No More A-Roving

Four-wheeling doesn’t cut it on Mars. After several months of spinning its four remaining operable wheels in deep, soft sand with no luck, NASA’s Spirit Mars rover is now officially, permanently stuck.

The rover “has encountered a golfer’s worst nightmare,” said Doug McCuistion, director of the Mars Exploration Program in Washington, D.C., “the sand trap that, no matter how many strokes you take, you can’t get out of it. Its driving days are likely over, for all intents and purposes.”

Engineers and scientists at NASA’s Jet Propulsion Laboratory, many of them profiled in our February/March cover story, would probably keep trying to free Spirit were it not for the steady creep of the Martian winter. From Spirit’s perspective, at a latitude 16 degrees below the Martian equator, the days will continue to get shorter until the winter solstice in mid-May. Worse, the rover’s solar panels happen to be tilted about nine degrees toward the southern horizon (away from the sun), further reducing the solar energy that recharges its batteries.

Even if JPL’s rover team did extract Spirit from its sand trap, serious questions linger about what kind of mobility the rover would have as it tried to drag its lame right front and right rear wheels across the sandy landscape.

So the rover team has gone to Plan B: Try to tilt Spirit more toward the sunny northern sky, and hope that when it goes into hibernation mode, it will have enough onboard power to make it through the six-month winter.

If it does survive, there’s still plenty of exciting science that Spirit could do as a stationary platform, principal investigator Steve Squyres says optimistically. “The one I’m most excited about is tracking the radio signal from Spirit,” he says.  Tracking the signal will give precise data on the planet’s wobble as it orbits the sun, which in turn gives clues about its internal structure. “If Mars has a solid core of iron, it’ll wobble in a certain, well-defined way,” says Squyres. “But if that core is liquid, it’ll wobble in an ever so slightly different way. And by tracking the rover’s position long enough, carefully enough, we can distinguish between the two.”

Squyres and the science team would need about six months to determine if the core of Mars is liquid or solid. “Totally new science, never been done before,” he says. “This is something that I didn’t really think much about when we first put a rover on the surface of Mars, because we were thinking about the geology of the surface. But when you delve deeply into what this vehicle is capable of, you find new tricks.”

Other projects suited to a stationary robot involve studying how the atmosphere interacts with the surface over time. And Squyres can do a more comprehensive study of the unusually sulfate-rich soils around the rover, which are thought to have formed long ago, when steam vents were actively changing the surface chemistry. “We expect to be able to use the wheels on the rover come springtime to slightly reposition the rover, to swing it back and forth a little bit, to move it forward and backward, bringing new patches of soil within reach of the arm,” he says.

“The most immediate issue for Spirit right now,” says rover mission project manager John Callas, “is surviving the next Martian winter. We’re clearly seeing a decrease in energy levels for the rover.”

Spirit is generating just enough power for about three more weeks of rocking and jostling in the fine Martian fluff. In past winters—three in total—the team has always been able to position Spirit with its solar arrays tilted northward to maximize energy production as the bitter Martian winter sets in. Spirit’s twin, Opportunity, on the other side of the planet, lives almost on the equator, and receives enough light year-round that it doesn’t have to hibernate. Opportunity has traveled more than 12 miles, or about three times the distance Spirit has, since the two arrived in early 2004.

Spirit needs about 160 watt-hours each day to communicate with the ground. With the current tilt, onboard power levels will drop below that by March or April, as the sun sinks lower in the sky. Rover driver Ashley Stroupe is improving the tilt by trying to move Spirit, centimeter by centimeter, up the bank of an adjoining crater to raise the left rear wheel. “On our last drive we saw a significant improvement in our northerly tilt of about one to two degrees over a short distance of a few centimeters,” she says. Stroupe and her colleagues will also try to rotate the rover in place during the coming three weeks, to improve the tilt a bit more.

Because they’re still actively driving the vehicle to put it in a better position, the rover team is still fully staffed. During the coming winter hibernation, they may cut back, but depending on what happens in the Martian spring (August/September), they may need to staff up again, depending on the budget. It costs NASA about $20 million a year to operate two mobile rovers, and a bit less when one of them is immobile.

Unless the team can improve Spirit’s tilt angle, here’s how it will go: By mid-February, the rover will reach a point where it can no longer move. By March or April, it will need to draw power from its batteries just to stay awake. This will quickly cause the batteries to drop below a minimum charge level, and trigger a low-power fault in which the rover shuts down and goes into hibernation. Everything will get turned off except a master clock linked to a timer that wakes the rover once a day, just long enough to check the charge in its batteries. During hibernation, “Every photon that hits the solar arrays goes into charging the batteries,” says Callas. If they’re well charged, the rover will wake all the way up and communicate with the JPL team, or allow the team to communicate with it. If there’s not enough juice in the batteries, the rover will go immediately back to sleep without sending a signal.

“We have to be prepared to go through a period where we’re not hearing from the rover for an extended length of time,” says Callas.

The biggest concern, he says, is the cold. The rover’s electronics are designed to tolerate temperatures as low as -40 degrees Fahrenheit when they’re operating. When the rover wakes each day to check its batteries, it will do so around midday, when the temperature should be warmer than -40. Those same electronics can tolerate temperatures down to -67 when they’re not operating. The team thinks temperatures shouldn’t go lower than -50, in which case the rover will be safe.

“But,” says Callas, “I’ll caution: Those design limits were tested for a brand new rover fresh out of the box. And this is a rover that’s been on the surface of Mars now for over six years and has endured thousands of grueling temperature cycles. So there’s no guarantee that the rover would be able to survive these colder temperatures.

“These will be temperatures that are colder than anything we’ve seen before on the surface of Mars, and we will have this extended period of time of not hearing from the rover. So that’s going to be frustrating, and that’ll be, you know, challenging for the team. But it’s something we’ll just have to be disciplined about. And eventually, when power allows, the rover will wake up and will begin to talk to us and we can resume activities in the [Martian] spring.”

He hopes.