Extraterrestrial atmosphere other trace elements, including compounds based on hydrogen, nitrogen, sulfur, carbon, and oxygen. The atmosphere of Venus is much hotter and denser than that of Earth, though shallower. As greenhouse gases warm a lower atmosphere, they cool the upper atmosphere, leading to compact thermospheres. By some definitions, Venus has no stratosphere. The troposphere begins at the surface and extends up to an altitude of 65 kilometres (an altitude at which the mesosphere has already been reached on Earth). At the top of the troposphere, temperature and pressure reach Earth-like levels. Winds at the surface are a few metres per second,
Atmosphere of Venus surface, attention shifted towards other targets such as Mars. There have been a number of proposed missions afterwards however, and many of these involve the little-known upper atmosphere. The Soviet Vega program in 1985 dropped two balloons into the atmosphere, but these were battery-powered and lasted for only about two Earth days each before running out of power. Since then, there has been no exploration of the upper atmosphere. In 2002, the NASA contractor Global Aerospace proposed a balloon that would be capable of staying in the upper atmosphere for hundreds of Earth days as opposed to two. A solar