VENUS

Some people may think locking some volunteers in a tin can for a couple of months is enough preparation for a flight to Mars, but the NASA panel reviewing the agency's manned space program envisions a more ambitious set of training wheels: docking with asteroids and a flyby of Venus.

ESA's Venus Express spacecraft has picked up evidence that the molecule hydroxyl is lurking in the dense atmosphere of the hot planet.

The molecule is considered to be a crucial component of any planetary atmosphere because it is highly reactive - scientists say it combats pollutants in Earth's atmosphere, and may prevent carbon dioxide from transforming into carbon monoxide above Mars.

121.5 Number of years since Venus last crossed between Earth and the Sun, making it visible to the naked eye


8 Number of years until the next Venus transit


5 billion + Estimated number of people who will be able to see all or part of the 2004 transit. People in the western U.S., Hawaii and New Zealand are out of luck until 2012


23° N, 55° W Best view, just south of Abu Dhabi, United Arab Emirates


Sun Filter Equipment necessary to view transit without eye damage