Five years and counting for the intrepid Mars rovers
BY STEPHEN CLARK
Posted: January 24, 2009
Five years ago this weekend, the Opportunity rover made what scientists have dubbed an interplanetary hole-in-one, fortuitously landing in a small Martian crater containing rock-hard evidence that Mars was once a warmer, wetter planet.
Opportunity parachuted to the Martian surface and bounced to a landing inside the crater Jan. 24, 2004. Its landing zone was a sweeping sandy plain called Meridiani Planum.
Scientists named the 100-foot-wide crater Eagle after the golf term and the Apollo 11 lunar module that ferried the first astronauts to the moon. Bedrock exposed in the rim of the crater held firm mineralogical clues that Meridiani Planum was home to an ancient salty sea.
Opportunity joined its twin sister rover Spirit, which had touched down on Mars three weeks earlier at a place called Gusev Crater.
Since then, the rovers have driven more than 13 miles across alien deserts and hills. A team of scientists and engineers on Earth have operated the robots every step of the way, living vicariously through the nearly $1 billion mission.
"These two vehicles have become our human proxies on Mars," said John Callas, the rovers' project manager at NASA's Jet Propulsion Laboratory. "They have given us an experience on Mars as if we were there ourselves."
NASA chose to dispatch two identical rovers to Mars in 2003 to hedge against the possible failure of a single spacecraft. The decision ended up being a boon to planetary geologists, essentially doubling the scientific return.
The mission's cost has been bested by the rovers' long list of accomplishments, according to NASA officials.
"The public has gotten their money's worth in Spirit and Opportunity," said Doug McCuistion, director of the agency's Mars exploration program.
McCuistion said the mission's financial return is evident in investments in U.S. industries and the development of new engineering technology made possible by the rovers.
But that's just the beginning, McCuistion said.
"There have been significant discoveries that have truly changed the textbooks and changed the public and scientific view of Mars. The returns are almost immeasurable at that point," McCuistion said.
The robots are responsible for much of what scientists know about Mars today. That knowledge simply wasn't available five years ago, Callas said.
"They have revealed that Mars was Earthlike at one time in its ancient past. Mars had sustained liquid water on its surface that was flowing and evaporating. There were environments like hydrothermal vents that created places that can be classified as habitable," Callas said.
These revelations raise two fundamental questions about Mars, according to scientists. Was there life on Mars? Why did Mars change?
The answers to those questions have even more significance for life on Earth. If Mars changed, could the same thing happen to Earth?
"Those are the questions that the rovers have revealed to us, and it will take future missions to really answer those kinds of questions," Callas said.
Spirit and Opportunity have climbed hills, explored craters and traversed unstable sand dunes to make the discoveries.
More challenges await the rovers as Spirit studies Home Plate, a rocky plateau scientists believe was once a hot spring. Opportunity is trekking toward Endeavour Crater, a vast hole in the ground spanning 13.7 miles across.
Spirit has explored Home Plate off-and-on since 2006, and the rover has been struggling to drive atop the outcrop since it emerged from a life-threatening dust storm in November.
Before examining Home Plate, Spirit climbed the Columbia Hills to study rocks at the summit.
Unlike Opportunity, which is closer to the Martian equator, Spirit has to be parked and tilted toward the sun during the peak of winter. The efforts are aimed at collecting as much power as possible through the solar panels.
Opportunity is currently driving toward Endeavour, the fifth crater on the rover's Martian sightseeing list. The craft already conducted detailed science investigations and captured jaw-dropping images at Eagle, Endurance, Erebus and Victoria craters.
The journey to Endeavour began in September and officials predicted the trip could take up to two years.
"This crater is staggeringly large compared to anything we've seen before," said Steve Squyres, the rovers' principal investigator at Cornell University. "I would love to see that view from the rim, but even if we never get there, as we move southward we expect to be getting younger and younger layers of rock on the surface."
Opportunity has already begun pausing periodically to study interesting rocks on the way to Endeavour, which is currently about seven miles south of the rover.
Another legacy of the mission will be its longevity.
The rovers have outlasted even the most optimistic predictions. The wheeled robots have lived more than 20 times longer than their original three-month missions.
"I was an optimist from the beginning," Callas said. "But I don't think anyone thought that they could last three Martian winters and keep going."
