Six months after the Mars rover Spirit became trapped in a patch of soft soil, its controllers are preparing to send a set of commands that they hope will free the robotic explorer, NASA announced Thursday.

In what Mars Exploration Program director Doug McCuistion called a "bittersweet press conference," mission managers offered some details of the planned extraction maneuver but noted that its success was far from guaranteed.

Back in April, Spirit experienced what McCuistion called "the equivalent of falling through the ice on a frozen pond," its wheels punching through a hard crust of soil to a troublesome soft patch underneath. By early May, attempts to move the rover had partly buried its wheels in the dirt. That caking of soil on the wheel treads, noted rover project manager John Callas, has made regaining traction a taller order. To make matters worse, Spirit may be lodged against a rock that is inhibiting the movement of one of its five functional wheels, and the rover has been dragging a sixth wheel for years since it stopped working in 2006.

Both Spirit and its twin rover Opportunity have far exceeded their planned lifetimes. Originally tapped for 90-day missions, the rovers will celebrate their six-year anniversaries of reaching the Red Planet in January.

The first commands for the extrication plan will be sent to the rover Monday. The goal is to move the rover forward, retracing the route it took in backing into a location now known as Troy. In so doing, Spirit should avoid having to climb uphill and will not have to break any new ground.

The team cautioned that Spirit may not be freed from its predicament for weeks or even months, if ever, repeatedly noting that the rover's current location may prove to be its final resting place. "Unfortunately Spirit may have met its match in this one," McCuistion said.

Photo: NASA/JPL-Caltech