The interplanetary dune buggies have lasted years longer than planned.
NASA marked the fifth anniversary of the landing of its rovers Spirit and Opportunity on Mars this week.
The Spirit rover landed on the Red Planet Jan. 3, 2004, and the Opportunity rover landed 21 days later. The rovers still operate today, although NASA didn't expect them to work nearly this long.
"The American taxpayer was told three months for each rover was the prime mission plan," Ed Weiler, associate administrator for NASA's Science Mission Directorate, said in a statement. "The twins have worked almost 20 times that long. That's an extraordinary return of investment in these challenging budgetary times."
The Mars rovers have uncovered information about Mars and its environment and helped scientists gain a better understanding about the planet's ancient history. They have captured and delivered about 250,000 images and sent more than 36 GB of data to the Mars Odyssey orbiter. They have toured 13 miles and covered mountainous terrain, navigated sand traps, and descended into craters through extreme conditions and sand storms.
"These rovers are incredibly resilient considering the extreme environment the hardware experiences every day," John Callas, JPL project manager for Spirit and Opportunity, said in a statement. "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."
High winds have actually helped the rovers last beyond expectations by clearing dust from solar panels, but winds are unreliable and dust has settled on solar panels on Spirit, which barely survived its third southern-hemisphere winter last month, NASA said.
"This last winter was a squeaker for Spirit," Callas said.
Both rovers will continue to explore Mars' surface this year as the planet enters its spring and summer, which bring more sunlight to power the solar panels. NASA has delayed plans for its next-generation rover until 2011.