Roman Amour of the BBC's program "Material World" interviewed (by phone) Dr. Michael A'Hearn, the Principal Investigator for the NASA Deep Impact mission, and Dr. Duncan Steel University of Salford, in the BBC studio in Manchester, England, on May 31, 2001.

RA: The term "shooting the sun" is seafaring slang for measuring the sun's altitude with a sextant. "Shoot the moon" is both a quaint-old term for doing a midnight flit in a quaint old Alan Parker film. And "shoot the comet", well, that one's new. Very new. In the last week NASA have announced plans to, for the first time, combine space ship, with marksmanship in their 200 million pound "Deep Impact" mission, which aims to fire a 350 kg projectile into the heart of the Comet Tempel 1 as it passes close to earth. Now it's definitely a mission with a "wow" factor, but what about the why factor? What's to be gained in scientific rather than publicity terms from this act of cosmic vandalism? Well, on the phone somewhere in America is Mike A'Hearn, director of the Deep Impact mission and in our Manchester studio is Duncan Steel a Reader in Space Technology at Salford University and the author of the scarily titled "Rogue Asteroids and Doomsday Comets: The Search for the Million Megaton Menace That Threatens Life on Earth," which helped inspire those planetary disaster movies, "Deep Impact" and "Armageddon." Although we won't hold that against him. Mike A'Hearn, so, what's Comet Tempel 1 ever done to you?

MA: Well this particular comet hasn't done anything to me. It just happened to be in the right place at the right time.

RA: Or the wrong time from the comet's point of view probably.

MA: Well, perhaps, yes. Hard to get a mission to a comet unless it is reasonably close to the sun and is close to the plane of the Earth's orbit. And this comet satisfies that criteria and it allows us to launch when NASA said we could have a launch window.

RA: All right, so, you've hunted around for a suitable comet and this was the one that best fitted your criteria.

MA: Exactly.

RA: Duncan Steel, what do we know about Tempel 1?

DS: Actually, it's a great target comet, as Mike said, there's a limited number of opportunities, but this one really is ideal. It's been observed now for 130 odd years. It was observed three times coming back in the 19th century then it disappeared until the 1960's when after a, I think, close approach to Jupiter it got flipped into a slightly different orbit, which brings it a bit closer to the sun. And as a result of that it kind of got active again, and was spotted again. So in many ways, it is a great target. It's one we know the orbit of, very precisely and obviously Mike needs to know exactly where this comet is going to be on July 4th in four years time and so it's a great target all around.

RA: Yeah, I suppose you do need to know exactly. You don't want to be going left a bit, up a bit, down a bit, right a bit - fire - do you on the big day. Mike, what was the origin of the idea? This is a very different sounding kind of mission. Is it genuinely different?

MA: Yeah, this is an entirely different kind of mission. Most of the missions that NASA runs, and all the space agency runs are passive ones which go someplace and make passive observations. Occasionally they'll do an experiment on a microscopic scale - analyze a single rock or analyze a little parcel of gas in the atmosphere of a planet. But doing a real experiment on a planet scale where you affect another planetary body on a macroscopic scale is really rare. The only other good example I can think of is when we dropped a lunar module on the moon and measured the effect with the seismometers that had been scattered around by the astronauts.

RA: Where I come from, the opposite of passive is aggressive. Is that what you're calling this kind of mission?

MA: I was thinking of active and passive - in the sense of verbs.

RA: Take me through exactly what will happen then. You launch the rocket and the rocket then launches the missile effectively? It's a two-stage operation?

MA: Well, it's actually two independent spacecraft and they fly together for a year and a half. We do one year around the sun - practice and calibrate our instruments and practice the targeting algorithms on the moon. And then we go 6 months to the comet, and in fact we don't launch one from the other we separate as gently as we possibly can about one day before impact. Then the impactor takes over it's own control. It is a complete spacecraft. And it does this a little up, a little left, a little to the right, in order to impact on an illuminated part of the nucleus of the comet - on the side where the flyby spacecraft will go by. The flyby has to slow down a little bit in order to give us a 15-minute window of observation before we fly past.

RA: I take it as in all the best movies; you just get one shot, do you?

MA: That's right. One shot is it.

RA: And you have to be pretty confident about your aim then? That's what all the calibrating about?

MA: Yes - exactly.

RA: And any figures on the speed of the collision between the impactor and the comet?

MA: 10.2 kilometers per second.

RA: Which is twenty something thousand miles an hour, if my math is up to it?

MA: That's correct.

RA: Duncan Steel, presumably, true to the mission's title, if it's going at that speed - we can expect a "deep impact?"

DS: Yes, correct, in fact, some people seem to imagine that this projectile, or the separate spacecraft might actually be carrying explosive charges. That's totally incorrect. In fact there's absolutely no point. At that sort of speed, Mike mentioned just over 10 kilometers per second, this big lump of copper - and a few instruments on board, actually has more energy than about 10 times that mass of TNT - just because of the high impact speed its kinetic energy, as physicists call it. And so indeed, it'll be a big bang. When that hits and it vaporizes, it's going to blow out a good size hole in the surface of that comet. And that's brilliant because the whole aim of this mission is to see what comets are really made of, what have they got underneath their skin. Because up until now, all we've seen is the skin and we really want to know what's underneath.

