Venus

Venus is the second solar system planet from the Sun. By many characteristics it is the most-Earth-like solar system planet, but with some distinct differences as well. Characteristics:

• 0.8 Earth masses.
• 0.95 Earth radii.
• 117 Earth days - solar day.
• 225 Earth days - orbital period (Venusian Year).
• 243 Earth days - rotation period (sidereal day, often cited as negative because its rotation is retrograde).
• 0.72 AU - orbital radius.
• 0.007 - orbital eccentricity.

It has no moons. It is a rocky planet with an extremely dense atmosphere (91 atm) with more than 95% carbon dioxide, but has a minimal magnetic field. Its rotation is strikingly slow and retrograde (opposite of its orbit; among the other solar system planets, only Uranus's rotation is retrograde). Its surface temperature is 737 K, far hotter than any other solar system planet including Mercury, apparently the end effect of a runaway greenhouse effect, consistent with the striking amount of a greenhouse gas (carbon dioxide) in its atmosphere.

There is some thought that Venus formerly had liquid water on the surface, potentially for billions of years, presumably long enough for life to take hold and evolve, before the extreme greenhouse effect took hold.

There has been relatively little exploration of Venus (as compared to Mars), due to the extreme challenge of operating equipment at its temperature. The Soviet Union launched the majority of missions specifically to study Venus; NASA launched a few and also has had a number of incidental flybys allowing for some observation, in part to use Venus for gravity assists in flights to other solar system destinations.