We have learned a lot about Venus since Galileo first observed it with his telescope 400 years ago. We know that it is similar in mass and size to Earth, leading to its designation as Earth's sister planet.
However, this is not a sister you want to visit. There would be no oxygen for you to breathe because Venus's atmosphere is mainly carbon dioxide. Furthermore the temperature is 465 degrees Celsius day and night, all year long! And even if you could endure these conditions, the atmospheric pressure is 90 times higher than on Earth, so you would be crushed - and that is exactly what happened to a Soviet spacecraft that penetrated Venus's atmosphere in 1967. It was not robust enough to withstand the high pressure. After that, they built stronger ships and had a number of successful landings in the 1970s and 1980s. In fact, some of the probes transmitted photographs back to Earth. These photos showed that Venus has a rocky surface.
Scientists understand why these two planets are so different.
Here on Earth, liquid water and the development of life influenced the the evolution of our atmosphere. Earth used to have just as much carbon dioxide as Venus, but much of it dissolved in the oceans and subsequently formed into sedimentary rocks. In addition, plant life has removed carbon dioxide from the atmosphere by the process of photosythesis, which releases oxygen. Our plant life produces the oxygen we breathe.
The sequence of events on Venus was different because it formed closer to the Sun where the temperature was higher. Consequently, its water started to evaporate, producing water vapour which is a greenhouse gas like carbon dioxide. This caused the temperature to rise and even more water to evaporate and the cycle continued. Scientists call this a "runaway greenhouse effect". Eventually all the water evaporated and escaped from the planet, leaving a dense carbon dioxide atmosphere. Today Venus is 500 degrees hotter than it would be if it had no atmosphere.
Knowing what happened to Venus raises concerns about the future of Earth as we continue to increase the carbon dioxide abundance in our own atmosphere. Nevertheless it must be acknowledged that a small amount of greenhouse effect is not a bad thing. The carbon dioxide and water vapour in our atmosphere have already raised Earth's temperature by 35 degrees. Without them, it would be much colder - something to ponder on a cold winter day. However, we don't want a runaway greenhouse effect on Earth. We have to take care of our planet.
So be glad that you reside on Earth not Venus. From now until the end of March 2009, you can enjoy observing Venus from afar as it shines brilliantly in the southwest sky in the early evening.