In 1976, NASA's Viking Project sent the first images from a Mars lander back to Earth; its landing-site lessons influenced later U.S. Mars missions

Viking 2 on the surface of Mars. Image Credits: NASA/JPL-Caltech

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Viking's orbiters captured this sweeping view of Candor Chasma, part of the vast Valles Marineris system. Image Credits: NASA/JPL-Caltech

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On July 20, 1976, Viking 1 became the first American spacecraft to land safely on Mars and return a picture from the surface, an achievement marking its 50th anniversary this year. It was not easy to get there safely. Two scientists involved in selecting that touchdown site, Harold Masursky and Norman Crabill, later explained in a paper titled ‘The Viking Landing Sites: Selection and Certification’ published in Science that the original landing zone was abandoned just days before landing, over safety concerns. The last-minute scramble helped establish a process that NASA still uses today whenever it selects a landing site on Mars.About 40 minutes after touchdown, a single black-and-white photo of rocks and dusty soil, relayed to NASA's Jet Propulsion Laboratory, showed the ground in an area called Chryse Planitia, or 'Golden Plain.' It was the first picture ever taken from the surface of Mars, though not the first from another planet altogether; the Soviet Venera 9 lander had already returned the first image from the surface of Venus, in October 1975. Viking 1 did not work alone for much longer. Viking 2, its sister ship, landed on September 3, 1976, at a spot called Utopia Planitia. Both landers operated beyond their planned lifetimes. Viking 1 was built to last for 90 days, but returned data for more than six years, and the landing-site process refined under pressure in 1976 outlived the hardware itself, shaping how NASA has picked every Mars site since. Neither lander found definitive evidence of life, but they did find something almost as tantalizing: strong evidence of ancient riverbeds and flooding, on a planet that used to look very different from the cold, dry one we see today.Mission planners had already selected and analyzed the proposed location for Viking 1's landing based on images obtained by Mariner 9 during its reconnaissance flyby. However, the more detailed examination made by Viking 1’s orbiter and by ground-based radar during June and July 1976 revealed that the initial site was not suitable for a safe landing. The team had to search the nearby terrain almost from scratch. They settled a compromise spot about 900 kilometers away, one that struck a balance between what cameras could see and what radar suggested about the ground beneath, according to the study. It wasn’t the best option on paper, but it was safe enough. Viking 1 successfully landed there on July 20, 1976.That scramble wasn't for nothing. It was a case study for future missions. As NASA geared up for the Mars Pathfinder mission, nearly two decades later, engineers were still using Viking orbital images and radar data to model what the new landing site would look like on the surface, says a NASA technical report on the Mars Pathfinder landing site selection process. Pathfinder's team used a two-step approach: first, they mapped engineering risks and scientific value separately, and then they held an open workshop for scientists worldwide to weigh in on the final decision. That same NASA technical report traces the open-workshop model back to lessons Viking's team learned during the 1976 site scramble.NASA’s later Mars missions followed a similar pattern: collect orbital data, invite the science community to debate candidate sites, and let safety concerns have the last word. Research from the Smithsonian National Air and Space Museum says the process used to select landing sites for the Spirit and Opportunity rovers was later used to select Gale Crater for Curiosity and Jezero Crater for Perseverance. Each round used newer tools, including sharper images from the Mars Reconnaissance Orbiter, but the notion of holding open community workshops before locking in a site is reminiscent of what Viking's team cobbled together under pressure decades ago.NASA has used some version of this orbit-map-then-workshop process for every Mars rover and lander site chosen since Viking, and mission planners have said it will likely guide the search for a crewed landing site as well. Fifty years after that first grainy photo, rovers now drive themselves and orbiters map the planet in far greater detail, but the first question any Mars mission must answer remains the one Viking's team faced in 1976: is a given patch of Martian ground safe enough to risk an entire mission on?