This image of the Martian surface was taken by NASA's Spirit rover,… (NASA )
After six highly successful years of exploring the red sands of Mars, NASA's rover Spirit will rove no more.
With its six wheels stuck in powdery sand and two wheels no longer working at all, the resilient little explorer will become an immobile scientific observatory -- if it can survive the harsh temperatures of the upcoming winter.
"Its driving days are likely over," Doug McCuistion, director of NASA's Mars Exploration Program, said in a telephone news conference Tuesday.
If Spirit can be awakened after what could be a six-month hibernation, researchers will use it to attempt to answer one of their most pressing questions: whether the planet has a solid iron core or a liquid one.
If the vehicle can't be revived, it will still have far surpassed scientists' original expectations and its design life of three months, traveling nearly 12 miles across the barren surface of Mars and finding strong evidence that water once altered the planet's terrain.
Spirit's twin, Opportunity, is still moving across the Martian surface farther north nearer the equator and on the other side of the planet, and continues to send back valuable data.
Opportunity has successfully weathered every Martian winter so far because "it is in a different thermal environment," McCuistion said, and the team that controls it doesn't expect any troubles for it this winter.
Spirit's problems began nine months ago as it was driving toward a pair of volcanic features named Von Braun and Goddard in the southern hemisphere. Its wheels broke through the thin Martian crust and sank into powdery sand.
Breaking free proved difficult because one of the rover's six wheels had broken down three years earlier. A second wheel became immobilized during the extrication attempts, leaving the vehicle with three good wheels on its left side and only one on its right. So far, the efforts to free it have only dug the wheels in deeper.
About a week and a half ago, with winter approaching, the team shifted its emphasis from extricating the rover to positioning it so that its solar panels would receive more sunlight, rover driver Ashley Stroupe of the Jet Propulsion Laboratory said at the news conference.
The rovers were designed and built at the La Cañada Flintridge laboratory, and engineers have been guiding them from that location.
The most likely scenario is that Spirit's power supply will get lower and lower and eventually it will shut down and go into hibernation mode until spring brings more sunshine. The question is whether engineers will be able to revive it to use that sunshine.
NASA engineers expect temperatures around Spirit to fall into the minus 40s this winter. The craft was designed to operate in temperatures as low as minus 40 degrees and to survive temperatures as low as minus 67 degrees.
"But that is with a brand-new rover fresh out of the box," said John Callas, JPL's project manager for the rovers.
If Spirit does survive, researchers hope to get many more scientific results from it, said Steve Squyres of Cornell University, the project's principal investigator. By tracking Spirit's radio signal precisely over a long time -- perhaps six months or more -- the team will be able to monitor Mars' "wobble" in its orbit. That will allow scientists to determine whether the planet has a solid or liquid core.
"This is totally new science, really fundamental stuff" that can be achieved only with a stationary platform, he said.
By looking around the craft for a long period, he added, the team will also be able to monitor how the planet's atmosphere interacts with its surface.
And finally, by continuing to dig at the current site, the team will be able to characterize the soil much more thoroughly than has been achieved anywhere else on Mars.
"The bottom line is, we are not giving up on Spirit," Squyres said.