"This is going to be a lengthy process, and there's a high probability attempts to free Spirit will not be successful," cautions Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters in Washington.

Spirit has been lodged at a site scientists call "Troy" since April 23, 2009. Spirit was driving backward and dragging its inoperable right front wheel when the rover's other wheels broke through a crust on the surface that was covering a slippery sand underneath.

After a few drive attempts to get Spirit out, the rover began sinking deeper in the sand trap. Driving was suspended to allow time for tests and reviews of possible escape strategies.

Data show Spirit is straddling the edge of a 26-foot-wide crater that had been filled long ago with sulfate-bearing sands produced in a hot water or steam environment: map. The deposits in the crater formed distinct layers with different compositions and tints, and they are capped by a crusty soil that Spirit's wheels broke through.

Engineers have now plotted an escape route from Troy that heads up a mild slope away from the crater.

"Our preparations to resume driving have been extensive and thorough," said John Callas, project manager for Spirit and Opportunity at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

"We've used two different test rovers here on Earth in conditions designed to simulate as best as possible Spirit's predicament. However, Earth-based tests cannot exactly replicate the conditions at Troy."

Researchers expect the extraction process to be long and the outcome uncertain based on tests they've done here on Earth. "After the first few weeks of attempts, we're not likely to know whether Spirit will be able to free itself," notes McCuistion.

Spirit has six wheels for roving the Red Planet. The first commands will tell the rover to rotate its five working wheels forward approximately six turns. Engineers anticipate severe wheel slippage, with barely perceptible forward progress in this initial attempt.

Spirit will return data the next day from its first drive attempt. The results will be assessed before engineers develop and send commands for a second attempt. Using results from previous commands, engineers plan to continue escape efforts until early 2010.

"Mobility on Mars is challenging, and whatever the outcome, lessons from the work to free Spirit will enhance our knowledge about how to analyze Martian terrain and drive future Mars rovers," McCuisition said.

Even if Spirit remains in place, it could continue making important discoveries about Mars. The "sand trap" turns out to be a region of great scientific interest.

"The soft materials churned up by Spirit's wheels have the highest sulfur content measured on Mars," says Ray Arvidson, a scientist at Washington University in St. Louis and deputy principal investigator for the science payloads on Spirit and Opportunity.

"We're taking advantage of its fixed location to conduct detailed measurements of these interesting materials." The rover's work at Troy augments earlier discoveries it made indicating ancient Mars had hot springs or steam vents, possible habitats for life.

Spirit and its twin rover landed on Mars in January 2004. They have explored Mars for five years, far surpassing their original 90-day mission. Opportunity currently is driving toward a large crater called Endeavor.

Stay tuned to Science@NASA for updates on NASA's efforts to free Spirit.

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