Venus's complete cloud cover and deep atmosphere make it difficult to study from Earth. Most knowledge of the planet has been obtained through the use of space vehicles, particularly those carrying probes that descend through the atmosphere. The first flyby was that of
Mariner 2, launched by the United States in 1962, followed by Mariner 5 in 1967 and Mariner 10 in 1974. The former Union of Soviet Socialist Republics developed several entry probes, some combined with flybys or orbiters:
Venera 4 and 5 (1967), 6 (1969), 7 (1970), 8 (1972), 9 and 10 (1975), 11 and 12 (1978), 13 and 14 (1981), and 15 and 16 (1983);
Vega 1 and 2, sent toward Halley's comet in 1984, also flew by Venus and released descent capsules. Several of these probes successfully reached the planet's surface. The United States sent two Pioneer Venus missions in 1978. Pioneer Venus 2 sent four probes to the surface, while the remaining craft explored the upper atmosphere.
Pioneer Venus 1, an orbiter, measured the upper atmosphere for 14 years. The
Magellan probe, launched toward Venus in 1989, transmitted
radar images of the planet from 1990 to 1994.
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