The Associated Press Published Monday, Jan. 21, 2013 8:23AM EST

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 8,000 kilometres away from the limelight surrounding Curiosity's every move, Opportunity this week quietly embarks on its tenth 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. 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 that it eventually recovered from.

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NASA's older Mars rover begins 10th year of exploration