The rovers have faced down harsh, cold winters, dust storms and mechanical breakdowns. Both robots are now showing signs of fatigue.
Dust has accumulated on Spirit's solar panels, reducing the rover's power output to about 25 percent of normal levels. Wind storms have cleaned dust from the solar arrays before, but the terrain around Spirit's current location lessens the likelihood of similar events in the future.
Spirit is also suffering from a broken right front wheel, and engineers have been driving the rover backwards because it is easier to pull the lame wheel than to push it.
The dragged wheel has dug a shallow trench behind the rover since its failure three years ago, exposing underground silica that is evidence of ancient Martian hot springs, according to scientists.
Opportunity's robotic arm also has a suspect joint, limiting its movement.
"It's kind of like having your arm in a sling," Callas said. "You can still drive a car and sign your name, but you can't play tennis."
Two imaging spectrometers are also facing problems with interference from dust and a decaying radioactive power source.
Some mission personnel say the rovers have even developed their own personalities, and the robots' resumes match their names, which were chosen to honor the legacy of American exploration.
"Opportunity really has been incredibly opportunistic and lucky. It basically found water from its lander and found its own heat shield," said Ashley Stroupe, a rover driver at JPL.
The heat shield inspection provided valuable information on how the structure fared during the fiery, high-speed entry into the Martian atmosphere, according to engineers.
"Spirit really has had to have a lot of spirit to keep going," Stroupe said. "When we were hit by dust storms, we got cleaned off and that's what's enabled the rover to survive this long. When we broke the right front wheel, we made one of the major scientific discoveries."
A team of about 100 scientists and engineers still work on the project at JPL. Most are part-time contributors, Callas said.
That number is down from the operations and support team of 300 people in place during the prime mission in 2004.
Some members of the original team still work on the mission, but most, like Callas, have been promoted to new positions.
Callas was the project's science manager during development and early surface operations. He was later promoted to deputy project manager and became project manager in 2006.
Another 150 scientists from other institutions are also on the team led by Squyres. That team has had much lower turnover, according to Squyres.
The rovers cost about $20 million per year to operate. The mission must maintain a larger staff of navigation experts and scientists than would be required for fixed landers or orbiters, McCuistion said.
That higher cost could make it a target for budget cuts.
NASA officials ordered cuts to the mission last March in an effort to find funding for the over-budget Mars Science Laboratory, which was scheduled for launch later this year. The next-generation rover, still reeling from ballooning costs, has now been delayed until 2011.
After a public backlash and the sudden resignation of Alan Stern, the former associate administrator of the agency's science mission directorate, the ordered cut was rescinded.
NASA has instituted a formal review process to gauge the science value of projects in extended mission phases. The review board is run by independent scientists, according to McCuistion.
"We could get to the point where the operational costs do exceed the science value," McCuistion said.
NASA executives said the cost-benefit analysis would likely continue to tilt in favor of continuing the mission unless the rovers became immobile or lost multiple science instruments.
"Nobody's immune to budget cuts, nor should they be," Squyres said. "But as long as the rovers are able to drive and do good science, I expect funding to continue."
If officials ever opt to shut down the mission, the ending could be poignant for the army of scientists and engineers choreographing the rovers' daily lives. The robots would continue to operate, dutifully listening for new orders from Earth.
"The rovers don't have an off switch," Callas said. "They will continue to be active until they break. The only question is will we continue to effectively utilize them while they're available to us. My intention is to utilize them as much as possible."
If federal funding remains intact, the rovers themselves will be the determining factors in the mission's life expectancy.
Engineers have uplinked new flight software programs to increase the robots' autonomy and safety margins. But officials are unable to fix broken or worn-down parts.
Callas said the team is pushing the rovers as far as possible to maximize the science return.
"Our objective is to wear these rovers out," Callas said. "We intend to push them as far as we possibly can."
Squyres identified power and mechanical breakdowns as his biggest fears for Spirit and Opportunity, respectively. Squyres cited the dust on Spirit's solar panels and Opportunity's looming long-distance drive to Endeavour Crater.
"Five more years seems ridiculously optimistic," Squyres said. "But you could easily go broke betting against these rovers.
"We'll just see."