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This illustration provided by NASA shows the Perseverance rover, bottom, landing on Mars. Hundreds of critical events must execute perfectly and exactly on time for the rover to land safely on Feb. 18, 2021. Entry, Descent, and Landing, or "EDL," begins when the spacecraft reaches the top of the Martian atmosphere, traveling nearly 12,500 mph (20,000 kph). EDL ends about seven minutes after atmospheric entry, with Perseverance stationary on the Martian surface. (NASA/JPL-Caltech via AP)