Launched on an Atlas-Agena rocket, Mariner 2 was a backup built from the same design as Mariner 1, which had been destroyed shortly after liftoff when its rocket veered off course. The 447-pound probe flew past Venus on December 14, 1962, passing within about 21,600 miles of the planet. Its instruments showed surface temperatures of several hundred degrees and found no detectable magnetic field, ending hopes that Venus's clouds concealed a temperate world.
In the early 1960s the United States and the Soviet Union were racing to reach the planets, and both had already failed. NASA's Jet Propulsion Laboratory adapted its Ranger lunar spacecraft into a lighter Venus probe, and after Mariner 1 was destroyed by a range safety officer when its launch vehicle strayed off course, its twin, Mariner 2, rode an Atlas-Agena rocket off the pad at Cape Canaveral. The spacecraft weighed just 447 pounds and carried a small suite of instruments, including microwave and infrared radiometers, a magnetometer, and detectors for charged particles and cosmic dust.
The cruise was not smooth. The probe suffered intermittent electrical problems and overheating as it moved closer to the Sun, and engineers on the ground could only watch the telemetry and hope. On the way, Mariner 2 confirmed that the solar wind, a stream of charged particles flowing from the Sun, blows continuously through interplanetary space. On December 14, 1962, it swept past Venus at a distance of roughly 21,600 miles, scanning the planet for about 42 minutes.
The results overturned a romantic vision. Some scientists and science fiction writers had imagined a humid, swampy world beneath Venus's perpetual clouds; Mariner 2's radiometers instead indicated a surface hot enough to melt lead, with no gaps in the cloud cover and no detectable magnetic field. Controllers lost contact with the probe on January 3, 1963, and it remains in orbit around the Sun. As the first spacecraft to return data from another planet, it set the pattern for every planetary mission that followed.