Two robots the size of golf carts were given 90 days to squeeze as much science as possible from the barren, dust-swept terrain of Mars. After that, scientists expected nothing more from them than death.
Nearly four years after their warranties expired, however, the Mars Explorations Rovers (MERs) "Spirit" and "Opportunity" continue to play productively in the red dirt.
Spirit celebrates its fourth anniversary of Martian work on Jan. 4, the day it landed in 2004, followed by Opportunity on Jan. 25. Those four Earth years since landing convert to 2.25 Martian years, or 1,501 Martian days called "sols."
"We never thought we'd still be driving these robots all over Mars," said Mark Lemmon, a planetary scientist at Texas A&M University and member of the rover science team. "We joked about driving Opportunity into Victoria Crater, but now we're there, and we're looking at doing even more science. Each day they still work is an amazing one."
Since the rovers bounced onto Mars' surface, they have collectively driven more than 11.8 miles (19.1 kilometers) and snapped more than 210,000 images. That's roughly 55 standard DVD movies worth of uncompressed data.
Scientists have used this information through the years to crank out more than 100 studies about the planet's geologic past "with many more in progress," Lemmon said.
"It's been a great year for the rovers and we're getting deeper into Martian history than we've ... done before," Lemmon said. "These robots have entirely changed the way we view Mars."
Those views include support for the existence of water on Mars, at least in the past, in the form of silica and meteorites.
In addition to that evidence, the year 2007 inflicted a global dust storm on the rovers. Although indirect sunlight powered the rovers through the dusty conditions, more than 96 percent of direct sunlight to their solar panels was filtered out.
"It was scary there for a while," Lemmon said of the low-light conditions that nearly drove the rovers to a permanent standstill.
Despite the nerve-wracking task of keeping both rovers power-positive — and their electronic circuits from snapping in the Martian cold — Lemmon explained that new science is still trickling out as a result of the weather event.
"The Mars orbiters looked down on the dust storm when it happened, but they didn't measure changes on the ground like the rovers did," he said. "The rovers are really helping us to better understand these storms."
Winter parking spot
Now that the dusty, five- to six-month Martian summer is waning and winter is creeping up, earthly operators have pinned down an over-winter parking spot for Spirit.
The rover suffered software glitches early in the mission, and now drives backward as its front right wheel is indefinitely stuck. Making matters worse is the literal fallout from the recent dust storm.
"Right now, we're working with the dustiest rover we've ever had," Lemmon said, who does not expect whirling dust devils to clean off the rover's coated solar panels any time soon. "As a result, we pretty much consider Spirit parked."
Lemmon said Opportunity, however, is in good shape to continue exploring and the team has no definitive date for parking the adventurous machine.
"Opportunity has much cleaner [solar panels] than Spirit," Lemmon said, "so there's no discussion of racing it to a north-facing slope for the winter."
Scientists used the north-facing-slope trick in the past, which helps maximize direct sunlight to the rover's solar panels during the dim Martian winter.
While Opportunity continues to maneuver around Victoria Crater, Spirit is presently resting on a slope of Home Plate — a layered outcrop of rock in the shape of a baseball home plate. "It'll stay in one place for a long time, but we'll still be able to do some science," Lemmon said.
That science includes watching the sky for water-crystal clouds and taking atmospheric measurements, but the rover may also witness a potential asteroid impact later this month.
"I'm not optimistic for the rovers seeing anything ... [but] we have some hope of seeing the impact cloud as it disperses around the planet," Lemmon said. "I like the thought of a birthday present from Mars. It'll certainly contribute more excitement to the mission."