Both outlived and outperformed expectations, on opposite sides of Mars.
Opportunity hasn't communicated with engineers since June 10. The Martian skies eventually cleared enough for sunlight to reach the rover's solar panels, but there was still no response. The golf cart-size rovers were created to operate as geologists for just three months, after bouncing onto our planetary neighbor inside cushioning air bags in January 2004.
They were rocketed from Cape Canaveral a month apart in 2003.
Astrophysicist Gemma Lavender tweeted: "Think we may be saying goodbye to the Opportunity rover".
The manager of the project, John Callas, described the decay of the machine as like "a loved one who's gone missing".
"But each passing day that diminishes, and at some point you have to say enough and move on with your life."
Nasa believes the device's lengthy outing will have scrambled its clock, which will have drained the rover's batteries.
Inspired, Dr Fraeman went on to become a planetary scientist, joined NASA's Jet Propulsion Laboratory and ended up deputy project scientist for Opportunity.
Deputy project scientist Abigail Fraeman said, "It gives you an idea just how long this mission has lasted".
Global dust storms typically kick up every few years, and "we had gone a long time without one", Mr Callas said. Unlike NASA's nuclear-powered Curiosity rover still chugging along on Mars, Opportunity and Spirit were never created to endure such severe weather.
"You could have lost a lot of money over the years betting against Opportunity", said the professor. Opportunity's mission has led to many discoveries about the Red Planet, but perhaps the most exciting was when Opportunity found evidence that Mars once had water and supported conditions for sustaining microbial life. It's especially frustrating, according to Callas, not knowing precisely why Opportunity-or Spirit-failed.
The scientist said the machine "has given us a larger world".
"Mars is now part of our neighbourhood".
A new rover, named InSight, has already arrived on the planet to continue the rover's legacy.