JENNIFER LUDDEN, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Jennifer Ludden.

Tonight, around 2 AM Eastern time, if all goes according to plan, a 820-pound NASA spacecraft will collide with a comet, setting off an explosion that could produce a crater the size of a football field. The collision will take place 83 million miles from Earth and it's intentional. A second spacecraft will be taking pictures. Scientists hope the mission, called Deep Impact, will reveal the raw material inside comets. NPR's David Kestenbaum spoke with three of the scientists involved. He found the success of the mission is not at all guaranteed.

DAVID KESTENBAUM reporting:

Carey Lisse has been working on the Deep Impact mission for six, going on seven years of his life. He says he's a logical guy, but he was still nervous when the spacecraft was sitting on the launchpad. And, frankly, though he can think of no reason to doubt that everything will go as planned, he's a little nervous about the collision, which will happen at 23,000 miles an hour.

Mr. CAREY LISSE: The hard part of our experiment is making sure the spacecraft hits the comet. It's the size of the Appalachian Mountains. It sounds big to you and me, but it's tiny compared to the solar system. It's about 14 kilometers acr--long and about maybe seven wide, and we're doing this 90 million kilometers away from us. The hard part is actually making sure we hit the thing.

KESTENBAUM: The spacecraft was released last night from the Deep Impact mother ship. Scientists call the little one the Impactor. It's carrying over 300 pounds of copper, and its job is to get in the path of the comet. The comet is called Tempel 1, and scientists want to hit it sunny side so that they can get a good view of the explosion and what they hope will be a giant crater. In the final hours, the Impactor will have to steer itself. David Spencer is the mission manager and an engineer at NASA's Jet Propulsion Laboratory. He says the comet is a bit of a moving target.

Mr. DAVID SPENCER (Jet Propulsion Laboratory, NASA): We're encountering Tempel 1 just as it reaches its closest point to the sun in its orbit. And at that point, the comet's at its most active. It's got volatile material that's been underneath the surface, and as it gets heated up, there are small explosions or outbursts on the surface of the comet. Each one of those explosions makes the comet dance around. So trying to impact this comet is kind of like trying to hit a knuckleball. It's not going in a smooth path; it's dancing around.

KESTENBAUM: Total mission cost is $333 million. David Farnham, another scientist on the mission, has faith in the spacecraft and the engineers. What he's not sure about is the comet.

Mr. TONY FARNHAM: Well, I'm confident that we're going to hit the comet, so that's not really a worry. I think my biggest worry is that it's going to kind of go ka-thunk like a rock into mud and not really do anything. I think that's my biggest fear.

KESTENBAUM: A ka-thunk with just a sputtering of debris would still be interesting he says. Comets are sometimes called dirty snowballs, but no one knows if they have the consistency of densely packed snow or ice or something airy and light. The material inside is thought to be among the oldest in the solar system, relics from a time over four billion years ago when the planets formed, material scientists have never really seen before.

Mr. FARNHAM: The problem is is that what we see from the ground and even with fly-bys of comets is--all we see are the upper layers, and those have been processed over time. Every time it comes in toward--makes an approach to the sun, material is vaporized off and it changes the surfaces. And what we hope is that this experiment is going to break through that surface and get us deep enough that we can actually see the primordial material.

KESTENBAUM: OK, OK. So there is important science about the origin of the Earth going on here. But do we need to hit a perfectly nice comet? And what if by hitting the comet we put it on a collision course with Earth? Mission manager David Spencer says he's been asked that one before.

Mr. SPENCER: Well, the impact with the comet is kind of like a gnat hitting the windshield of a 737. It really is not going to change the course of the comet at all.

KESTENBAUM: David Spencer will be watching from the Jet Propulsion Laboratory in California, but telescopes all over the Earth will be trained on the comet. Tony Farnham will be at Kitt Peak in Arizona; Carey Lisse is on Manua Kea in Hawaii. The Hubble telescope will be watching from orbit, and the Deep Impact mother ship will take close-ups to beam back to Earth.

Tempel 1 is currently too dim to see with the naked eye. The impact may cause it to brighten a bit. Scientists say after the collision, a pair of binoculars might do the trick.

David Kestenbaum, NPR News. Transcript provided by NPR, Copyright NPR.