The rover's task in the next few weeks will be to shift itself into a more favorable position. The solar panels are now tilted 9 degrees to the south. An even tilt or a tilt to the north is needed to power the solar panels during the severe Martian winter, since during this time the Sun stays in the northern sky.

At its current angle, Spirit probably would not have enough power to keep communicating with Earth through the Martian winter. Energy levels will drop below the 160 watt hours per day necessary to maintain communications.

If the team is unable to shift the rover's position and power levels drop, the rover will shut down and essentially hibernate. The rover will be able to check its battery status once a day, and if the batteries are not charged enough, the rover will go back to sleep. Spirit could be in this state for 6 months.

Keeping the rover powered on helps keep it warm. If Spirit hibernates, the rover's internal temperature will drop. The rover's sensitive electronics are designed to withstand temperatures as low of minus 40 degrees C while operating, and minus 50 degrees C when not operating.

Based on the previous winters Spirit has endured on Mars, the mission scientists say the electronics could reach as low as minus 45 C during the coldest depths of winter. Even though that temperature is within the design limits, the rover may be less durable after having gone through many extreme temperature cycles on Mars.

Winter will begin in May. If Spirit does go into hibernation, the rover team should know by this August or September if it survived. Several science experiments are planned in the hopes the rover will awaken in the spring.

"Spirit is not dead; it has just entered another phase of its long life," said Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington. "We told the world last year that attempts to set the beloved robot free may not be successful. It looks like Spirit's current location on Mars will be its final resting place."

Sliding Into Home
Ten months ago, as Spirit was driving south beside the western edge of a low plateau called Home Plate, its wheels broke through a crusty surface and churned into soft sand hidden underneath.

After Spirit became embedded, the rover team crafted plans for trying to get the six-wheeled vehicle free using its five functioning wheels -the sixth wheel quit working in 2006, limiting Spirit's mobility.

The planning included experiments with a test rover in a sandbox at NASA's Jet Propulsion Laboratory in Pasadena, Calif., plus analysis, modeling and reviews. In November, another wheel quit working, making a difficult situation even worse.

Recent drives have yielded the best results since Spirit became embedded. However, the coming winter mandates a change in strategy. It is now mid-autumn at Spirit's location on Mars. Solar energy is declining and expected to become insufficient to power further driving by mid-February. The rover team plans to use those remaining potential drives for improving the rover's tilt.

"We need to lift the rear of the rover, or the left side of the rover, or both," said Ashley Stroupe, a rover driver at JPL. "Lifting the rear wheels out of their ruts by driving backward and slightly uphill will help. If necessary, we can try to lower the front right of the rover by attempting to drop the right-front wheel into a rut or dig it into a hole."

Even a few degrees of improvement in tilt might make enough difference to enable communication every few days. The rover will gain 5 to 6 watt hours for each degree improvement in northerly tilt.

"Getting through the winter will all come down to temperature and how cold the rover electronics will get," said John Callas, project manager at JPL for Spirit and its twin rover, Opportunity. "Every bit of energy produced by Spirit's solar arrays will go into keeping the rover's critical electronics warm, either by having the electronics on or by turning on essential heaters."

Stationary Science
"There's a class of science we can do only with a stationary vehicle that we had put off during the years of driving," said Steve Squyres, a researcher at Cornell University and principal investigator for Spirit and Opportunity. "Degraded mobility does not mean the mission ends abruptly. Instead, it lets us transition to stationary science."

One stationary experiment Spirit has begun studies tiny wobbles in the rotation of Mars to gain insight about the planet's core. This requires months of radio-tracking the motion of a point on the surface of Mars to calculate long-term motion with an accuracy of a few inches.

"If the final scientific feather in Spirit's cap is determining whether the core of Mars is liquid or solid, that would be wonderful - it's so different from the other knowledge we've gained from Spirit," said Squyres.

Tools on Spirit's robotic arm can study variations in the composition of nearby soil, which has been affected by water. Stationary science also includes watching how wind moves soil particles and monitoring the Martian atmosphere.

Spirit and Opportunity landed on Mars in January 2004. They have been exploring for six years, far surpassing their original 90-day mission. Opportunity currently is driving toward a large crater called Endeavor and continues to make scientific discoveries. It has driven approximately 12 miles and returned more than 133,000 images.

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