Venus orbits the Sun every 225 days at a distance of 108 million km; it can pass closer to Earth than any other planet, within 40 million km. With a diameter of 12,100 km, only 650 km smaller than the Earth, it is almost a twin of our own planet in size. But the main reason for the brilliance of Venus in the sky is not its size or proximity but its cloak of unbroken clouds that reflect two-thirds of the light hitting them. While making Venus so prominent, these clouds also prevent astronomers from seeing its surface. Unable to see the planet's surface, astronomers could only guess at the rotation period of Venus until the 1960s - and they guessed incorrectly. As with Mercury, radar observations provided the surprising truth. It turns out that Venus rotates on its axis from east to west, the opposite direction from the Earth and other planets, and it does so very slowly: once every 243 days, longer than the 225 it takes to orbit the Sun. Its clouds, though, rotate every four days, also retrograde (east to west), a result of high-speed winds in the upper atmosphere. Before space probes arrived there, theories about the nature of the planet's surface abounded. Since Venus was so similar in size to the Earth, it was tempting to speculate that conditions there might be Earth-like. One charming notion was that Venus resembled our planet as it had been in Carboniferous times, with steaming jungles and even dinosaurs. Some astronomers proposed that the planet was covered with water, while others imagined it to be a world of deserts. None of the theories came close to anticipating the uniquely hostile conditions on Venus. Radio astronomers provided the first clue in the late 1950s when they detected radio-wave emissions from the planet, which implied that it was very hot - even hotter than boiling water. By comparison, the deserts of the Earth are only mildly warm. These readings, doubted at the time, were confirmed by the American probe Mariner 2 which scanned the planet as it flew past it in 1962. Conditions on Venus were experienced directly for the first time by a Soviet probe, Venera 4, when it parachuted into the atmosphere in October 1967. It found that Venus's atmosphere is made almost entirely of unbreathable carbon dioxide gas, but the probe was destroyed by the intense heat and crushing pressure long before it reached the surface.Venera 7 was the first probe to land intact on the surface, in 1970 on December 15. It registered a temperature of 475°C and an atmospheric pressure 90 times that on Earth. Venera 7 landed on the night side of the planet; in 1972 its successor Venera 8 landed on the day side, finding identical conditions. The reason why Venus should be so hot, despite the fact that its clouds reflect over three-quarters of the incoming sunlight, is the greenhouse effect, which works far more effectively on Venus than on Earth. About 1 per cent of the incoming sunlight penetrates to the planet's surface, so that it is as gloomy there as on a heavily overcast day on Earth. That incoming sunlight is absorbed by the surface and is re-radiated at longer wavelengths, in the infrared. Although the carbon dioxide of the atmosphere is transparent to visible light, it traps infrared; since infrared is heat energy, the temperature of the atmosphere rises. It turns out that Venus and the Earth have similar amounts of carbon dioxide, but on Earth most of it is locked away in rocks such as limestone. Whereas the amounts of carbon dioxide on both planets are similar, Venus is almost entirely devoid of water - whatever water it originally possessed has long since evaporated and been lost to space. Only a trace of water vapour remains, but that is sufficient to boost the effect of the carbon dioxide in causing the greenhouse effect of the atmosphere of Venus. A final contribution to the greenhouse effect is provided by the clouds of Venus. These are made not of water vapour, as are the clouds of Earth, but of sulphuric acid of 80 per cent concentration, stronger than in a car battery. Sulphuric acid, too, absorbs infrared. Taken together, these three factors of carbon dioxide, water vapour and sulphuric acid turn Venus into a perfect suntrap. The clouds add to the nastiness of Venus in another way: from them descend showers of corrosive sulphuric acid rain. Despite its heavenly name, Venus turns out to be an incarnation of hell. Although the clouds of Venus mask its surface from view, astronomers have nevertheless been able to map the planet's features by radar, which penetrates the clouds. Radar observations from Earth during the 1970s first revealed some features, and detailed maps of the entire planet have since been made by spacecraft, notably the Magellan probe that went into orbit around Venus in 1990. Venus is mostly rolling plains, but there are three main continental areas. One, called Ishtar Terra, the size of the United States, has a mountain range, Maxwell Montes, which towers 12 km above the mean surface level, higher than Mount Everest on Earth. The largest continental area of all, Aphrodite Terra, the size of South America, is cut by a system of rift valleys that extends for thousands of kilometres. Magellan's radar 'eye' spotted impact craters ranging in size from over 100 km across down to 3 km, demonstrating that large meteorites can get through the dense atmosphere without burning up. Most exciting of all were volcanic mountains with fresh-looking lava flows on their flanks, notably Maat Mons, at 8 km high the second-highest peak on the planet, which lies near the equator in Aphrodite Terra. Surrounding it are lava flows estimated to have been no more than ten years old when Magellan observed them. Clearly, Venus is still an active planet, with highlands formed by volcanic action.