In May 1989, space shuttle 'Atlantis' released in outer space the space probe 'Megal-lan', which is now on her 15-month and one-billion-kilometer flight to Venus. A new phase in space exploration has begun. The planet Venus is only slightly smaller than the earth; it is the only other object in the solar system, in fact, that even comes close to the earths size. Venus has a similar density, so it is probably made of approximately the same stuff, and it has an atmosphere, complete with clouds. It is also the closest planet to the earth, and thus the most similar in distance from the sun. In short, Venus seems to justify its long-held nickname of 'the earths twin'. The surface temperature of Venus reaches some 900 degree F. Added to that is an atmospheric pressure about 90 times the earths. High overhead in the carbon dioxide(CO2)that passes for air is a layer of clouds, perhaps 10 to 20 miles thick, whose little drops consist mostly of sulfuric acid(H2SO4). Water is all but nonexistent. Born with so many fundamental similarities to the earth, how did Venus get to be so radically different? It is not just an academic matter. For all its extremes, Venus is a valuable laboratory for researchers studying the weather and climate of the earth. It has no the earths oceans, so the heat-transport and other mechanisms are greatly simplified. In addition, the planet Venus takes 243 earth-days to turn once on its axis, so incoming heat from the sun is added and distributed at a more leisurely, observable pace. Question: The main idea of this passage is about______.