I'm staring at the great red spot, a giant ruddy hurricane on Jupiter, slowly rotating in front of me as though I were looking through the porthole of a future manned spacecraft. Seconds later, after taking a quick spin around the vast planet, I hover over the volcanoes on the moon Io, as if preparing for descent. Instead, I throttle back to take in the entire moon, which looks like a bruised olive orbiting above Jupiter's permanent storms. Then I set off for icy Europa, a neighboring moon, and fly over the crater formation of Callanish. As I view the landscapes, text overlays explain the latest theories about them. These computer simulations are served up by the free Web-based software program Celestia, www.shatters.net/celestia, which features graphics so convincing that you feel as if the age of space tourism has arrived. Better yet, in the digital universe of Celestia, anyone can be a guide. Unlike other popular astronomy programs such as Red-Shift and Starry Night, Celestia allows ordinary space fans to create their own customized tours through the galaxies and share those tours on the Internet.
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