https://www.scientificamerican.com/article/how-nasa-aims-to-achieve-perseverances-high-stakes-mars-landing/

If NASA’s $2.7 billion flagship mobile science laboratory, Perseverance, successfully touches down in Jezero Crater on Mars on February 18, the feat will not only open a new chapter in exploration of the Red Planet, but also mark the triumphant culmination of four decades of increasingly challenging landings there. Replete with sedimentary rocks that might contain fossilized creatures from the planet’s warmer, wetter, more habitable past, Perseverance’s destination—the dried-up delta-and-lake system of Jezero Crater—seems so ideal for sniffing out signs of ancient life that one might wonder why it as of yet has remained unvisited. The answer is simple: Attempting a landing in such complex terrain has been a recipe for disaster. At least, until now.

When the robotic exploration of Mars kicked off in the 1970s, the best available pictures of the planet’s surface were so crude that targeting where to go was akin to playing a blindfolded game of darts. NASA’s first landing efforts, the Viking 1 and 2 missions of 1976, had to snap images from orbit before mission planners could pick landing sites, and even then any semblance of safety was far from guaranteed. The vintage tech of the Viking missions could only ensure that each lander would come to rest somewhere within an ellipse 300 kilometers long and 100 kilometers wide, hopefully close to a central point scientists had flagged for investigation. Ultimately, NASA officials put this “landing ellipse” on the safest places they could see from orbit, setting down each Viking in a smooth, near-featureless plain stretching more than a thousand kilometers.