Comet collision!

This Fourth of July, one exploding attraction will truly be out of this world. On that day, NASA scientists will smash a device into a comet, or a celestial body made of ice, rock, and dust that circles the sun.

Last January, the device launched into space aboard the craft Deep Impact. This device-carrying craft was set on a course toward Tempel 1, a comet that orbits the sun every five and a half years. As it nears the comet, Deep Impact will send the impacting device reeling into Tempel 1. The collision will gouge a stadium-size crater in the comet--spewing forth gases and rocky particles trapped below the comet's surface.

Donald Yeomans, a NASA scientist, hopes the collision will answer questions about comets. "Is [Tempel 1] layered like an onion? Is it a pile of rubble? Watching the impact will give us clues," he says. Plus, special detectors will analyze the ejected material to determine what lies beneath the comet's surface.

ORBITAL PERIODS DF COMETS

Every comet has its own orbital period: Each takes a different length of time to complete a full orbit around the sun. About how many more years does Comet Halley spend in one orbit compared with Comet Tempel-Tuttle?

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LONG TRIP: Deep Impact will circle the sun (orange line) before hitting the comet.

WEB EXTRA

To keep an eye on the summer show, visit: http://deepimpact.nasa.gov