LOS ANGELES — Opportunity, NASA's other Mars rover, has tooled around the red planet for so long it is easy to forget that it's still alive.
Some 5,000 miles away from the limelight surrounding Curiosity's every move, Opportunity this week quietly embarks on its 10th year of exploration — a sweet milestone since it was only planned to work for three months.
"Opportunity is still going. Go figure," said mission deputy principal investigator Ray Arvidson of Washington University in St. Louis.
True, it's not as snazzy as Curiosity, the most high-tech interplanetary rover ever designed. It awed the world with its landing near the Martian equator five months ago.
After so many years crater-hopping, Opportunity is showing its age: It has an arthritic joint in its robotic arm and it drives mostly backward due to a balky front wheel — more annoyances than show-stoppers.
For the past several months, it has been parked on a clay-rich hill along the western rim of Endeavour Crater that's unlike any scenery it encountered before. It plans to wrap up at its current spot in the next several months and then drive south where the terrain looks even riper for discoveries.
Long before Curiosity became everybody's favorite rover, Opportunity was the darling.
The six-wheel, solar-powered rover parachuted to Eagle Crater in Mars' southern hemisphere on Jan. 24, 2004, weeks after its twin Spirit landed on the opposite side of the planet.
During the first three months, there were frequent updates about the twin rovers' antics. The world, it seemed, followed every trail, every rock touched and even kept up with Spirit's health scare from which it eventually recovered.
Opportunity immediately lived up to its name, touching down in an ancient lake bed brimming with minerals that formed in the presence of water, a key ingredient for life. After grinding into rocks and sifting through dirt, Opportunity made one of the enduring finds on Mars: Signs abound of an ancient environment that was warmer and wetter than today's dusty, cold desert state.
Spirit, on the other hand, landed in a less-interesting spot and had to drive some distance to find geologic evidence of past water. After six productive years wheeling around, it fell silent in 2010, forever stuck in Martian sand.
Opportunity went on to poke into four other craters, uncovering even more hints that water existed on Mars long ago.
The rover "is not like a lander staring at the same real estate. We've gone to different terrains, explored different geology and answered different questions on Mars," said project manager John Callas of the NASA Jet Propulsion Laboratory, which runs the $984 million project.
"Mars is big enough for more than two rovers to explore," he said.