Rover managers will send the first in a new set of computer commands on Monday in an effort to maneuver Spirit out of the fluffy, loose soil where it's been stuck for the last six months. In a teleconference briefing for reporters, the Mars rover team said it was "optimistic" that Spirit would be able to resume its peregrinations across the Martian surface.
But in admitting that this is by far the most serious threat Spirit and its twin, Opportunity, have faced in the nearly six years they've been exploring Mars, the team members seemed to be preparing for the inevitable goodbye.
"This is bittersweet," said Doug McCuistion, director of the Mars exploration program at NASA. "Spirit did the equivalent of falling through the ice and has not been able to pull itself out."
Ashley Stroupe, a rover operator at the Jet Propulsion Lab in La Cañada-Flintridge, where the project is managed, spoke of "the tremendous bond" that has developed between team members and the computerized machines. The robots have gathered vast quantities of data about the Red Planet and its history while outliving every expectation for their survival.
Stroupe said she'd come to think of the rovers "as children you send off into the world. . . . We are very hopeful, but we're very concerned."
Expected to last just 90 days when they landed on Mars in 2004, the two rovers have survived more than five years and three Martian winters. They have surmounted not just planet-wide dust storms, but also technical glitches, including one that left Spirit with a gimpy front wheel that forced rover drivers to drive it backward. It was while maneuvering backward along the edge of a rocky plateau dubbed Home Plate -- in the giant Gusev Crater just south of the equator -- that Spirit got stuck.
The rover had broken through a surface crust, something like the brittle covering on a creme brulee dessert, and its wheels had sunk deeply into the sulfate-rich sand underneath. Unlike sand on Earth, Mars' dry atmosphere and lower gravity prevents the particles from bonding, making the subsurface soil almost as fluffy as cornstarch.
After the rover got stuck in April, JPL scientists simulated the mishap with a sandbox and test rover. But tests to devise an escape route for Spirit were not completely successful.
"We haven't found a clear solution for how to get Spirit out of its predicament," said John Callas, the rover project manager.
For now, the plan calls for the rover to try to back out the way it went in. The first commands will turn the wheels six times before stopping so scientists can assess the situation.
Besides the fluffy soil, rover scientists are concerned about a small rock under the robot. Further efforts to free the rover could cause the undercarriage to snag on the rock. Tests showed that if that happens, the wheels will lose traction and the rover could become permanently stuck.
In such a scenario, Spirit could still do science, but as a station, not a rover. Also, without the ability to move into a position that gets good sunlight to wait out the harsh Martian winter, its batteries could be drained, dooming the robot.
Even if the initial efforts Monday are unsuccessful, operators will continue their efforts to salvage Spirit at least through February, when a NASA review panel is scheduled to discuss the rovers' fate. If Spirit is still stuck, the panel could call off the rescue.
"If Spirit cannot make the great escape from this sand trap, this might be where Spirit ends its adventure on Mars," McCuistion said.
Over their nearly six years of exploration on Mars, the two rovers have helped unravel the planet's geological past. They also found evidence that water once flowed on the surface.
Opportunity is currently on the opposite side of Mars driving toward a large crater called Endeavor.
The rovers have attracted a worldwide fan base that has followed their every move and hardship. There's even a “Free Spirit” campaign, which has its own logo emblazoned on T-shirts sold at the JPL store.