A solar-powered rover called Spirit is shaking off the cold and dust of a nearly fatal Martian winter. At the same time, its twin, Opportunity, is inching its way across a desolate 10-mile-wide plain toward a deep, richly layered crater.
The golf-cart-size explorers were built to last at least 90 days, perhaps a year. Instead, they have lasted five years, and they have far exceeded their expected payoff in research.
The rovers have roamed the surface of Mars examining its geology and have shed light on the role that water played in the planet's history.
Phil Christensen, a scientist at Arizona State University, built a key part of the rovers' brains that allow them to evaluate the composition of rock.
NASA and the Jet Propulsion Laboratory in California, which manages the rover program, will mark the fifth anniversary of the landings later this month.
The rovers have been a particular point of pride for NASA and JPL, drawing worldwide attention when they were launched in 2003 as part of NASA's mission to "follow the water" on the Red Planet.
The program cost upward of $800 million and produced more than a quarter-of-a-million images, fascinating scientists and the public.
Spirit and Opportunity, named by a Scottsdale girl in an essay contest, hurtled through space for seven months before landing on Mars on Jan. 3 and Jan. 24, 2004, respectively.
The rovers' success represents 10 years of work for Christensen and the team who built the thermal-emissions spectrometers to identify minerals by examining light spectrum.
"It's great stuff, but in terms of understanding the history of Mars and discovering life on Mars, we're really just beginning," Christensen said.
Engineers were confident that once the rovers landed safely, they would keep sending information back to Earth for as long as six months to a year. Instead, Spirit and Opportunity are still communicating.
Martian winds occasionally have cleared Spirit and Opportunity of suffocating dust, which was expected to coat their solar panels eventually and make them useless.
"So, that's part of the reason: darn good engineering and a little bit of luck," Christensen said.
Rover achievements
In the same year they landed, the rovers made headlines by finding unmistakable geological evidence that Mars had been drenched with water at some point in its history.
The journal Science named that discovery the most important scientific achievement of 2004.
It was luck that led to one of the rovers' most exciting discoveries. Before it powered down in early 2008 to survive the Martian winter, Spirit was dragging a faulty wheel and accidentally dug a trench in the soil. The trench contained silica deposits and other minerals, which suggests Mars once bubbled with hot springs and possibly life.
Spirit also took the highest-resolution and most detailed photos ever snapped at the planet's surface. It turned its camera skyward, as well, and sent back photos of Earth, which appears on Mars as an evening star.
Opportunity is closer to the planet's equator, where its solar panels get a bit more sun and a little less dust. It sent back photos and information about its own heat shield after it survived entry. It then rolled 21 months across the planet before pulling up to the edge of a crater called Victoria, sending back its spectacular views and geological history.
It recently left Victoria and is headed toward a larger crater at a speed of about 100 yards a day.
Although the yearlong journey is risky at Opportunity's advanced age, the second crater promises to reveal more secrets of the planet's past and possible signs of water and life.
Christensen's role
Christensen is a Mars celebrity. He is operating three instruments on four craft roving or orbiting the planet, the most of any scientist. He published a controversial theory in 2003 that water once flowed on Mars and could still flow today. Other discoveries support his theory.
Images from his Mars-orbiting cameras are the building blocks for the Google Mars Internet site. One of his orbiting cameras helped determine the landing site for Opportunity.
Not all of it has been triumphs. A spacecraft carrying an earlier version of the rovers blew up when it was three days from landing on Mars. Successes still amaze Christensen, though, particularly the landing of these rovers.
"Parachutes are coming out and air bags are popping, and it's all happening in seconds. It's incredible that it works," Christensen said. "And when the thing lands and the first pictures come back, it's hard to put into words."
When Spirit landed on Jan. 3, 2004, Christensen heard cheering from the normally staid engineers around him. He thought about the hours they all had dedicated to the project.
"The payoff: It's a very emotional time," he said.
Christensen is working every day with scientists in this country and in Europe who determine how each rover will communicate and move. Some of Christensen's graduate students, who talk with the rovers and move them along the surface of Mars, are the brightest in the country, he said. They were high-school students when the rovers landed.
"There aren't many professors who can say, 'Come work for me, and you can play on Mars,' " Christensen said. "So, you get really talented people."
Find more information about Spirit and Opportunity at marsrovers.jpl.nasa.gov/home/index.html.
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