Twenty years ago this month, twin spacecraft made dramatic planetary landings and set about creating an unrivaled legacy of science and exploration.
NASA's Spirit and Opportunity rovers were supposed to operate on the surface of Mars for just three months. But both solar-powered robots far outlived their warranties, setting travel and longevity records, making weird discoveries and forever changing Mars science and exploration.
The two Mars Exploration Rovers launched separately in June and July of 2003 on Boeing Delta II rockets. Spirit was the first onto the Martian surface on Jan. 3, 2004, bouncing around 30 times on a cluster of airbags, which deflated after the spacecraft came to a stop. Opportunity followed in a similar fashion three weeks later, on Jan. 24.
The intrepid rovers set about their work to look for evidence of past water on Mars, finding this and much, much more. Opportunity discovered "blueberries," or spherical hematite pebbles, soon after landing, indicating past acidic water. Spirit found signs of ancient hot springs, suggesting potential past habitats for microbial life. Other discoveries included the "jelly-filled doughnut" and the first meteorite ever discovered on another planet.
Spirit got caught in soft sand in 2009 and continued making science measurements until contact was lost in 2010. Opportunity would continue much longer; it was formally declared lost in 2019, after a planet-wide dust storm in 2018 covered its solar arrays. Across its lifetime, "Oppy" covered more than a marathon distance, racking up a total 28.06 miles (45.16 kilometers) on Mars — an off-world driving record.
"This was a paradigm shift no one was expecting," said former project manager John Callas of NASA's Jet Propulsion Laboratory (JPL) in Southern California, which managed the Mars Exploration Rover mission, in a statement. "The distance and time scale we covered were a leap in scope that is truly historic."
Their scientific accomplishments and robust engineering secured a special place in exploration history for Spirit and Opportunity. The duo achieved their objectives and paved the way for the development and practical operation of larger rovers. The experiences from these missions also improved the techniques for exploring Martian terrain, including the use of specialized software and 3D goggles for navigation.
"Our twin rovers were the first to prove a wet, early Mars once existed," said former project scientist Matt Golombek, also of JPL. "They paved the way for learning even more about the Red Planet's past with larger rovers like Curiosity and Perseverance."
These two SUV-sized rovers are continuing Mars exploration today, respectively looking for the chemical ingredients needed for life and collecting rock samples that will be examined for signs of ancient microbial life, once brought to Earth by the Mars Sample Return mission campaign.
Spirit and Opportunity also live on through their inspiration. Abigail Fraeman, a high school student who was invited to JPL on the night of Opportunity's landing back in 2004, is now a deputy project scientist for Curiosity.
"The people who kept our twin rovers running for all those years are an extraordinary group, and it's remarkable how many have made exploring Mars their career," Fraeman said.
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Andrew is a freelance space journalist with a focus on reporting on China's rapidly growing space sector. He began writing for Space.com in 2019 and writes for SpaceNews, IEEE Spectrum, National Geographic, Sky & Telescope, New Scientist and others. Andrew first caught the space bug when, as a youngster, he saw Voyager images of other worlds in our solar system for the first time. Away from space, Andrew enjoys trail running in the forests of Finland. You can follow him on Twitter @AJ_FI.