A decade after landing on Mars, the rover Opportunity is still chugging along.
Sure, it has some wear and tear. One of its six wheels and two instruments stopped working long ago. It has an arthritic joint. Its flash memory occasionally suffers a senior moment.
But these problems are considered minor for a journey that was supposed to be just a three-month adventure.
"No one ever expected this - that after 10 years a Mars exploration rover would continue to operate and operate productively," project manager John Callas said.
The challenges for Mars rovers was underlined yesterday by news China's Jade Rabbit is in trouble after just two months.
Nasa has scrutinised our planetary neighbour for decades, starting with quick flybys and later with orbiters, landers and rovers. Opportunity touched down on January 24, 2004 - several weeks after its twin, Spirit.
Both rovers outlasted their warranty by years, but Spirit stopped phoning home in 2010 after getting stuck in sand. Meanwhile, Opportunity has logged 39 kilometres crater-hopping. The solar-powered Nasa rover is now in a sunny spot on the rim of Endeavour Crater, where it's spending its sixth winter poking into rocks and dirt. Its power levels have unexpectedly improved. A recent "selfie" showed dust on its solar panels was later wiped away by blowing winds.
Early discoveries by the two rovers pointed to a planet that was once tropical and moist. However, the signs of water suggested an acidic environment that would have been too harsh for microbes.
More recently, Opportunity uncovered geological evidence of water at Endeavour Crater that's more suited for drinking - a boon for scientists searching for extraterrestrial places where primitive life could have thrived. The crater is the largest of five examined by Opportunity. This reinforces similar discoveries made by fellow lander Curiosity on the other side of the planet.
A new study published by the journal Science - on Opportunity's 10th anniversary - determined the rocks from the crater are the oldest yet found - about four billion years old. The rocks interacted with water during a time when the environmental conditions were favourable for microscopic organisms.
"This is really a neat area," said deputy project scientist Ray Arvidson of Washington University in St Louis.
In 2012, Opportunity was joined on Mars by Curiosity, which quickly became the world's favourite rover and is currently rolling across bumpy terrain towards a mountain.
Opportunity snatched some of the attention back earlier this month when it discovered a rock shaped like a jelly doughnut that suddenly appeared in its field of view. Scientists said it's unlike any rock they've seen on Mars.
It costs about US$14 million a year to maintain Opportunity. Nasa periodically reviews missions that have been extended to decide where to invest scarce dollars. The next decision is expected this year for Opportunity and other extended missions.
"From all the missions that we have, they're very productive and it would be a shame not to have enough to afford the continuation of those missions," said Michael Meyer, Nasa's lead scientist for Mars exploration.
In several months, Opportunity will decamp from its winter haven and head south to what scientists are calling the mother lode - a clay-rich spot that should yield more discoveries.
"As long as the rover keeps going, we'll keep going," said chief scientist Steve Squyres of Cornell University.