Mariner 2 became the first spacecraft to fly successfully past another planet when it reached Venus in 1962.
NASA launched Mariner 2 on August 27, 1962. The spacecraft passed Venus on December 14 at a distance of about 34,800 kilometres and transmitted scientific measurements back to Earth. It was the first fully successful interplanetary mission and the first successful planetary flyby.
Mariner 2’s instruments found that Venus was extremely hot and confirmed that the planet rotated very slowly. Its microwave and infrared radiometers measured temperatures that helped establish that Venus has a dense, heat-trapping atmosphere.
The mission followed the failure of Mariner 1, whose launch vehicle veered off course and had to be destroyed. Mariner 4 later became the first spacecraft to fly past Mars and return close-up pictures of another planet, but it was not the first successful planetary flyby.