Washington - All media attention may have focused on Curiosity ever since the 2.5-billion-dollar rover landed on Mars in August 2012, but its older and more modest cousin Opportunity continues to defy expectations.
Incredibly, the original stated mission length was 90 Martian days, but the little rover refuses to quit. On Monday, Opportunity celebrates the 10th anniversary of its launch to the Red Planet from Kennedy Space Centre at Cape Canaveral, Florida.
"I don't think there's anyone on this planet that would have imagined this rover lasting this long when we started 10 years ago," said John Callas, project manager of the NASA Opportunity mission.
"It is in incredibly good shape for its age."
The approximately 185-kilogram, six-wheeled rover was launched from Florida on July 8, 2003, and landed in the so-called Eagle Crater on Mars in January 2004.
Since the, Opportunity has traversed more than 36 kilometres on the Martian surface, searching for traces of water including sedimentary structures and mineral traces, to help scientists better understand the drastic climate change that occurred in Mars' past.
The rover completed its three-month primary mission in April 2004 before exploring nearby craters, ridges and dunes. Opportunity can take photographs, as well as chip away and drill into the planet's surface.
The last decade has not gone entirely without incident.
Opportunity got stuck in sand at one stage and could only be freed with the help of some complicated manoeuvres. Opportunity's sister rover, Spirit, ceased functioning in 2009 after suffering a similar fate.
A sand storm damaged Opportunity's solar panels, meaning the battery no longer fully charges, while NASA scientists now drive the rover in reverse because one its the wheels no longer rotates perfectly. The rover's robotic arm has only reduced functionality.
To help it survive its landing, Opportunity was stripped down to its essential equipment, with just one processor and a single power supply -if anything happens to either the explorer will cease to function.
"The processor is slowly beginning to show signs of wear because of so much use," Callas said.
The newer, more modern and technically capable Curiosity is obviously the future of Mars exploration, as evidenced by the numerous discoveries it has already made that have astounded scientists.
Yet the plan remains for Opportunity to continue rolling for as long as possible. At the moment, the rover is moving along the edge of the Endeavour Crater, where it will shelter from the Martian winter and recharge its batteries.
Callas is not yet sure what happens after that: "Every extra day is a bonus."