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Spirit Struggles to Survive the Martian Winter

April 04, 2006

This image shows two twin tracks of light tan to white, overturned soil moving from top to bottom toward the viewer. Surrounding soil is dark tan on the surface.
Spirit's wheels have churned up light-toned, subsurface soil deposits on Mars that may record the past presence of water. Image credit: NASA/JPL-Caltech/Cornell
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Two years and three months after landing on Mars, Spirit can't help but dig trenches in the martian sand. The right front wheel of NASA’s Mars Exploration Rover is no longer working.

As a result, the rover's handlers on Earth are having a difficult time getting the rover to a suitably north-facing slope to survive the cold, dark, martian winter. The rover must face its solar panels northward to collect enough solar energy as the sun sinks low above the horizon.

Essentially, it's a race against time. The period of minimum sunshine in the martian winter is more than 100 days away, but Spirit currently gets only enough power for about one hour of driving on flat ground. And, Spirit literally has an up-hill battle.

This black-and-white image shows Spirit's two front wheels at the bottom of the frame. Stretching upward from the base of the wheels toward the top of the frame are intersecting trenches and deep tracks in the soil.
Spirit has slipped in difficult terrain as the right front wheel has churned up the martian soil. Credit: NASA/JPL-Caltech
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Safety is Priority #1

"It is too early to tell how serious this is," said Project Manager John Callas. "The nature of the terrain is a dominant factor."

"The climate is changing rapidly, and we have to put our primary emphasis on keeping the rover safe," remarked principal investigator and Cornell University geologist Steve Squyres. "Assuring survival has to take priority over science until we've got the vehicle on safer ground."

That means the team has stopped trying to move Spirit along an uphill route that proved too daunting. Instead, Spirit is in the process of driving back downhill before beginning a different trek toward a slope that will provide maximum sunlight--and thus power--to survive the martian winter. (On the other side of Mars and nearer to the martian equator, Spirit's twin Opportunity does not face the same power concerns.)

This image shows an overhead topographic map, labeled '3D energy map,' with contour lines winding from the top left corner toward the lower right corner, representing slope intervals of 1 meter. A key to the left of the map provides four ranges of values between 0 and 1 that correspond to colors on the map to the right. Red areas have the lowest values of 0.33 to 0.74. Orange areas are next lowest, from 0.74-0.77. Green represents moderately favorable values of 0.77 to 0.86. Blue represents the highest energy values of 0.86 to 1. The rover will move from its current position in a red, unfavorable area back to an earlier location in the green area.
This map shows a three-dimensional view of slopes in Spirit's vicinity. Blue areas are best because they have the greatest northward tilt toward the sun. Green areas are second-best for collecting solar energy during the martian winter. Image credit: NASA/JPL-Caltech/USGS/OSU
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Rover Loses Traction in Soft Soil

Progress has been intermittent. Spirit drove 10 meters (33 feet) in one day after the wheel stopped working, then lost traction on following days because of wheel slippage in difficult terrain. The rover team continues to evaluate all potential directions of travel, as well as mechanical tricks for tilting the rover sunward.

The rovers' suspension systems, which keep all six wheels in contact with the surface, have helped make both Spirit and Opportunity hardy explorers on steep slopes and sandy terrain as their investigations have continued far beyond their original 90-day warranties.

"Engineers designed the rovers so well that Spirit's weight remains evenly distributed on all six wheels," explained Al Herrera, a computer sequencing specialist for Columbus Technologies and Services Inc. who oversees commands uplinked to the rover. "That keeps the rovers balanced so they don't tip or roll backward."

While such balance is good for keeping the rovers safe and steady, it does not provide relief against the dragging right front wheel as there continues to be equal weight on it.

This image shows a rock-strewn, sand ripple-covered slope that ends at the horizon two-thirds of the way to top of the frame. The slope rises from left to right. Just beneath the top edge on the right is a dark, protruding outcrop of layered rocks.
Spirit was initially headed toward a layered rock outcrop known as "Korolev," to the east of the rover's current position, when the wheels began experiencing a high rate of slippage. Credit: Caltech/Cornell
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Wheel Stalled While Turning

The problem with Spirit's right front wheel emerged on the rover's 779th martian day, or sol (March 13, 2006), when the motor began drawing more electrical current and then stopped drawing current altogether as the rover was turning to adjust the orientation of the communications antenna. Engineers determined that the stall occurred in a drive actuator, one of 10 motors that operate the rover's six wheels.

Following tests in a rover facility at the Jet Propulsion Laboratory, engineers determined that an open connection precludes further use of the motor. This conclusion is different from the initial problem with Spirit's right-front wheel some five months after landing on Mars in January 2004. In that case, engineers were able to apply their ingenuity successfully. Driving Spirit backward redistributed lubricant and returned the wheel to normal operation.

Since an open connection means that power can not be delivered to the motor any longer, the team will need to use some of the same engineering innovation for five-wheel driving from now on. They will be tirelessly applying their skills to simulate and plan the rover's activities with the new handicap.

This image shows an overhead elevation map. The rover's current location is in relatively low terrain in the upper left of the image, colored lavender. Winding upward across the image from the lower left to the upper right corner is a yellow-colored strip of terrain of intermediate elevation. On the lower right is high, hilly terrain colored red in this view.
This bird's-eye view shows the topography of Spirit's location in Gusev Crater on Mars. Blue areas are lower in elevation. Red areas on the lower right represent the topographically higher slopes and peaks of "McCool Hill." Image credit: NASA/JPL-Caltech/OSU
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Proceeding on Five Wheels

Issues with the rovers at this stage are to be expected. Spirit has lasted almost 9 times longer than planned. The motors on Spirit's wheels have rotated more than 13 million times, far more than called for in the rovers' design. Whether Spirit will be able to continue to drive successfully on five wheels while dragging a sixth in the sand, only time will tell.

That said, while reaching a favorable north-facing slope is the current priority, there is always a sunny side for science: twin trails of fluffy soil churned up in Spirit's wake reveal a salty chemistry, which may indicate signs of a past water in the area.
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