Image: Chart of distances driven by moon and Mars vehicles / Karl Tate
Distances driven by wheeled vehicles on the moon and Mars.
updated 7/30/2010 5:45:57 PM ET 2010-07-30T21:45:57

NASA's beleaguered Mars rover Spirit, which has been hibernating on the surface of Mars since March, is facing its toughest challenge yet the harsh conditions of the Martian winter. And the rover may lose.

Spirit has been stuck in Martian sand for more than a year, and in January, NASA abandoned all attempts of extricating the long-lived rover, rechristening it instead as a stationary probe.

But Spirit entered a hibernation-like state on March 22, and while mission controllers are cautiously optimistic about its chances of survival, they still have yet to hear any communication from the rover.

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

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

"Our estimation 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 up for the energy being used by the rover," John Callas, project manager for Spirit and Opportunity at NASA's Jet Propulsion Laboratory in Pasadena, Calif., told

Cold and powerless on Mars
The rover team anticipated that Spirit would go into its low-power hibernation mode in the lead-up to the Martian winter, which runs from May through November.

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

During this hibernation, the rover suspends communications and other activities in order to preserve available energy to recharge and heat batteries, and to keep the mission clock ticking.

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

"The rover may also experience a mission-clock fault," Callas said. "It's like when you turn your hair dryer on and the lights blink momentarily. As the rover actually starts to wake up, and the heaters and loads come on, this could cause the clock to reset, and if that happens, the rover loses track of absolute time."

That would be unfortunate. If Spirit loses track of time, it could stretch its hibernation 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.

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

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

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

"Instead of just listening, we send commands to the rover to respond back to us with a communications beep," Callas explained. "If the rover is awake and hears us, she will send us that beep."

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

Spirit is likely experiencing its coldest internal temperature yet, as the rover's heaters were not being powered through the winter.

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

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

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

The earliest date the rover could generate enough power to send a signal to Earth was originally calculated to be around July 23. So far, Spirit has not sent a peep.

If Spirit does wake up, the rover will undergo a complete health check on its instruments and electronics, mission managers said.

The spirit of Spirit
Based on the conditions from previous Martian winters, the rover team anticipates the increasing haziness in the sky over Spirit to offset longer daylight for the next two months. After that, the amount of solar energy available to the rover will increase until the southern Mars summer solstice in March 2011.

If mission managers have not heard anything from Spirit by March, it is unlikely that the rover will ever come back to life.

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

And if Spirit does wake, the rover could contribute even more to scientific research on the surface of Mars.

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

In other words, if Spirit is driving around on the surface of Mars, its radio signal follows the motion of the rover. But, if the rover is stationary (or near stationary), the radio signal follows the motion of the planet itself. So, by tracking the slight permutations of the rover's radio signals, scientists can essentially observe the planet's faint wobble, giving them insight into the planet's interior.

"There's that 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 wobbles a bit, then it's raw. Planets actually do the same thing, although the effect is much, much smaller."

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

"If Mars 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 rover after the winter, and they're very profound scientific investigations. These are questions we've had about Mars for decades."

Spirit and Opportunity began exploring Mars in January 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.

Opportunity covered more distance in 2009 than in any previous year on the Martian surface. This month Opportunity spotted its first dust devil on Mars, though Spirit has seen the tornado-like wind phenomenon in the past.

Both rovers have made important discoveries about wet environments on ancient Mars that may have been favorable for supporting microbial life.

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