Opportunity passed its warranty date long ago, but the rover is still healthy and changing our perceptions of Mars after years of work on the Red Planet.
Initially intended to last 90 Martian days or "sols," Opportunity has now driven more than 20 miles and explored two large craters: Victoria and Endeavour. Along the way, the rover has found multiple signs of water – while surviving a sand trap and bad dust storm.
Carrying an orphan's name
Opportunity and its twin rover, Spirit, received their names from 9-year-old Sofi Collis. She was the winner of a naming contest NASA held (with assistance from the Planetary Society and sponsorship from Lego) to find monikers for the Mars Exploration Rovers. Siberian-born Collis was adopted at age two and came to live with her new family in Scottsdale, Arizona.
"I used to live in an orphanage," Collis wrote in her winning essay. "It was dark and cold and lonely. At night, I looked up at the sparkly sky and felt better. I dreamed I could fly there. In America, I can make all my dreams come true. Thank you for the 'Spirit' and the 'Opportunity.'"
The Mars Exploration Rovers launched in 2003 on a 283-million-mile (455.4 million km) journey to hunt for water on Mars. The $800-million cost for the two of them covered a suite of science instruments. Site survey tools included a panoramic camera, as well as a mini-thermal emission spectrometer that was supposed to search for signs of heat. Each rover also had a small arm with tools such as spectrometers and a microscopic imager.
Cruise to Mars
Opportunity left Earth July 7, 2003, aboard a Delta II rocket en route to a landing site at the Martian equator called Meridiani Planum. NASA was intrigued by a layer of hematite that the orbiting Mars Global Surveyor spotted from above. As hematite (an iron oxide) often forms in a spot that had liquid water, NASA was curious about how the water got there in the first place and where the water went.
Opportunity rolled to a stop inside a shallow crater just 66 feet (20 meters) across, delighting scientists as the first pictures beamed back from the Red Planet. "We have scored a 300-million mile interplanetary hole-in-one," quipped Cornell University's Steve Squyres, principal investigator for the rover's science instruments, in a press release in the days after the landing.
Early sols of science
Opportunity and Spirit (which had landed successfully three weeks earlier, on Jan. 3, 2004) had a primary goal to "follow the water" during their time on Mars. They would hunt for any environments that showed evidence of water activity, particularly looking for minerals that may have been left behind after water came through.
Both rovers met that goal quickly. In early March, just six weeks after landing, Opportunity identified a rock crop that showed evidence of a liquid past. The rocks at "Guadalupe" had sulfates as well as crystals inside of niches, which are both signs of water. Spirit found water evidence of its own that same week.
Two weeks later, Opportunity found hematite inside some small spheres that NASA dubbed "blueberries" because of their size and shape. Using a spectrometer, Opportunity found evidence of iron inside a group of berries when comparing it to the bare, underlying rock.
The month wasn't yet over when Opportunity found more evidence of water, this time from images of a rock outcrop that probably formed from a deposit of saltwater in the ancient past. Chlorine and bromine found in the rocks helped solidify the theory.
All in all, it was a positive start to Opportunity's mission — and it hadn't even left the crater where it had landed yet. Before Opportunity's 90-day prime mission was over, the golf-cart size rover clambered out of Eagle Crater and ventured to its next science target about half a mile away: Endurance Crater. It spotted more water signs there in October.
Stuck in the sand
One of Opportunity's most dangerous moments came in 2005, when the rover was mired in the sand for five weeks. NASA had put the rover into a "blind drive" on April 26, 2005, meaning the rover was not checking for obstacles as it went. Opportunity then plowed into a 12-inch-high (30 cm) sand dune, where the six-wheeled rover initially had trouble getting out.
To save the stranded rover, NASA ran tests on a model of the rover in a simulated Martian "sandbox" at the Jet Propulsion Laboratory. Based on what they learned in the sandbox, the rover drivers then sent a series of commands to Opportunity. It took the rover about 629 feet (191 meters) of wheel rotations before it was able to move forward three feet, but it cut itself free in early June 2005.
NASA chose to move the rover forward in more careful increments, which was especially important because Opportunity lost the full use of its right-front wheel (because of a seized steering motor) just days before it got stuck in the sand. The rover could still move around just fine with its other three steerable wheels, NASA said.
Opportunity's experience in the sand came in handy in October 2005, when NASA detected unusual traction problems on Sol 603. Just 16 feet into a planned 148-foot drive, a slip check system on board automatically stopped the rover when it went past a programmed limit. Two Martian days later, Opportunity backed itself out of the problem and kept on going.
A marathon on Mars
In late September 2006, Opportunity wheeled up to Victoria Crater after 21 months on the road. It circled the rim for a few months snapping pictures and getting a close look at some layered rocks surrounding the crater. NASA then made a gutsy decision in June 2007 to take Opportunity inside the crater. It was a risk to the rover as it might not have been able to climb up again, but NASA said the science was worth it.
"The scientific allure is the chance to examine and investigate the compositions and textures of exposed materials in the crater's depths for clues about ancient, wet environments," NASA stated in a press release. "As the rover travels farther down the slope, it will be able to examine increasingly older rocks in the exposed walls of the crater."
The trek down was interrupted by a severe dust storm in July 2007. Opportunity's power-generating capabilities dropped by 80 percent in only one week as its solar panels became covered in dust. Late in the month, Opportunity's power dipped to critical levels. NASA worried the rover would stop working, but Opportunity pulled through.
It wasn't until late August that the skies cleared enough for Opportunity to resume work and head into the crater. Opportunity spent about a year wandering through Victoria Crater, getting an up-close look at the layers on the bottom and figuring that these were likely shaped by water.
Opportunity climbed out successfully in August 2008 and began a gradual journey to Endeavour, an incredible 13 miles (21 km) away. It took about three years to get there, as the rover was stopping to look at interesting science targets on the way. But Opportunity successfully arrived in August 2011.
— Elizabeth Howell, SPACE.com Contributor
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