Venus

**Venus** is the second planet from the Sun. It is a rocky planet and the only Solar System object with a size and mass close to that of Earth. It has an atmosphere, which is by far the thickest of any rocky body in the Solar System. Venus is the planet with the nearest orbit to Earth's orbit. Orbiting inferiorly (inside of Earth's orbit), it appears in Earth's sky always close to the Sun, as either a "morning star" or "evening star". While this is also true for Mercury, Venus appears as such much more prominently, since it is the third brightest object in Earth's sky, after the Moon and the Sun, appearing brighter than any other star-like classical planet, and any fixed star of Earth's sky. With such prominent appearances in Earth's sky, Venus has been a common and important object for humans, in both their cultures and astronomy.

Venus retains, despite having only a weak induced magnetosphere, an especially thick atmosphere of mainly carbon dioxide, creating an extreme greenhouse effect together with its global sulfuric acid cloud cover. Because of this, the atmosphere reaches at its bottom a mean temperature of 737 K (464 °C; 867 °F) and a crushing pressure of 92 times that of Earth's at sea level, turning the air into a supercritical fluid, though at cloudy altitudes of 50 km (30 mi) Earth like levels are found. Conditions possibly favourable for life on Venus have been identified at its cloud layers, while recent research has found indicative, but not convincing evidence. Early in Venus' history water might have been abundant enough to form oceans, before it probably evaporated when greenhouse effects cascaded and then was taken away into space by the solar wind. Internally Venus is thought to consist of a core, mantle, and crust, the latter releasing internal heat through its active volcanism, shaping the surface with large resurfacing instead of plate tectonics.

Venus has no moon (like Mercury). It rotates retrograde (like Uranus), against its orbital direction. Having been slowed by the strong currents and drag of the atmosphere it completes a sidereal rotation, relative to the stars, in 243 Earth days. Therefore it rotates slower than it is orbiting, having a solar year of 224.7 Earth days, this results together with the retrograde rotation in having a solar day of 117 Earth days. Venus and Earth approach each other in synodic periods of 1.6 years, while coming closer to each other at inferior conjunction than any other pair of planets, both on average staying closer to Mercury than to any other planet. That said the gravitational potential between Earth and Venus and the needed speed to transfer between them is the lowest than between any other planet from Earth. This and its proximity to Earth has allowed Venus to be the most accessible destination and attractive gravity assist waypoint for interplanetary flights.

In 1961, Venus became the target of the first interplanetary flight in human history, followed by many essential interplanetary firsts, confirming in 1970 Venus' inhospitable surface conditions with the first soft landing on another planet. Venus as a place for humans to be was a popular topic in early science fiction. Actual proposals have suggested to send crews particularly on flybys, used as gravity assists for crewed missions to Mars, or to enter the atmosphere and stay afloat at altitudes with conditions most comparable to Earth's surface, including radiation and gravitation, than anywhere in the Solar System. Currently robotic probes are studying and will be sent to study Venus, having been providing crucial knowledge particularly about greenhouse effects, informing predictions about global warming on Earth.