After months of tests and analysis, engineers plan to beam commands to NASA's Spirit Mars rover Monday, kicking off a long-awaited attempt to free the hardy craft from the talcum powder-like soil of a hidden crater that trapped it last April.
"Spirit's facing the most challenging situation it's seen yet on the surface of Mars," Doug McCuistion, director of NASA's Mars exploration program, said Thursday. "We know a lot of people around the world...view Spirit with great affection, exploring the Red Planet along with it, experiencing the excitement, seeing new and exciting vistas, seeing new landscapes, uncovering some incredible new knowledge about our sister planet.
"I'd like everybody to be hopeful, but I'd also like them to be realistic," he said. "If Spirit cannot make the great escape from this sand trap, it's likely that this lonely spot, straddling the edge of this crater, might be where Spirit ends its adventures on Mars."
Designed to operate for just three months on the frigid surface of Mars, Spirit and its twin rover, Opportunity, have been exploring opposite sides of the planet since early 2004, collecting data in concert with orbiting spacecraft to help scientists understand the role of water in the Martian environment.
Chalking up a steady stream of discoveries over the past five years, the unexpectedly long-lived rovers are held in high esteem by the scientists and engineers who drive them across the surface of Mars and eagerly await the data they send back.
"In many ways, we think of these rovers kind of as our children that we've sent off into the world way too early," said Ashley Stroupe, a rover driver at the Jet Propulsion Laboratory in Pasadena, Calif. "And like most parents, when their kids go off to college, we can't reach out to help them every time they really need us. So it really is a bond, not just between us and the rover, but also the team has become a very close family as well."
Last April 23, the six-wheel Spirit was slowly rolling backward on the western side of a feature known as "Home Plate," heading toward the south and a pair of volcanic structures that scientists wanted to examine. The rover was driving backward because its right front wheel stopped working in 2006.
The ground to the south of Spirit looked normal, but as it rolled along, its wheels broke through an upper-crust-like layer of soil and into a softer, unseen material.
"Essentially, the rover was driving on what we call a dirt crust," said John Callas, the project manager of the Mars exploration rovers at JPL. "It was a hard surface that we broke through, and underneath this material, camouflaged underneath, was this loose, fine material where the rover is challenged right now."
Scientists later determined that Spirit's path was straddling the rim of an ancient, 26-foot-wide crater just beneath the surface. The crater was filled in with sulfate sands that formed layers with different compositions.
Initial attempts to drive out in a crablike fashion by turning the front and back wheels in the same direction only made matters worse.
Pictures from navigation cameras on the rover show its forward and rear wheels almost buried in the soil, their treads caked with a powdery coating that reduces traction. Even worse, photographs show a pyramid-shape rock sticking up from the soil directly below Spirit's body that threatens to rub against the belly, possibly lodging in an indentation. If the rock ends up bearing any of the weight of the rover, traction could be reduced even more.
NASA managers decided to halt any additional attempts to free Spirit until engineers could complete a thorough analysis using a full-scale mockup and simulated Martian soil.
"Unfortunately, Spirit may have met its match in this one," McCuistion said. "We will see if we can get it out of this talcum powder-type soil that laid beneath a seemingly innocuous surface crust that we broke through...The rover teams have been working very hard since April, they've been testing, strategizing, analyzing, and modeling to figure a way out. We even called experts in soil mechanics and mechanical systems in to try to help us understand the environment. But there's only so much you can do on Earth to simulate Mars."
Late Monday, commands will be uplinked to Spirit in an attempt to drive north, back along the furrows its wheels dug as the rover moved into the sand trap last April. Engineers will find out how the move went on Tuesday. No one expects a quick extraction, and engineers said it likely will take weeks or months to either free the rover or determine that it can't be done.
"Our best plan at this point is to try to drive forward, retracing our steps as we drove in," Stroupe said. "And we believe this is our best plan for several reasons. One is that we believe this softer material may be easier to plow through than trying to break through the crust and cut new tracks. So if we follow our old tracks out, we may be able to make better progress.
"We have very little ground clearance under the vehicle. Wheel turns cause us to sink further into this material, and there is no guarantee that any plan we come up with will succeed in extricating the vehicle," she said. "This is going to clearly be a very long process, to either get to extrication or perhaps even to determine if extraction is going to work."
The team's progress will be assessed in February. Depending on the success or failure of the work at that point, NASA could opt to continue with additional attempts or decide to call it off. Even in that worst-case scenario, scientists could still use Spirit's instruments to study nearby rocks and soil, and to monitor the martian weather.
But Stroupe hopes it won't come to that.
"I think a lot of us, while we're waiting for that plan to execute (Monday), will not get a lot of sleep," she said. "But regardless of the outcome, none of us can have anything but primarily positive emotions about this mission. It's been such an incredible experience, we've come so far beyond what we thought we would accomplish...We're so proud of them, and we're so thrilled to have been part of this project. It will be sad to see them go. But we're not ready to let go yet, and we don't plan to let go yet. We still have a lot of work to do."