LOS ANGELES — Opportunity, NASA’s other Mars rover, has tooled around the red planet for so long it’s easy to forget 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 tasked 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. Curiosity 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 because of a balky front wheel.

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 has encountered before. It plans to wrap up there 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.

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.

What’s still unknown is whether Mars ever had the right environmental conditions to support microscopic organisms - something Curiosity is trying to answer during its two-year mission.

Front Section, Pages 2 on 01/22/2013