Launched in August 1962, Mariner 2 swept past Venus after a 109-day voyage, the first successful interplanetary mission in history. Its instruments detected surface temperatures around 800°F beneath a thick, unbroken cloud layer, dispelling romantic notions that Venus might harbor oceans or jungle. The tiny probe, cobbled together after its twin Mariner 1 was destroyed by a rocket malfunction, gave humanity its first hard data about a neighboring world.
Mariner 2 was something of an underdog mission, assembled quickly as a backup after Mariner 1 veered off course during launch in July 1962 and had to be destroyed by range safety officers barely five minutes into flight. Engineers scrambled to ready its twin, and on August 27, 1962, Mariner 2 launched successfully, beginning a 109-day, 180-million-mile journey.
On December 14, 1962, it passed within about 21,600 miles of Venus's surface, its instruments sweeping across the planet for just 42 minutes since it had no camera and could not orbit. What it found ended decades of speculation: microwave radiometers detected surface temperatures near 800 degrees Fahrenheit, far too hot for liquid water, and no evidence of a magnetic field or radiation belts like Earth's.
The mission weighed barely 447 pounds, a featherweight by later standards, yet it proved that robotic probes could survive interplanetary space and return usable science. It set the template for the Mariner program's later triumphs at Mars and Mercury, and it quietly killed off the pulp-fiction image of Venus as a humid jungle world, replacing it with the reality of a hellish, cloud-choked furnace.