Spirit Struggles to Survive the Martian Winter

April 04, 2006

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 Larger Image

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.

	Spirit has slipped in difficult terrain as the right front wheel has churned up the martian soil. Credit: NASA/JPL-Caltech Large Image

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 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 Larger Image

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.

	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 Large Image

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 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 Larger Image

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.