The effort to free NASA's Spirit rover, currently mired in sand on Mars, will begin in earnest today, when engineers team send the first escape commands to the stuck robot to try to move out of its trap late tonight.

Spirit has been stuck in the Martian dirt since April, when drove into a spot of soft terrain called "Troy" back in April.

Mission managers have spent the past six months devising strategies to move the rover out of the sand pit. They tested them with model rovers back on Earth that are essentially replicas of Spirit and its twin, Opportunity.

• SLIDESHOW: Mars Rover Spirit's best photos.

Rover drivers decided that the best strategy would be to have Spirit backtrack, moving forward to retrace the tracks that brought it into its current predicament. (The rover's broken right front wheel has meant that Spirit's primary mode of driving is backwards.)

NASA announced the strategy last week, and the rover team plans to write the commands for Spirit to drive out of Troy today and send them up to the rover in the wee hours of Tuesday morning.

The results of the drive are expected to come back sometime on Tuesday, after which the team will spend at least a day analyzing them before sending up any more commands.

The precarious nature of the embedding — Spirit's wheels are dug in to their hubs, one wheel has stopped spinning and a rock is sitting underneath the rover's belly, possibly even touching it — means that the team has to be very careful about any movements they make with the rover.

"This is by far the most complicated and complex" embedding the team has had, said John Callas, project manager for the Mars Exploration Rovers (MER) at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Callas said the team expects any motion the rover makes to be small at first and the process to take several weeks.

If Spirit has not yet been freed by the time or the rovers' annual review rolls around in February, NASA officials will weigh whether to keep trying, to keep Spirit where it is and continue doing science there or to call it quits.

The team is upfront about the chances of getting Spirit out. Ray Arvidson, deputy principal investigator for the rovers (and based at Washington University in St. Louis) has told fans of Spirit to be "hopeful, but realistic."

And whether or not Spirit makes it out, NASA considers the mission an unqualified success, as both MER rovers have last 24 times as long as initially planned, as they come close to rounding out their sixth year on the red planet.