V enus was the first planet to be visited by a passing spacecraft, and is the most visited as well. The first spacecraft to pass by Venus was the U.S. Mariner II which passed within 22,000 miles of Venus on December 14, 1962 after a period of 3.5 months of space travel. Its purpose was very basic, just measuring a few different conditions on the planet like surface temperature and air pressure. In 1966 the Russians sent two unmanned spacecraft, the first being Venera 2 which came within 25,000 miles of Venus on February 27 and the second being Venera 3 which crashed into Venus on March 1.

T he next set of Russian and United States spacecraft reached Venus in October 1967. Venera 4 deployed a capsule filled with scientific instruments which landed on the surface of Venus on the 18th, and Mariner V passed within 2,500 miles of Venus a day later. Both probes reported the same finding, that large amounts of carbon dioxide are present in the atmosphere. The Russian probe Venera 7 landed on the surface of Venus on December 15, 1970 to conduct more thorough tests. The next U.S. probe, Mariner X, reported that Venus had no magnetic field when it flew by the planet on February 5, 1974.

A breakthrough came on October 1975, when the Russian unmanned spacecraft Venera 9 landed on Venus and provided the first detailed photographs of the surface of Venus. Venera 10 was sent to Venus to take more pictures and determine its atmospheric pressure. In 1978 there was a renewed interest in exploring Venus and four space craft were sent. The first was the U.S. Pioneer 1 which transmitted radar photographs and found the temperature of the upper clouds on Venus. Five days later on December 9, the U.S. Pioneer 2 descended on the planet and measured its density and chemical composition. Then on the 21st the Russian Venera 12 landed on Venus and