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NASA Comet Mission On Track for July 4 Collision

Scientists at the U.S. space agency NASA say its Deep Impact spacecraft is on course to reach the comet Tempel 1 on the fourth of July, America's Independence Day. NASA hopes the impact will trigger a spectacular explosion, much like the fireworks with which America marks its national holiday.

NASA scientists say the flight system on Deep Impact has performed well since its January launch from Cape Canaveral Space Center in Florida, and the mission is on-track for its July 4 collision date with the Tempel 1 comet.

The Deep Impact spacecraft is made up of two sections - the so-called "flyby" and the impactor. The impactor has the difficult task of placing itself in the path of the speeding comet, while the flyby will transmit photographs of the collision and expected explosion back to Earth.

Comets are pieces of ice, rocks, dust and gases.

"From ground-based observations, we think its largest extent - about nine miles - is about the size of Washington, D.C., and it's sort of a jet-black, pickle-shaped, icy, dirt ball. That's our best guess of what it will look like when we arrive with the spacecraft," explained Don Yeomans, a scientist on the Deep Impact team

Michael A'Hearn, the chief investigator on the team, says the mission's main goal is to study the differences between the surface and interior of the comet . He says the spacecraft's collision with the comet is simple in concept, but technically difficult to carry out.

"We get one chance lasting 800 seconds to take all of the key data from impact until we've flown past and the challenge of getting all this to happen autonomously has been the really big effort that has gone on over the last couple of years," he said.

NASA will have many eyes trained on the collision. In addition to Deep Impact's flyby spacecraft, at least 30 high-powered telescopes - including Hubble and Spitzer - will be watching, as will dozens of professional astronomers in about 20 countries.

Mr. Yeomans says he hopes the Deep Impact mission will also help unlock the mysteries of how planets, such as Earth, form in outer space.

"Not only are comets thought to be leftover bits of outer solar system formation process, but they are thought to have brought to the early Earth much of the water and carbon-based molecules that allowed life to form. So they hold the keys to the birth of solar system, and perhaps to life itself - providing the building blocks," he said.

If the mission is successful, it will be the first time a spacecraft has ever touched the surface of a comet, and NASA scientists hope that impact will provide answers to some of their questions about the evolution of the solar system.