What do you hope to learn from this project?
There are a couple of different things. We're hoping to learn what's inside a comet. There are ices inside, as well as dust--silicate dust--which you see coming out of the comet, but we don't know what the proportions of the pristine ices are. I say ices very meaningfully, because in addition to water ice, there's carbon dioxide ice, carbon monoxide ice, methane ice and probably a whole bunch of different ices that are frozen into the comet. Those were frozen, we believe, about 4 1/2 billion years ago when the solar system formed. By learning how much of each different ice is present, we hope we can learn more about the conditions of that earliest phase in the solar system.
What other knowledge might you gain?
The other level is that we hope to learn about the mechanical properties of the comet--that is, whether it's loose, grainy stuff, (or) whether it's cemented together and hard as concrete. We've seen comets from a distance, but we've never actually gone out and touched them--or punched them, in this case--to learn about how strong it is (and) what the mechanical properties are.
Why did you choose to ram Tempel 1?
We picked Tempel 1 as our target simply because at our encounter, it will be both close to the sun--as close as it ever gets--and it will also be close to the Earth. That's important, because we plan to do many of the observations during impact and afterwards from the ground, so we wanted it to be as close as possible. Now, it's not really close; it's still outside the orbit of Mars, and there's no danger of any pieces of it coming and hitting the Earth.
How difficult is it to shoot an impactor from a spacecraft hundreds of millions of miles away and hope it hits the comet?
Well, it's a challenging thing to do. ... The impactor is not a dumb impactor. We actually proposed this twice. The first time around, we had a dumb impactor, and we thought we could steer the mother ship into a collision course, release this big mass of copper and then veer off--and it would hit. The experts at JPL (Jet Propulsion Laboratory) downgraded the proposal, because they weren't convinced we could actually do that. In hindsight, they were absolutely right. ... What we're flying is an impactor that has its own guidance system and a modest propulsion system, so that it can steer itself into collision.
The impactor also has to hit a specific place--is that right?
It's not just (steering) into collision; it's a collision in a sunlit area and an area that the flyby can see. And that's important, because we know this comet is about three times longer than it is wide, so it's a very long thing--kind of like a bowling pin.
What will the mother ship do as it flies by?
It has an array of telescopes that will be focused on the expected impact point.
What happens to the mother ship after the collision?
If everything works, we should have a spacecraft with better telescopic capability than any other interplanetary spacecraft and plenty of fuel. One of the peculiarities with the way NASA works is that if you're doing a mission, you may not think or spend any money on thinking about what you would do when the mission is over, even though the spacecraft is functioning. ... But, of course, we'd be crazy if we hadn't looked into--without charging NASA anything--the possibilities of what we could do next. We have a list of about six other comets we can reach within about five years. We can't reach all six, but we can reach about two or three of them. And although we won't crash into any of them, we can make images with our cameras as we do close flybys.