NASA's Mariner 2 mission confirmed Sagan's conclusions about Venus's extremely hot surface temperature in 1962.
Before the spacecraft encounter, Sagan had analyzed radio observations and argued that Venus was not a mild, Earth-like world but a hot planet with a dense atmosphere. Mariner 2, launched on August 27, 1962, became the first successful mission to another planet when it flew past Venus on December 14.
Its microwave and infrared radiometers measured Venus's thermal emission during several scans. The results indicated temperatures around 500°C and showed little meaningful difference between the planet's day and night sides, supporting the idea of an optically thick atmosphere and a powerful greenhouse effect. Mariner 2 was a flyby, not a lander, so it did not directly touch the surface; its remote measurements nevertheless decisively validated the broad picture Sagan had proposed. Later Soviet Venera landers made direct surface measurements.