Venus

Venus is the second solar system planet from the Sun. By many characteristics it is the most-Earth-like of other solar system planets, 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, sometimes cited as a negative time-interval, a means of indicating Venus's retrograde rotation).
• 0.72 AU - orbital radius (semi-major axis).
• 0.007 - orbital eccentricity.

Venus has no moons. It is a rocky planet with an extremely dense atmosphere (92 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. Other than carbon dioxide, the atmosphere is mostly nitrogen with traces of sulfur dioxide (SO₂) and clouds of sulfuric acid (H₂SO₄).

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, including some very-short-lived landers; NASA launched a few non-landing Venus missions and also has had a number of incidental flybys allowing for some observation while using Venus for gravity assists during flights to other solar system destinations.