Both outlived and outperformed expectations, on opposite sides of Mars.
Nasa wrote: 'The solar-powered rover last communicated with Earth on June 10, 2018, as a planet-wide dust storm was blanketing the Red Planet'.
The six-wheeled vehicle was built to operate just three months. They rocketed from Cape Canaveral a month apart in 2003.
Astrophysicist Gemma Lavender tweeted: "Think we may be saying goodbye to the Opportunity rover".
Project manager John Callas said, "It's just like a loved one who's gone missing, and you keep holding out hope that they will show up and that they're healthy ..." The Martian skies eventually cleared enough for sunlight to reach the rover's solar panels, but there was still no response.
Deputy project scientist Abigail Fraeman was a 16-year-old high school student when Opportunity landed on Mars; she was inside the control center as part of an outreach program.
Engineers speculate the rover's internal clock may have become scrambled during the prolonged outage, disrupting the rover's sleep cycle and draining on-board 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.
Callas believes it is "good luck that we skirted so many possible storms" over the years.
NASA's nuclear-powered Curiosity rover was created to endure severe weather but Opportunity and Spirit were not so the mere fact they lasted so long is testament of the durability of the models.
Mission scientists have been trying to rouse the rover since dust storms subsided in October, but have been unsuccessful so far.
"You could have lost a lot of money over the years betting against Opportunity", Professor Squyres said.
Opportunity and its long-dead twin rover, Spirit, found evidence that ancient Mars had water flowing on its surface and might have been capable of sustaining microbial life. It's especially frustrating, according to Callas, not knowing precisely why Opportunity-or Spirit-failed.
As for Opportunity: "It has given us a larger world", Dr Callas said. "Mars is now part of our neighborhood".
A new rover, named InSight, has already arrived on the planet to continue the rover's legacy.