have been “rotted” right through, leaving their empty shells posed like canopies above the Martian soil. Intriguing as they are, Soderblom explained, it is too early to speculate about the composition or history of these “rotting rocks.”

On the other side of the Martian globe, at Meridiani Planum, Opportunity is gradually moving into Endurance Crater. For the past few sols mission controllers have been carefully testing the ground, trying to find the best and safest method to drive the rover into the crater and back out again. Several times they edged Opportunity a few meters into the crater at a particular angle, and then reversed course and brought it back up. Finally, explained Scott McLennan, science team member from the State University of New York at Stony Brook, the team decided that the best course would be to send the rover straight down into the crater, and then bring it up again by the same route. Opportunity is now on its way, having descended 5 meters below the crater’s rim.

McLennan and his colleagues are particularly excited about investigating the various sedimentary layers that are visible on the Endurance Crater’s wall. “Color differences suggest that at least three lower, older layers are exposed below Opportunity ’s [current] location” McLennan explained. The rover’s current target is a flat rock, 36 by 15 centimeters in size, whose elongated shape has earned it the nickname “Tennessee.” Scientists are now preparing to analyze the rock using the spectrometers and microscopic imager on the rover’s robotic arm. When this is completed, Opportunity will descend further to study the lower stratigraphic layers.

“If we can get to the lower units, this will be the first detailed stratigraphic section ever done on another planet,” said McLennan. “We’re doing exactly what a field geologist would be doing.”

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