Opportunity and Spirit were built to last 90 days, but Opportunity far exceeded that number, roving for almost 15 years.
The solar-powered rover's demise was no surprise: It fell out of contact with controllers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., last June - due to a globe-girdling Martian dust storm that kept Opportunity from charging its batteries. "And our beloved Opportunity remains silent", Thomas Zurbuchen, associate administrator of the Science Mission Directorate, told a news conference Wednesday afternoon.
The two rovers' greatest achievement was finding firm proof that water that could have sustained microbial life had once flowed on the Martian surface. Dust storms are common on Mars, but a global event like the one we saw in 2018 only happens every few years.
The small rover first arrived on the Red Planet on January 25, 2004.
Spirit and Opportunity moved, of course, over surprising spans of time and space.
NASA's Opportunity Mars rover landing site in 2004. Spirit, too, lasted much longer than planned, finally going dim forever in 2010 after a sand trap crippled the machine, limiting its ability to reorient and receive adequate sunlight to power itself. "It's a team sport, and that's what we're celebrating today", Zurbuchen said.
"These rovers have transformed the way humanity looks at Mars", said Steve Squyres of Cornell University, the scientific principal investigator of Spirit and Opportunity's Mars Exploration Rover (MER) mission. And so some argued that Opportunity - affectionately known as "Oppy" by some - wasn't being given the same chance. Instead, the rover has persisted for 15 years, sending back incredible data and photos from Mars to help uncover the Red Planet's secrets.
Opportunity was exploring Mars' Perseverance Valley, fittingly, when the fiercest dust storm in decades hit and contact was lost.
Scientists consider this the end of an era, now that Opportunity and Spirit are both gone. If Opportunity fails to respond by Wednesday, it will be officially declared defunct. Steve Squyres, a professor at Cornell University in the U.S. studying the history of water on the Red Planet, called Mars a "dry, desolate world".
Opportunity not only leaves behind a legacy of discovery, but also became the longest-running rover.
"I can not think of a more appropriate place for Opportunity to endure on the surface of Mars than one called Perseverance Valley", said Michael Watkins, director of JPL. It had a heater that was draining energy, and the clock was scrambled by loss of power, so it didn't know when to sleep. Together they landed on Mars the following year on a quest to determine the history of the water on the planet, NASA said on their website.
The programme has had an extraordinary record of success: 45.2km traversed, more than the Soviet Union's Lunokhod 2 moon rover during the 1970s and more than the rover that USA astronauts took to the moon on the Apollo 17 mission in 1972.
Now it is up to Curiosity and the InSight lander to carry on the legacy, Mr Callas said, along with spacecraft in orbit around Mars.
And one day, NASA hopes rover tracks will be alongside human footprints on the Red Planet.