delta-v needed to transfer between them than to any other planet. This has made Venus a prime target for early interplanetary exploration. It was the first planet beyond Earth that spacecraft were sent to, starting with Venera 1 in 1961, and the first planet to be reached, impacted and in 1970 successfully landed on by Venera 7. As one of the brightest objects in the sky, Venus has been a major fixture in human culture for as long as records have existed. It has been made sacred to gods of many cultures, gaining its mainly used name from the Roman goddess of love and beauty which it is associated with. Furthermore, Venus has been a prime inspiration for writers, poets and scholars. Venus was the first planet to have its motions plotted across the sky, as early as the second millennium BCE. Plans for better exploration with rovers or atmospheric missions, potentially crewed, at levels with almost Earth-like conditions have been proposed. Physical characteristics Venus is one of the four terrestrial planets in the Solar System, meaning that it is a rocky body like Earth. It is similar to Earth in size and mass and is often described as Earth's "sister" or "twin". The diameter of Venus is —only less than Earth's—and its mass is 81.5% of Earth's. Conditions on the Venusian surface differ radically from those on Earth because its dense atmosphere is 96.5% carbon dioxide, with most of the remaining 3.5% being nitrogen. The surface pressure is , and the average surface temperature is , above the critical points of both major constituents and making the surface atmosphere a supercritical fluid. Atmosphere and climate Venus has a dense atmosphere composed of 96.5% carbon dioxide, 3.5% nitrogen—both exist as supercritical fluids at the planet's surface—and traces of other gases including sulfur dioxide. The mass of its atmosphere is 92 times that of Earth's, whereas the pressure at its surface is about 93 times that at Earth's—a pressure equivalent to that at a depth of nearly under Earth's oceans. The density at the surface is , 6.5% that of water or 50 times as dense as Earth's atmosphere at at sea level. The -rich atmosphere generates the strongest greenhouse effect in the Solar System, creating surface temperatures of at least . This makes the Venusian surface hotter than Mercury's, which has a minimum surface temperature of and maximum surface temperature