As folks here on Earth prepare to mark the passage of another year, two NASA robots a world away are creeping up on a big milestone of their own: seven years on the surface of Mars.
The golf-cart-size rover Spirit landed on Mars on Jan. 4, 2004. Its twin, Opportunity, hit the planet's red dirt three weeks later, on Jan. 25. The rovers were originally supposed to tool around the Martian surface for a mere 90 days, looking for evidence of the planet's past water activity, but both have far outlasted their warranties.
In 2009, however, Spirit got trapped in soft sand and in March 2010, the rover stopped communicating with Earth. Still, mission scientists think the intrepid rover may wake up in the next few months. Meanwhile, Opportunity is still going strong, making its slow, steady way toward a huge crater called Endeavour.
In their nearly 2,500 days on the Martian surface, the two rovers have fundamentally changed scientists' understanding of Mars, finding lots of evidence that the Red Planet was once a much wetter, warmer place. Spirit and Opportunity have also paved the way for future rover missions by testing out technologies and showing just what is possible, researchers said. [Q & A with Mars rover manager John Callas]
And, on a more abstract level, the rovers have brought Mars closer than ever before, making another world accessible to scientists and laypeople alike.
"In addition to all the scientific discoveries, these rovers have made Mars a familiar place," John Callas, Mars Exploration Rover (MER) project manager at NASA's Jet Propulsion Laboratory in Pasadena, Calif., told SPACE.com. "Mars is now our neighborhood."
Following the water
Beginning with NASA's Mariner 9 spacecraft in the 1970s, multiple orbiters have detected evidence that liquid water once flowed on the Martian surface. Spirit and Opportunity were sent to investigate these clues, to look for more, and to put everything into a better geological context, researchers said.
The six-wheeled rovers accomplished all of this, and more. Both robots found minerals that form in the presence of water ? solid evidence that areas around their disparate landing sites had once been immersed. [Photos From Spirit and Opportunity]
"We were sent there under the paradigm, 'Follow the water,' to find the evidence for wet conditions," MER deputy principal investigator Ray Arvidson, of Washington University in St. Louis, told SPACE.com. "And we've done that."
Most of that evidence points to a wetter Mars billions of years ago. But some of Spirit's recent discoveries ? made after the rover got trapped in soft sand in 2009 ? suggest that liquid water may have trickled across the Martian surface much more recently, perhaps just hundreds of thousands of years ago.
Key find by a crippled rover
Spirit's right front wheel stopped working in 2006, impelling mission scientists to drive the rover backward instead. The rover probably would not have gotten stuck in the sand ? where it remains today ? if all six wheels had been working properly, Callas said.
But the wonky wheel proved something of a blessing. It dug a shallow trench through the Martian soil as Spirit chugged along. In 2007, one of Spirit's drag-furrows exposed subsurface deposits of pure silica, which forms when hot water reacts with rocks.
Spirit thus unearthed compelling evidence of hydrothermal systems on Mars, perhaps similar to the geysers of Yellowstone National Park, located primarily in the U.S. state of Wyoming, with sections that extend into Montana and Idaho. The find was intriguing, suggesting that large amounts of energy ?possibly life-sustaining energy ? once coursed through some Martian environments.
"Not only was there liquid water on Mars, but there were energy sources coincident with that liquid water," Callas said. "So you have a system that could potentially support an ecosystem."
Setting the technological bar high
Spirit and Opportunity were each designed to last about three months on the Martian surface, and to travel about 0.6 miles (1 kilometer). Spirit lasted more than six years, and could still wake up. To date, it has put about 4.8 miles (7.7 km) on its odometer, NASA officials said.
Opportunity has been roving for almost seven years now, covering about 16.5 miles (26.5 km) as of late December, scientists have said.
"You get what you pay for," Arvidson said. "They're just well-made American vehicles that have been thoroughly tested before launch."
Arvidson said that part of the rovers' legacy will be their tremendous longevity and the solid engineering that produced them, which should be instructive for future rover missions.
"In terms of engineering accomplishments, we've set the bar so high," he said. "These vehicles are so far out of warranty, and they've traveled so far."
The rovers are also testing and working to advance technologies for future rover missions, including NASA's Mars Science Laboratory (MSL) ? also known as Curiosity ? which is scheduled to hit the red dirt in August 2012.
Curiosity's baseline flight software, for example, was tested first on Spirit and Opportunity, Callas said. Mission scientists also recently loaded Opportunity with software called Aegis, which allows the rover to sift through its own images, find interesting targets and take follow-up pictures, autonomous from the rover team back on Earth.
"That software is going to be baseline for MSL as well," Callas said.
Will Spirit wake up?
The crippled, trapped Spirit went silent on March 22, 2010, after failing to maneuver into a position that would slant its solar panels toward the sun over the course of the Martian winter. But now spring has arrived on Mars, and the rover team is holding out hope that Spirit will warm up, wake up and check in.
"We're listening now," Callas said. "We've been listening every day."
If Spirit does bounce back, it can continue to make valuable observations even while bogged down in the dirt, scientists said.
For example, researchers hope to track the radio signal from a stationary Spirit, using the rover's movement as a proxy for the rotation of Mars, Callas said. Scientists could thus get very precise measurements of the planet's rotation, which could help them figure out how big Mars' core is ? and perhaps reveal other important details.
"If we see a slosh, we can tell whether it's a solid or a liquid core, and that would be a really, really key finding for Mars," Callas said.
Of course, there is no guarantee that Spirit will wake up. The Martian sun will be at its strongest in March 2011, so if the rover has not risen from its slumber by then, the team's optimism will likely wane.
"That would be a bad sign," Arvidson said.
Opportunity: Seven more years?
Despite continuing to boldly trek across the surface of Mars, Opportunity is showing some signs of its advanced age. Its robotic arm is arthritic in one joint, Callas said, though the other four joints are still working well. And in the summer of 2008, the gearbox in the rover's right front wheel experienced a spike in electrical current, similar to one that preceded the malfunction of Spirit's wheel.
While Opportunity was not crippled, scientists have been driving the rover backward for more than two years, Callas said, to distribute the wear more evenly amongst the rover's gear mechanisms.
Yet despite these hiccups, the rover keeps chugging along, and most of its instruments are still in remarkably good condition, as are those on Spirit.
"Our cameras are in excellent shape," Callas said. "The rovers still have 20/20 vision all around."
So, will Opportunity make it to Endeavour, which is about 3.7 miles (6 km) away at this point? How much longer will the rover tool around the Red Planet's frigid, bone-dry surface?
It's anybody's guess, Callas and Arvidson said. But the rover has turned a 90-day mission into a seven-year marathon, so it's probably wise not to bet against it ? or to count Spirit out.
"I think there's still quite a bit of adventure left for these rovers," Callas said.
- Making Mars a Familiar Place: Q and A With Mars Rover Manager John Callas
- 11 Things Americans Will Be Doing in Space in 2011
- 6 Facts About NASA's Next Mars Rover
You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall