JEFFREY KAYE: NASA has plans to create its own fireworks show on the Fourth of July. That's when the space agency hopes to smash a probe into a speeding comet the size of Washington, D.C., with a force equal to four and a half tons of TNT exploding.
The kamikaze-style mission, called Deep Impact, might seem like cosmic vandalism, but there's a serious scientific objective: To better understand the makeup and origins of comets and, by extension, the early history of the solar system.
DONALD YEOMANS: This one, we're going to reach out and smack it and understand what is in the inside.
JEFFREY KAYE: Donald Yeomans, an expert on comets and asteroids, is a scientist with the Deep Impact mission at NASA's jet propulsion laboratory near Pasadena.
DONALD YEOMANS: They are the least changed objects within the solar system, so they do offer the opportunity for understanding the chemical mix from which the solar system formed.
JEFFREY KAYE: Yeomans says comets are like celestial time capsules. They contain material from the ancient swirling stew of gas, dust and debris that developed into our solar system.
DONALD YEOMANS: Comets are the leftover bits and pieces from the outer solar system formation process. So if we wish to understand the initial conditions from which the outer solar system formed, the chemical mix and the structure of the particles that came together to form Jupiter, Saturn, Uranus and Neptune, then we'd like to study comets because they haven't changed a great deal in the intervening four and a half billion years.
JEFFREY KAYE: Composed of gas, dust and ice, comets are massive and dirty celestial snowballs, swooping around the sun in long elliptical orbits. As they get closer to the sun, ice evaporates, causing streams of gas and dust -- comets' signature tails -- which can extend for tens of thousands of miles. By smashing into a comet and peeking inside, scientists hope to learn more about their composition.
DONALD YEOMANS: Is it a solid ice ball, like a clump of ice you'd take out of your refrigerator? Is it solid? Or is it a rubble pile that's bits and pieces held together by little more than their own self gravity.
JEFFREY KAYE: The target comet is called Tempel 1, named for Wilhelm Ernst Tempel, a European astronomer who observed it in 1867. NASA has used space telescopes in space to determine Tempel 1's shape and to pinpoint its location. On the day of the planned collision, the comet will be more than 83 million miles from Earth. Deep Impact is a $330 million mission. The probe consists of two connected spacecraft -- seen here as engineers prepared for their launch this past January.
One of them, the aptly dubbed "Impactor," is designed to strike the solid part of the comet -- its nucleus. Its mate is a flyby spacecraft about the size of a small car packed with computers, cameras and communications gear. Mission manager David Spencer is responsible for Deep Impact's day-to-day operations.
DAVID SPENCER: Just 24 hours before we get to Tempel 1, we're going to release the "Impactor," and it's going to be in free flight on its own.
JEFFREY KAYE: Using an onboard navigation system and thrusters, the "Impactor" is designed to guide itself towards its explosive rendezvous with the comet.
DAVID SPENCER: The "Impactor" spacecraft is taking images all the way in, actually right up to the point of impact. So the result of that is we're going to, if everything goes as planned, have some really spectacular images, very high-resolution images close up to the surface of a comet. The first time that's ever been done.
JEFFREY KAYE: Meanwhile, the flyby craft should maneuver within about 300 miles of the comet to take pictures of the crater created by the crash.
DAVID SPENCER: It's got some very good imagers on board, so it's going to be taking pictures of the comet before, during and after the impact. But it also serves a purpose of relaying the data back from the "Impactor."
JEFFREY KAYE: Even though they've practiced their rendezvous with Tempel 1 again and again, Deep Impact's mission control personnel anticipate many white knuckle moments.
DAVID SPENCER: Deep Impact is a very challenging mission. First off, we've got two fully functional spacecraft that we have to operate simultaneously.
Secondly, we've got a target out there: The comet that's very small. It's difficult to see. We don't understand it very well and we don't know how it's rotating. We don't know whether it's rotating around its axis or whether it's kind of tumbling, and so the target's always shifting.
JEFFREY KAYE: For a successful mission, the "Impactor" has to hit the pickle-shaped comet in just the right spot as it hurtles through space at six miles a second. In addition, the flyby vehicle has to survive its close encounter.
DAVID SPENCER: The flyby spacecraft could be damaged by debris. We're flying the flyby spacecraft 500 kilometers, about 310 miles away from the nucleus of the comet.
We think that's far enough out so that we shouldn't hit any really large dust particles, but we haven't been to enough comets to really know for sure, so there's always a risk.
JEFFREY KAYE: Despite their beauty, comets have cast a sinister shadow over human history, myth and folklore. Across centuries and civilizations, comets were often viewed as harbingers of doom, their unpredictable arrival in the night sky seen as foreshadowing wars, plagues and natural disasters.
In recent times, concerns have turned to the possibility of a comet striking the Earth and extinguishing civilization.
DONALD YEOMANS: Comets and asteroids get a little bit of bad press because the only time we hear about them is when they're close to us and threaten us.
JEFFREY KAYE: Yeomans says all living things on Earth probably owe a debt of gratitude to comets for bringing the spark of life to this world early in its history.
DONALD YEOMANS: What folks don't often understand is that we're all made of cometary stuff. The carbon-based molecules in the water that make up our persons are all brought to the early Earth via comets.
JEFFREY KAYE: Deep Impact's planned encounter won't be visible to the naked eye, but telescopes in space and observatories large and small will be trained on Tempel 1 to study the collision. And NASA says if you're on the West Coast or Hawaii and use a telescope or strong binoculars, you should see the comet brighten at impact -- about 2 in the morning Eastern time, July 4.
DONALD YEOMANS: You'll see a very faint streak.