Venus is the second planet fromthe Sun and is named after the Roman goddess of love and beauty. Asthe brightest natural object in Earth's night sky after the Moon,Venus can cast shadows and can be visible to the naked eye in broaddaylight. Venus's orbit is smaller than that of Earth, but itsmaximal elongation is 47°; thus, at latitudes with a day-nightcycle, it is most readily visible for up to a few hours following thestart of sunset or before sunrise. At times, it has been seen in acompletely dark sky. Venus orbits the Sun every 224.7 Earth days. Ithas a synodic day length of 117 Earth days and a sidereal rotationperiod of 243 Earth days. Consequently, it takes longer to rotateabout its axis than any other planet in the Solar System, and does soin the opposite direction to all but Uranus. This means that the Sunrises from its western horizon and sets in its east. Venus does nothave any moons, a distinction it shares only with Mercury among theplanets in the Solar System.
The third smallest planet in the SolarSystem, 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 radicallydifferent from Earth in other respects. It has the densest atmosphereof the four terrestrial planets, consisting of more than 96% carbondioxide. The atmospheric pressure at the planet's surface is about 92times the sea level pressure of Earth, or roughly the pressure at 900m (3,000 ft) underwater on Earth. Even though Mercury is closer tothe Sun, Venus has the hottest surface of any planet in the SolarSystem, with a mean temperature of 737 K (464 °C; 867 °F). Venus isshrouded by an opaque layer of highly reflective clouds of sulfuricacid, preventing its surface from being seen from Earth in light. Itmay have had water oceans in the past, but after these evaporated thetemperature rose under a runaway greenhouse effect. The water hasprobably photo-dissociated, and the free hydrogen has been swept intointerplanetary space by the solar wind because of the lack of aplanetary magnetic field. Because of the lethal surface conditions,the planet is sometimes referred to as Earth's "evil twin".
As one of the brightest objects in thesky, Venus has been a major fixture in human culture for as long asrecords have existed. It has been made sacred to gods of manycultures and has been a prime inspiration for writers and poets asthe "morning star" and "evening star".Venus was the first planet to have its motions plotted across thesky, as early as the second millennium BCE.
Its proximity to Earth has made Venus aprime target for early interplanetary exploration. It was the firstplanet beyond Earth visited by a spacecraft (Venera 1 in 1961) andthe first to be successfully landed on (by Venera 7 in 1970). Theplanet's thick clouds render observation of its surface impossible inthe visible spectrum, and the first detailed maps did not emergeuntil the arrival of the Magellan orbiter in 1991. Plans have beenproposed for rovers or more complex missions, but they are hinderedby Venus's hostile surface conditions. The possibility of life onVenus has long been a topic of speculation; in recent years, thetopic has received active research.
Physical characteristics
Venus is one of the four terrestrialplanets in the Solar System, meaning that it is a rocky body likeEarth. It is similar to Earth in size and mass and is often describedas Earth's "sister" or "twin". Thediameter of Venus is 12,103.6 km (7,520.8 mi)—only 638.4 km (396.7mi) less than Earth's—and its mass is 81.5% of Earth's. Conditionson the Venusian surface differ radically from those on Earth becauseits dense atmosphere is 96.5% carbon dioxide, with most of theremaining 3.5% being nitrogen. The surface pressure is 9.3mega-pascals (93 bars), and the average surface temperature is 737 K(464 °C; 867 °F), above the critical points of both majorconstituents and making the surface atmosphere a supercritical fluid.
