The rover may have stopped roving, but its Spirit lives on.
It's been quite a run for NASA's golf-cart-sized, solar-powered rover originally designed for a 90-day jaunt on the martian surface. Earlier this month, Spirit passed its sixth year exploring the red planet. But at a news conference today, wistful mission controllers announced what many had suspected: With two of its six wheels malfunctioning, Spirit is intractably stuck in loose soil in an area of Mars named Troy (see photo, left). There the rover will stay until its batteries die or its systems experience a catastrophic failure.If Spirit can survive the coming martian winter, it will begin functioning as a stationary platform, conducting science studies that had not been part of its original itinerary but will be important nevertheless.
"The most immediate issue is surviving the next martian winter," said John Callas, the mission's project manager of NASA's Jet Propulsion Laboratory in Pasadena, California. Over the three past winters, mission controllers have been able to tilt the rover enough toward the sun to generate sufficient electrical power to keep its electronics from freezing to death. But now, in addition to being stuck in the sand—"like a golfer's worst nightmare," Callas said—Spirit is tilted 9 degrees away from sun. "We're clearly seeing a decrease in energy levels," he said. If that situation can't be improved, Spirit's power levels could drop below what's needed to continue operating, about 160 watt/hours per day.
Even if that happens, Callas explained, Spirit would go into what's called a low-power fault: All systems automatically shut down except for the rover's master clock. Then, each morning, Spirit would wake up slightly and check its power levels. If its batteries have accumulated enough charge, Spirit's radio would check in briefly with NASA. Otherwise, the rover would go back to sleep until the next day. This routine could persist for the next 6 months, Callas said, and controllers must be prepared not to hear from the rover for an extended period.
"We have hope that Spirit will survive the cold, dark winter," echoed Steve Squyres of Cornell University, principal investigator for both rover missions. When Spirit's six wheels were operating, he said, the rover team felt a "relentless imperative" to drive it. But now mission controllers must "focus on new classes of science," the most exciting of which is using Spirit's radio signal to track the motion of Mars both in its orbit and in its spin on its axis. That wobble, Squyres explained, reflects the martian internal structure. "If the core is made of iron, Mars will wobble one way; if it's liquid, it will wobble a different way." And by tracking the radio signal for the next 6 months or so, scientists think they can characterize the martian core. "We didn't think much about this at beginning of the mission," he said, "but now with a stationary rover we can focus on it."
Spirit can continue to serve science in two other ways, Squyres told reporters. First, its instruments can conduct the most intense study yet of the martian atmosphere. And second, it turns out that the place where Spirit got trapped is "extraordinarily rich" in sulfate salts—minerals that were ejected by hot, caustic steam vents. In addition, "when we look very carefully at soil we see evidence of layering," he said. That suggests thin films of water have formed at the location in the past, so studying the chemical composition can reveal much more about the martian past. "There's a really interesting story here," he said.