The Mars Exploration Rovers have weathered two drab winters on the Martian surface, and mission managers are already looking ahead to yet a third chilly season. All this from a mission that was only designed to last 90 days.
The Spirit rover is searching for a spot to stick it out during the upcoming Martian winter, which will last from March 2008 through October 2008, according to a statement from NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Although Mars has a tilt similar to Earth's, Martian seasons last longer because the planet takes almost twice as long to circle the Sun—almost 687 Earth days.
"When you're talking about the rovers surviving winter on Mars, planning many months in the future is really important," said Steve Squyres, principal scientist of the Mars Exploration Rover team at Cornell University in Ithaca, New York. "But it's too soon to tell where we might situate them."
Science on a hill
The solar-powered robots take an energy hit during the dim conditions, Squyres explained.
"The sun gets very low in the sky, giving us a lot less solar power to work with," Squyres said. Fewer gusts of wind are around to clean dust from the rovers' solar panels gathered during recent dust storms, he added, which compounds the problem.
So how do the rovers survive the winter while still doing plenty of science, as they did in 2004 and 2006?
"We try and park them on a broad, north-facing slope that helps them gather more sunlight," Squyres said. Scientists not only prefer a good slope, but a large and scientifically interesting one. "During the winter, it's essentially science on a hill," Squyres said. "If we're stuck on a small slope, we're much more limited in what we can investigate."
Even if a rover gets "stranded" on a tiny hill, however, it's not necessarily the end of the world.
"Being stuck in one place allows us to do science we normally can't because we're in a hurry, driving the rovers around," he said. "It also allows us to intensively study a single, small area, like landers do."
During the last winter, Squyres said, Spirit studied a palm-sized area of Martian soil for months.
"It's probably the best-studied patch of dirt in the solar system beyond Earth," he said.
Although Squyres said the rovers have led impressively long lives to this point—each now more than 1,324 Earth-days beyond their warranty—he said it's impossible to predict their ability to survive.
"Their long lives are experiments in progress," Squyres said. "They could die three years from now, or in the next hour. There's simply no way for us to know, so we're trying to do as much as we can with what time we might have left."
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