Since Temple1 is traveling at 23,000 miles per hour, the impactor spacecraft is expected to completely vaporize when it collides into the comet.

"Our mission in this project is to blast our way through the crust of a comet and expose the interior ices," says Jay Melosh, Deep Impact co-Investigator from the University of Arizona in Tucson.

"A number of people have asked me, 'Why don't you pack the spacecraft with explosives to blow a hole in it?' We're going to do better than that. Simply the speed of the spacecraft moving into the comet - 10 kilometers a second, or 10 times faster than a rifle bullet - means that the amount of energy of the spacecraft is about 10 times larger than an equivalent mass of TNT hitting the comet. So we could pack it with explosives, and it wouldn't make much of a difference. Simply by colliding with it, we're going to blast a big hole."

While the scientists are not sure exactly how large a crater the spacecraft will excavate, they estimate it will be the size of a small house to the size of a football stadium, and several stories deep.

Like all comets, the nucleus of Temple1 is made of rock and ice, but the scientists aren't sure exactly how firmly that material is bound together.

"It could be weak and fluffy like a bowl of corn flakes, or it could be like a concrete sidewalk that we're hitting," says Melosh. "Part of the challenge of the design of the impactor was to take account of either possibility."

The mothership will take photos of the comet before, during and after the impact, flying within 300 miles (482 kilometers) of the comet. It will complete the flyby in about 24 hours, and then transmit the data back to Earth over 30 days.

The mothership's high-resolution instrument will be used to see how high the impact debris shoots out of the crater and how far it travels out in space.

A medium resolution instrument will provide a wider perspective of the impact, and a spectrometer will allow scientists to determine the chemical composition of the ejected material. The impactor spacecraft has a camera as well, and will take pictures right up to the moment of collision.

The impactor spacecraft will have to hit the comet on the sunlit side so the visible light cameras on the mothership can see the impact. Maneuvering the spacecraft could be tricky, since the exact shape of Temple1 is still unknown. The scientists do know that the comet nucleus is elongated - its length is 3 times its diameter - and it is rotating slowly.

At the time of impact, the comet will be 85 to 90 million miles from Earth. This distance means that any signals sent from mission control will take 20 minutes to reach the spacecraft, so it is designed to think for itself during the final phase of the mission.

Comets are often called dirty snowballs because they are made out of water ice and organic materials. Comets may have played a major role in the origin of life on Earth, delivering a significant share of the Earth's water as well as carbon-rich organic compounds.

Most of the comets in the solar system formed 4.5 billion years ago, at the same time the planets were forming. Yet as a comet travels near the sun, the comet becomes heated and its surface chemistry changes. Scientists presume that the material beneath the surface is more pristine, and can provide information about what the solar system was like in its earliest years.

"One of the key points of Deep Impact is to understand how deep the changes go," says Mike A'Hearn, Deep Impact principle investigator from the University of Maryland.

"The material at the surface is certainly very different from anything a hundred feet deep. I am personally convinced that it's very different from anything ten feet deep. Whether it's different from something one foot deep or not, I honestly don't know."

The comet Temple1 formed in the Kuiper Belt, a gas and dust-filled region in the solar system located beyond Neptune's orbit. Temple1 now orbits the sun every 5.5 years, tracing a path between the orbits of Mars and Jupiter.

The comet currently is in the asteroid belt, so the sun's heat has not yet generated the coma of gas and dust that makes a comet visible in the night sky.

By July 4, the comet will be closer to the sun, so the Deep Impact spacecraft will have travel through gas and dust streaming off the comet. Although the scientists expect the dust to be very fine, they say there could be larger clumps of dust and ice.

Both the mothership and the impactor spacecraft have been equipped with several layers of copper shielding, and each individual shield should help break up any particles into smaller and smaller pieces.

Professional astronomers with the Deep Space Network will gather data during the event, and the Hubble, Chandra, and Spitzer Space Telescopes will point to the comet for even more data collection. Hundreds of amateur astronomers also plan to point their own telescopes at the comet on the morning of July 4.

Although comet Temple1 normally is not bright enough to observe with the naked eye, the impact could cause the comet to brighten considerably. Sky watchers should look for the comet near the bright star Spica and the planet Jupiter early in the morning on July 4.

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