NASA's Mars rover Spirit last Saturday passed the fifth anniversary of its arrival on the Red Planet on 3 January 2004 - an achievement which will be matched on 24 January by its twin Opportunity.
During their epic Martian jaunt, the pair have "returned a quarter-million images, driven more than 21 kilometers (13 miles), climbed a mountain, descended into craters, struggled with sand traps and aging hardware, survived dust storms, and relayed more than 36 gigabytes of data via NASA's Mars Odyssey orbiter".
A suitably chuffed Ed Weiler, associate administrator for NASA's Science Mission Directorate, said: "The American taxpayer was told three months for each rover was the prime mission plan. The twins have worked almost 20 times that long. That's an extraordinary return of investment in these challenging budgetary times."
John Callas, Jet Propulsion Laboratory project manager for Spirit and Opportunity, added: "These rovers are incredibly resilient considering the extreme environment the hardware experiences every day. We realize that a major rover component on either vehicle could fail at any time and end a mission with no advance notice, but on the other hand, we could accomplish the equivalent duration of four more prime missions on each rover in the year ahead."
NASA notes that Mars's wind has "provided unanticipated aid to the vehicles' longevity" by blowing dust off the vehicles' solar panels. It adds, though, that "Spirit has not had a good cleaning for more than 18 months" and that it "barely survived" its third winter in the southern hemisphere.
However, with the arrival of the Martian spring in December, Spirit is gathering strength for a new challenge. NASA explains the rover team plans to dispatch it to "a pair of destinations about 183 meters (200 yards) south of the site where Spirit spent most of 2008".
It elaborates: "One is a mound that might yield support for an interpretation that a plateau Spirit has studied since 2006, called Home Plate, is a remnant of a once more-extensive sheet of explosive volcanic material. The other destination is a house-size pit called Goddard."
Steve Squyres of Cornell University, principal investigator for the rover science instruments, said: "Goddard doesn't look like an impact crater. We suspect it might be a volcanic explosion crater, and that's something we haven't seen before."
Opportunity, meanwhile, is en route for the Endeavour Crater, described as "approximately 22 kilometers (14 miles) in diameter, more than 20 times larger than another impact crater, Victoria, where Opportunity spent most of the past two years".
While Endeavour is about 12km (7 miles) from Victoria in a straight line, the trek is actually "considerably farther as the rover drives on a route evading major obstacles". Opportunity has in four months travelled roughly a mile towards its destination.
Frank Hartman, a JPL rover driver, said: "We keep setting the bar higher for what these rovers can do. Once it seemed like a crazy idea to go to Endeavour, but now we're doing it."
Steve Squyres concluded: "The journeys have been motivated by science, but have led to something else important. This has turned into humanity's first overland expedition on another planet.
"When people look back on this period of Mars exploration decades from now, Spirit and Opportunity may be considered most significant not for the science they accomplished, but for the first time we truly went exploring across the surface of Mars." ®