The first two attempts to dislodge the stuck Spirit rover from a calamitous Martian sand pit yielded little progress, but the mission's project manager said he is encouraged the wheeled robot even moved at all.

Spirit's front hazard camera shows the rover's wheels dug into the Martian soil on Thursday. Credit: NASA/JPL

Engineers commanded the first attempt to extract Spirit on Tuesday, but tight constraints on the rover's tilt angle halted the drive as soon as it began. A second drive on Thursday resulted in the first discernable movement of Spirit in months.

The rover was instructed to drive straight ahead in two steps of two-and-a-half meters, or 8.2 feet. The second half of Thursday's drive did not occur because ground controllers had imposed a movement limit of 1 centimeter, or 0.4 inches.

"We did see the left front wheel climb a little bit, we saw the right front wheel shift forward slightly. But we also did see some sinkage on the rear wheels, so we want to balance all that out," said John Callas, project manager for the Spirit and Opportunity rovers.

Sensors detected the center of the rover moved 12 millimeters, or one-half inch, forward, 7 millimeters (0.3 inches) to the left, and 4 millimeters (0.2 inches) down, according to NASA.

"It's only one step, so one can't base any kind of trend on that," Callas sad. "But it's encouraging that the vehicle is moving, so we'll continue and the plan ahead is to do something very similar, command another five meters of driving, two two-and-a-half meter steps, and see how the vehicle does."

Officials planned the third drive Friday and Spirit was expected to execute the commands Saturday. The results should be received back on Earth by Saturday night or Sunday, Callas said in an interview Friday night.

"The right rear wheel will be commanded at a slightly slower speed to allow the right middle wheel to gain more traction," NASA said in an update posted on the mission's Web site. "This is to mitigate the potential for a wheelie by the right middle wheel. There will also be tighter limits on the rover's change in direction (yaw) and the angles of the suspension system."

During this week's drive attempts, controllers observed unexpected movement of a bogey in Spirit's rear suspension system. Callas said ground teams want to better understand the motion before committing to more ambitious drives.

"We're still taking baby steps because we want to understand the nature of the vehicle," Callas said. "We want to understand that characteristic before opening up the envelope here and continuing to drive further."

A fourth drive is targeted for Monday, pending the results of Saturday's move.

Spirit has been trapped along the edge of a 26-foot-wide sand-filled crater since April. Scientists have dubbed the location "Troy."

"It's remarkable that we can have that level of control over a robotic system that's hundreds of millions of miles away in a hostile environment," Callas said. "It's frustrating in that we want to get Spirit unstuck. We want to continue driving and exploring on Mars. We don't know whether we'll be successful. We don't know how long it will take, but we're going to give it our best shot."

Spirit is wrapping up its sixth year on Mars, having trekked 4.8 miles across the bed of Gusev Crater and into an area of highlands named the Columbia Hills.