The Jet Propulsion Laboratory’s greatest minds expected its two identical Mars Exploration Rovers to last 90 days before dust clouded their solar arrays and drained their power.

On Sunday, one of those NASA rovers, Opportunity, rolled into its 12th anniversary on Mars. It’s now a veteran of surviving the harsh Martian landscape that most believed would limit its lifespan to months, not years. Opportunity’s sibling, Spirit, landed weeks earlier and lasted six years.

• MORE: Follow Opportunity’s journey using this interactive map of Mars

“Twelve years is a very long time to have this sort of a continuous presence,” said Matt Golombek, Mars rovers project scientist. “For a science team to be this involved, on a daily basis, for this long on Mars, is pretty much unprecedented.”

The golf-cart sized solar-powered robot is showing its age. Its joints ache and lock, two of its scientific instruments no longer work, and it suffers bouts of amnesia caused by faulty flash memory.

Opportunity moves at a turtle’s pace, and maybe slower now in its old age, but it keeps moving. In the past two years, it broke the record for the longest distance traveled on another planet and completed a full marathon’s distance. Today, its odometer is just shy of 27 miles total.

Opportunity, the precursor to the car-sized and equally resilient Curiosity Rover, launched in late 2003 and arrived on Mars on Jan. 24, 2005. No other rover mission operated as long in NASA’s history, yet its team members still find it enthralling.

“Every day, you’re looking at images that no one has ever seen before,” said John Callas, project manager. “In that sense, it’s always new, it’s always fresh.”

Keeping it moving

Opportunity isn’t steered by joystick, and it’s nothing like driving a car. The scientists at JPL in La Cañada Flintridge send code to the rover — with a 20-minute delay — to execute commands. That meant planning out Opportunity’s actions daily in the beginning. A Mars day, called a Sol, lasts about 40 minutes longer than on Earth. In California, the team’s schedule shifted and blackout curtains were installed, according to Callas.

“You’re essentially permanently jet lagged,” Callas said.

Some worked in the middle of the night, while others gave up their weekends to keep an eye on the twin rovers. As the mission extended, they planned actions on two- and three-day intervals to return employees’ to a more normal schedule, Callas said.

JPL uses a test rover on Earth to plan out more complicated movements. JPL built inclines to put the duplicate rover through its paces when Opportunity reached its first crater.

“The rovers were never designed to go up or down steep slopes,” Callas said. “We didn’t want it to be a ‘Thelma and Louise’ suicide mission.”

Endurance Crater gave Opportunity a chance to photograph deeper and older geology. The crater goes further into the planet’s history the deeper it goes, Callas said.

When Opportunity buried all six wheels in a Martian dune in its first year, the scientists at JPL spent five weeks trying to extricate the robot. On Earth, they buried the test rover in sand and diatomaceous earth, a crumbly soil used in pool filters. It took 629 feet of wheel rotations to move Opportunity one meter out of the trap.

In 2009, Spirit, on the other side of the planet from Opportunity, similarly became stuck, but it was unsuccessful in getting free. It eventually ran out of power, sending its final communication in March 2010. JPL continued to use its arm for science up until it shut down.

NASA used software updates over the years to improve the rover’s movements and visual detection capabilities. One upgrade made Opportunity more selective in its photography, another updated its hazard detection and gave it more autonomy.

A never-ending mission

Both Callas and Golombek joined the mission in the beginning, with Golombek selecting the landing site 15 years ago. For some on the team, Opportunity preceded and inspired their careers. The team’s youngest member was 11-years-old when Opportunity launched.

Lead spacecraft systems engineer Mike Seibert, 33, was a third-year undergraduate studying aerospace at the University of Colorado-Boulder. He recalled buying 3-D glasses to look at the images coming back from Opportunity and Spirit. Three days after graduation, he landed an interview at JPL.

Now 10 years and several title changes later, he’s still working on the mission, his first straight out of college.

“They dangled a Mars rover in front of me, and I lost my mind very quickly,” he said. “We’re all kind of emotionally invested in this mission.”

Seibert said he’d only leave Opportunity to follow his dream of becoming an astronaut. He’s never seen Opportunity in person — it left before he graduated — but he hopes to someday pay it a visit.

“I hope that’s a selling point — I’m familiar with the planet,” he said.

Scientific discoveries

The science done by Opportunity and Spirit changed age-old views of Mars from an eternally desolate planet to one that may have been more Earth-like than previously believed. Opportunity’s landing zone, dubbed Eagle Crater in Mars’ Meridiani Planum, put it practically on top of geological evidence of the Red Planet’s watery past. The rover during its mission found evidence that very salty water likely existed above the surface. It peeked into four craters where it studied salt flats, meteorites and rock compositions.

Opportunity travels roughly 600 meters a day, but it’s rarely in a straight line. The rover might spend weeks, months or years in the same areas. It took the rover nearly three years — with several detours — to reach what NASA calls its “long term goal,” a 13-mile wide crater named Endeavour.

At Endeavour, Opportunity found minerals that suggested Mars may have had less acidic and more life-friendly bodies of water billions of years ago, a shift from the mission’s earlier discoveries that suggested a wet but a much less habitable planet.

The next steps

Opportunity’s extensions cover two-year cycles, with the rover funded through the end of 2016. In the last review cycle, Opportunity received the highest rating of any ongoing Mars mission, including Curiosity. JPL is working on the next extension proposal.

Emily Lakdawalla, a senior editor at the Pasadena-based Planetary Society, called the years of extensions and operation a testament to the rover’s construction and JPL’s careful piloting. She said she suspects Opportunity will continue to receive funding as long as it remains operational.

“Generally speaking, if you have a spacecraft that is functional, that is responding to you, that is already at a distant planet, it is worth it to continue to put what effort you can in to get science out of it,” she said.

Lakdawalla said she writes a blog post every year about NASA’s aging Mars technologies, where she warns that a mechanical failure or operator error could end the missions at any time.

“I keep on betting against them, and I keep being wrong,” she said. “I’m extremely happy to be wrong about that.”