Venus

Venus is the second planet from the Sun and one of the four rocky inner planets. Its size and internal structure are broadly similar to Earth's, but its surface and atmosphere are markedly different. Venus completes an orbit in about 224.7 Earth days. It rotates slowly in a retrograde direction, opposite to the direction of most planets: one rotation relative to distant stars takes about 243 Earth days. Because of this reverse spin, a solar day on Venus lasts about 117 Earth days. The planet's thick cloud cover makes it appear bright in Earth's sky, especially around sunrise and sunset.

Venus has the most massive atmosphere among the terrestrial planets. It is composed mainly of carbon dioxide, with nitrogen and smaller amounts of other gases; high clouds contain concentrated sulfuric acid. The atmosphere traps outgoing heat, creating a strong greenhouse effect. Average surface temperatures are roughly 465 °C, making Venus hotter than Mercury despite orbiting farther from the Sun. The pressure at the surface is around 90 times Earth's sea-level atmospheric pressure. These extreme conditions make direct surface measurements difficult and cause landers to operate only briefly.

A persistent cloud layer hides Venus's surface from ordinary visible-light observation, so radar has been essential to mapping it. Spacecraft data show broad plains, highland regions, mountains, impact craters and numerous volcanic landforms. NASA's Magellan mission used radar to image the entire planet, revealing a surface strongly shaped by volcanism and other geological processes. How those processes have changed through time, and how much volcanic activity is occurring now, remain subjects of scientific investigation.

Robotic missions transformed Venus from a bright but cloud-obscured point of light into a world whose atmosphere and surface could be studied at close range. NASA's Mariner 2 made the first successful planetary flyby in 1962. Soviet Venera spacecraft later returned measurements and brief surface images. NASA's Magellan, launched in 1989, entered Venus orbit in 1990 and mapped the surface with radar before its mission ended in 1994. ESA's Venus Express arrived in 2006 and operated until 2014, studying the planet's atmosphere, clouds and surface temperatures. These missions combined flyby, orbital, atmospheric and lander observations, each revealing different parts of the planet.

Venus is a useful comparison for studying rocky planets. Its similarity to Earth in size, combined with its very different climate and surface conditions, gives planetary scientists a way to investigate how atmospheres and geological histories can develop along different paths. Radar mapping, atmospheric measurements and comparisons with Earth continue to refine understanding of the planet and of the processes that shape terrestrial worlds.