While most people tend to focus on Mars when thinking of exploring planets in Earth's neighbourhood, the European Space Agency is taking some time to move in the other direction by sending a probe towards Venus, our other neighbour.
Of all planets, Venus is the most similar to Earth in terms of size and gravity, but its enviroment is even more hostile to humans than that of Mars. It is dominated by a runaway greenhouse effect that keeps surface temperatures above 400°C and a pressure 90 times greater than that of Earth. Its atmosphere is composed mainly of carbon dioxide and is permanently clouded, hiding the surface from view (and, probably, keeping the temperature lower than it would otherwise be). One other unusual feature of the planet is the orientation of its axis, which is tilted so much that the planet rotates in the opposite direction as compared to all others (except Pluto, which is always an exception, and Uranus). Also, the rotation is so slow that one local year is equivalent to less than two local days.
ESA's Venus Express was launched yesterday morning (Australia time) from Kazakhstan, and will reach Venus in 162 days, on 21 April 2006. The probe is expected to remain in operation for two Venusian days (that is, approximately 15 Earth months) and to make several measurements of atmosphere dynamics and temperature distribution, trying to infer clues about the characteristics of the surface.
The former Soviet Union was the pioneer in the exploration of Venus, with 16 Venera probes launched between 1961 and 1983. Venera 3 was the first man-made object to land (or, more accurately, crash) in another planet, and Venera 4 the first to send data from an alien atmosphere. In all, the Venera probes returned an impressive amount of data, including chemical analysis of soil samples, surface images and video, and even sound recordings. Unfortunately, due to the severly hostile enviroment, none of the probes lasted for more than a few hours on the surface.
Venus Express is expected to help us understand the workings of the Venusian climate and, hopefully, of the origins of its greenhouse effect, without which it would have an enviroment not very dissimilar from ours. Since we may be starting to deal with a (much smaller) greenhouse effect of our own, Venus may teach us many lessons in the future.