Venus is a terrestrial planet and is sometimes called Earth's "sister planet" because of their similar size, mass, proximity to the Sun, and bulk composition. It is radically different from Earth in other respects.
It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet's surface is about 92 times the sea level pressure of Earth, or roughly the pressure at 900m underwater on Earth. Venus has, by far, the hottest surface of any planet in the Solar System, with a mean temperature of 464 °C, even though Mercury is closer to the Sun.
Due to its proximity to Earth, Venus has been a prime target for early interplanetary exploration. It was the first planet beyond Earth visited by a spacecraft (Mariner 2 in 1962), and the first to be successfully landed on (by Venera 7 in 1970). Venus' thick clouds render observation of its surface impossible in visible light, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more complex missions, but they are hindered by Venus's hostile surface conditions.
The first robotic space probe mission to Venus, and the first to any planet, began with the Soviet Venera program in 1961. The United States' exploration of Venus had its first success with the Mariner 2 mission on 14 December 1962, becoming the world's first successful interplanetary mission, passing 34,833 km above the surface of Venus, and gathering data on the planet's atmosphere. In the decades since, a number of robotic missions to Venus have taken place, including orbiters and landers.
Manned Venus Flyby was a 1967–1968 NASA proposal to send three astronauts on a flyby mission to Venus in an Apollo-derived spacecraft in 1973–1974, using a gravity assist to shorten the return journey to Earth; but this proposed mission was never realized.
As of March 2021, no human missions to Venus have taken place, and none are actively being planned, but recent concepts have included the High Altitude Venus Operational Concept, which would involve human crews exploring the Venusian atmosphere in dirigibles, and establishing floating outposts to allow for a long-term human presence on Venus. A detailed presentation on this proposal is available here.
When will the first human mission to Venus take place?
This question resolves as the first date on which conscious humans approach Venus within a distance of 1 million kilometres.
The humans must be awake and alert flesh-and-bone humans, not EMs or some non-corporeal instantiation of consciousness. They must not be in suspended animation, hibernation, or any sort of minimally-conscious state.