Their off-world exploits no longer dominate the headlines. Their mechanical joints are showing signs of age.
But nearly a year after touching down on Mars, NASA's twin 384-pound robot geologists, Spirit and Opportunity, are still quietly grinding their way across the Red Planet, one rock at a time.
As officials at NASA's Jet Propulsion Laboratory in Pasadena, Calif., discuss how to celebrate the $820 million mission's first anniversary, scientists are taking stock of what the rovers and other scientific instruments have taught us about Mars in the last 12 months.
"Mars is practically a new world," says Jeffrey Kargel, a planetary geologist at the U.S. Geological Survey in Flagstaff, Ariz., who was not a member of the rover team but relies on its data for his research. "There are very few people whose work has not been fundamentally altered by the rover's findings." Many of these findings have trickled out in conferences and journals such as Science, which published a special section today on Opportunity's discoveries. (The journal devoted similar space to Spirit in August.)
The biggest lesson from 2004: Although Mars, biologically speaking, remains a dead world, the prospects for life are getting a whole lot better.
The credit goes mostly to the rovers, which for the first time proved that one of the key prerequisites for life - water - puddled on the surface at some point in the planet's ancient past.
Serious talk about water on Mars has been going on in scientific circles since 1971, when NASA's Mariner 9 became the first spacecraft to swing into orbit around the planet and snap clear pictures of its surface. The photos showed river beds and other geological features that could have only been sculpted by water.
Or at least that's how it seemed from space. But, says Kargel, "there have always been skeptics." Over the years, the doubters have proposed explanations ranging from high wind to liquid carbon dioxide to account for the planet's features.
Then the rovers arrived. Spirit settled into Gusev crater on Jan. 3, while Opportunity touched down 22 days later at Meridiani Planum, a vast plain near the Martian equator.
Opportunity was the first to strike paydirt, stumbling on BB-sized minerals the scientists dubbed "blueberries." Analysis showed the blueberries were made mostly of hematite, a mineral that forms primarily in water.
