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Image: Rover on Mars
NASA / JPL-Caltech
This view from the Spirit rover's front hazard-avoidance camera shows the wheel at left elevated ever so slightly from its previous position. The rover has been stuck in a Martian sand trap since April. The shadow of its robotic arm can be seen at center.
By Alan Boyle Science editor
msnbc.com
updated 11/20/2009 8:57:02 AM ET 2009-11-20T13:57:02

For the first time in seven months, the Mars rover moved.

NASA's Spirit rover has been stuck in a slanting sand trap nicknamed "Troy" since April. The problem started when golf cart-sized robot broke through a crust of soil and slipped into softer stuff beneath. Ever since then, the rover team has been trying to figure out how to get it out.

The crucial commands telling Spirit to spin its wheels were sent up this week — and before-and-after photos sent back down to Earth on Thursday revealed "very slight forward movement," NASA reported.

That movement didn't come easy: For months, engineers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., have been working out a detailed strategy for extracting the rover from the fine, slippery sand in which it has been stuck. The job is more complicated because one of Spirit's six wheels broke years ago. That means the rover has had to drive backwards, dragging its front wheel behind.

The engineers finally decided the best course would be to have Spirit retrace its steps downslope by driving "forward" — rather like driving a car out of a winter snowdrift that it backed into. The driving commands that were uploaded to the rover called for making two spins of the wheels, each of which would be the equivalent of 8.2 feet (2.5 meters) of travel. NASA didn't expect the rover to move that far, but the space agency was hoping that the spins would make at least a small difference in Spirit's position.

The first time the maneuver was attempted, on Tuesday, the rover aborted the commands less than a second after the wheels started to spin. NASA said the commands set off an alarm in the rover's electronic brain because it looked as if the maneuver would change the machine's tilt too dramatically.

Slideshow: Month in Space As it turned out, Spirit had inaccurate information about how much it was tilted to begin with. The commands were revised to reflect a more accurate read of the rover's position and sent back up to Mars on Thursday.

This time, the commands worked, as shown in the before-and-after imagery.

"Spirit's left front wheel has become slightly less buried in the soft soil in which the rover had become embedded about six months ago," NASA reported in its mission update late Thursday. "The right front wheel, which has not been usable for driving since 2006, has been pushed perceptibly forward by the drive. The amount of forward motion is less than 1 percent of the distance that would have been covered on firm ground by the amount of wheel rotation commanded in the drive."

It could take months to get Spirit out of its predicament. "Extrication drives are expected to make slow, if any, progress in coming weeks, and the probability of success in escaping from Troy is uncertain," NASA said.

Spirit and its twin rover, Opportunity, landed on opposite sides of the Red Planet in January 2004. Mission planners expected them to last at least 90 days on Mars, and the robots have far exceeded those expectations. The rovers' missions have been extended several times at a fraction of the initial cost of $820 million. Along the way, Spirit and Opportunity have confirmed that liquid water once existed on what is now a cold, dry planet, in volumes that could have sustained life as we know it.

Even Spirit's forced immobility hasn't been a loss. The rover science team has been conducting detailed observations of the area around Troy even while engineers have been working on the escape plan. If Spirit fails to make its escape, it could continue to do useful science in place.

© 2013 msnbc.com Reprints

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