On May 19, 1971, the Soviet Union launched Mars 2, a spacecraft on a mission to reach the Red Planet. After traveling for nearly seven months across 300 million miles of space, Mars 2 arrived at Mars in November 1971. It released a small landing craft designed to touch down on the surface. But the descent went wrong. The landing system failed, and the craft slammed into the Martian surface at high speed. Despite the crash, Mars 2 earned a place in history. It was the first human-made object to reach the surface of another planet. Its companion spacecraft, Mars 3, landed twelve days later and briefly transmitted data before going silent after just 20 seconds. Landing on Mars is extraordinarily difficult. Unlike the Moon, Mars has a thin atmosphere. It is thick enough to create friction and heat during entry, but too thin for parachutes to work well on their own. Engineers must use a combination of heat shields, parachutes, and rocket thrusters to slow a spacecraft from 13,000 miles per hour to a gentle stop. Even today, about half of all Mars missions fail. The planet's distance from Earth makes real-time control impossible. Radio signals take between 4 and 24 minutes to travel between the two planets. Every landing must be automated -- the spacecraft makes all decisions on its own during what engineers call "seven minutes of terror."