NASA'sbeleaguered Mars rover Spirit, which has been hibernating on the surface ofMars since March, is facing its toughest challenge yet ? the harsh conditionsof the Martian winter. And the rover may lose.

Spirithas been stuck in Martian sand for more than a year, and in January, NASA abandonedall attempts of extricating the long-lived rover, rechristening it instead as astationaryprobe.

ButSpirit entered a hibernation-like state on March 22, and while missioncontrollers are cautiously optimistic about its chances of survival, they stillhave yet to hear any communication from the rover.

"Itwill be the miracle from Mars if our beloved rover phones home," said DougMcCuistion, director of NASA's Mars Exploration Program at NASA Headquarters inWashington, D.C. "It's never faced this type of severe condition before ?this is unknown territory."

Spiritand its robotic twin Opportunity have been exploring different parts of Marssince January 2004. Both rovers have far outlasted their initial 90-daymissions and are now in the middle of their seventh year exploring Mars. [Gallery:Latest Mars rover photos.]

"Ourestimation is that Spirit has experienced what is called a low-power fault,where there is not enough energy being produced by the solar arrays to make upfor the energy being used by the rover," John Callas, project manager forSpirit and Opportunity at NASA's Jet Propulsion Laboratory in Pasadena, Calif.,told SPACE.com.

Coldand powerless on Mars

Therover team anticipated that Spirit would go into its low-power hibernation modein the lead-up to the Martian winter,which runs from May through November.

Becauseit is stuck, Spirit was unable to get to a favorable slope for its fourthwinter on the planet, so the low angle of sunlight during these months limitedthe power generated from the rover's solar panels.

Duringthis hibernation, the rover suspends communications and other activities inorder to preserve available energy to recharge and heat batteries, and to keepthe mission clock ticking.

But,there's a further risk, said Callas.

"Therover may also experience a mission-clock fault," Callas said. "It'slike when you turn your hair dryer on and the lights blink momentarily. As therover actually starts to wake up, and the heaters and loads come on, this couldcause the clock to reset, and if that happens, the rover loses track ofabsolute time."

Thatwould be unfortunate. If Spirit loses track of time, it could stretch itshibernation time out to its maximum ? if it survives the Martian winter at all.Callas' expectation is that Spirit will not wake up for several months.

Ifthe rover has experienced a low power fault, he estimates Spirit could wake inlate September or early October. But, if Spirit has experienced a mission-clockfault, the rover may not come out of hibernation until November.

"Ifwe can have the rover respond to us, we can at least have a sense oftiming," Callas said. "When your alarm clock loses track of time, youcan go in and reset it. We would do the same thing with Spirit once we're ableto communicate regularly with the rover."

PagingSpirit on Mars

OnJuly 26, NASA's Mars rover team began using a paging technique called"sweep and beep," in which commands are sent repeatedly to the roverto check if it is awake and listening.

"Insteadof just listening, we send commands to the rover to respond back to us with acommunications beep," Callas explained. "If the rover is awake andhears us, she will send us that beep."

Yet,mission managers have good reasons to keep their expectations in check. Basedon models of Mars' weather and its effect on available power, there is adistinct possibility that Spirit may never respond to their communications.

Spiritis likely experiencing its coldest internal temperature yet, as the rover's heaters were not being poweredthrough the winter.

"Ourexpectation is that the rover will survive the winter, but I can't say withhigh confidence that that will be the case," Callas said. "The roveris experiencing the coldest temperatures it's ever been in ? equivalent toabout minus 55 degrees Celsius (minus 67 degrees Fahrenheit). It was designedto take those temperatures when it was a brand new rover, but right now, it'sbeen on the surface of Mars 20 times longer than it should have been."

Theharsh Martian winter

Duringthree previous Martian winters, Spirit sent signals back to Earth about once ortwice a week, and used its heaters to stay warm as it parked on a sun-facingslope for the duration of the harsh winter months. As a result, the heaterswere able to maintain internal temperatures above minus 40 degrees Fahrenheit(minus 40 degrees Celsius).

Spiritis designed to come out of its hibernation and communicate with Earth when itsbatteries are adequately charged. But, if the batteries have lost too muchpower, and the rover experiences a mission-clock fault, Spirit would start anew timer to wake up every four hours and listen for a signal from Earth for 20minutes of every hour while the sun is up.

Theearliest date the rover could generate enough power to send a signal to Earthwas originally calculated to be around July 23. So far, Spirit has not sent apeep.

IfSpirit does wake up, the rover will undergo a complete health check on itsinstruments and electronics, mission managers said.

Thespirit of Spirit

Basedon the conditions from previous Martian winters, the rover team anticipates theincreasing haziness in the sky over Spirit to offset longer daylight for thenext two months. After that, the amount of solar energy available to the roverwill increase until the southern Mars summer solstice in March 2011.

Ifmission managers have not heard anything from Spirit by March, it is unlikelythat the rover will ever come back to life.

"Thishas been a long winter for Spirit, and a long wait for us," said SteveSquyres, the principal investigator for Spirit and her twin rover Opportunity,who is based at Cornell University. "Even if we never heard from Spiritagain, I think her scientific legacy would be secure. But we're hopeful we will hear from her, and we'reeager to get back to doing science with two rovers again."

Andif Spirit does wake, the rover could contribute even more to scientificresearch on the surface of Mars.

"IfSpirit emerges from winter, we want to track the rover's radio signal,"Callas told SPACE.com. "Since Spirit was stationary ? or near stationary ?the motion of the rover is now a proxy for the motion of Mars."

Inother words, if Spirit is driving around on the surface of Mars, its radiosignal follows the motion of the rover. But, if the rover is stationary (ornear stationary), the radio signal follows the motion of the planet itself. So,by tracking the slight permutations of the rover's radio signals, scientistscan essentially observe the planet's faint wobble, giving them insight into theplanet's interior.

"There'sthat old trick where you spin an egg to see if it's raw or hardboiled,"Callas explained. "If it spins freely, it's hard-boiled, but if it wobblesa bit, then it's raw. Planets actually do the same thing, although the effectis much, much smaller."

Assuch, the rover's radio signals could provide valuable information about thedistribution of matter within the planet, and the fluidity of Mars' core.

"IfMars has a liquid core, the planet is going to have a bit of a slosh,"Callas said. "These are the kinds of measurements we'll do with the roverafter the winter, and they're very profound scientific investigations. Theseare questions we've had about Mars for decades."

Spiritand Opportunity began exploring Mars inJanuary 2004, on missions that were originally planned to last three months.While Spirit has been stationary since April, Opportunity is in good health,and is currently driving toward a large crater named Endeavour.

Opportunitycovered more distance in 2009 than in any previous year on the Martiansurface. This month Opportunity spotted its firstdust devil on Mars, though Spirit has seen the tornado-like wind phenomenonin the past.

Bothrovers have made important discoveries about wet environments on ancient Marsthat may have been favorable for supporting microbial life.