RA: But it's not that big a comet is it? Isn't there a possibility it might not just gouge a hole out of Tempel 1, it could send Tempel 1 into smithereens?

DS: Well comets are pretty fragile. We do see comets fall apart for no apparent reason, pretty well every year - quite a problem with one last year, Comet Linear smashed itself into lots of pieces in interplanetary space. So maybe it was thermal stress as the sun heated it and so on. So we know that comets are pretty fragile. And people, some people, some newspaper reports have cast some sort of worry about this particular comet saying, "Well, if it breaks apart, isn't that dangerous?" The answer is, no. Again, they picked a beautiful target comet. It never comes close to the Earth. It is always at least 50 percent further away from the sun than as the Earth. And as a result, and even if it is smashed into pieces - separate fragments - none of those fragments are going to come close to the Earth. So let's be absolutely clear here. There is no danger to us. This is the sort of mission, which needs to be done in order to work out what comets are made of. There are important, scientific reasons for knowing that. Most of you, most of our bodies are actually made of bits of comets. You know, we're 70 percent water, and that water was delivered to the Earth by comets over the last four billion years. And it will be nice to know what comets are made of, and hence, where the material came from that makes up us - ourselves.

RA: So assuming you do hit it, how do you go about analyzing whatever happens as a result of the collision?

MA: Well, the images that we take will show this cone of ejecta coming up from the surface. The morphology of that cone is diagnostic of the physical properties - the density and the strength of the material. We'll use images to look for large chunks that were strong enough to resist the shock wave. I don't expect to see any, but we do see them on asteroids, so there might be such chunks. We'll use a spectrometer onboard the flyby spacecraft to measure the spectrum of the dust and rocks that are ejected, and also of the gasses that are ejected. So this tells us the composition, and as we see gradually deeper and deeper down as the material is excavated, we get an idea of how the composition varies with depth. We'll also have a major Earth-based observing campaign with all the other instruments that are much too big and heavy to carry on our spacecraft.

RA: You mention about an Earth-based observing campaign so that answers one of my questions. We should be able to see the impact from Earth then?

MA: Yes.

RA: Not with the naked eye or anything, this is a telescope deal I presume.

MA: It's a telescope deal primarily, although a crude estimate of how much brighter the comet will get says that it should get up to at least binocular visibility. Possibly up to naked eye visibility, but we simply don't know enough about comets to make these predictions with any confidence.

RA: Although for those of us without telescopes, I think you have something planned, don't you? Which is blanket television coverage?

MA: Yes, in almost real time, we will be releasing some of the images that come back.

RA: When will all this be happening so we can write it in our diaries?

MA: July 4th 2005.

RA: Oh, July the Fourth, so you're out to get "Deep Impact" with the media coverage as well then, are you?

MA: Absolutely. Absolutely.

RA: Duncan, is this part of the modern NASA mission? You have to plan all these things. You have to have a name like "Deep Impact" which ties in with the movies. You have to have a nice sort of active element to it, and you have to have it climax in some sort of interesting way like on July the Fourth.

DS: Well, it all helps, doesn't it?

MA: It's not a required thing. In fact, the comet comes to perihelion on July 5th. That's the distance which is closest to the sun, and we have to do it within a week or two of July 5th anyway, so we said, why not July 4th to get just a little extra pizzazz out of it.

RA: But come on Mike, you've gotta accept this is part of the modern NASA. They have to be media savvy, don't they?

MA: Oh, yes. NASA is certainly very media savvy. And we need to be media savvy because the public is paying a lot of money to fly these missions and therefore it's important that the public understand what's going on and get a lot of exposure to it.

RA: Mike, one of the things that people will think they will get out of this mission, and you can tell me whether they will, is this idea that comes form the title, "Deep Impact" - that it might tell us whether or not we have the ability to blow up, ward off, shatter, or whatever it might be, rogue comets or asteroids that are heading our way.

MA: Sure. We have no idea how to divert some rogue comet or asteroid unless we know what their physical properties are and this mission provides basic physical data on a typical short period comet. It is crucial to understanding how you'd go about diverting it if it were coming to the Earth - which it's not, fortunately.

RA: And Duncan, perhaps you should reassure us that the risk of something like that happening is not that great, is it really? You should not go around with steel umbrellas all the time.

DS: Well this particular comet is not a danger to us, -

RA: Any, any.

DS: - as I have pointed out and Mike has pointed out. But if we do take a look at comets in general, yes, comets do hit the Earth from time to time. More important, there are asteroids. Asteroids hit the earth more frequently than comets and we do have a need to look for these things. I'm sorry to disabuse you of the notion that there is a very slim chance. There is a significant chance that you will die due to an asteroid impact than you will die in a jet-liner crash. I take that very seriously. I've spent much of the last ten years trying to get various governments to take it seriously. The UK government is now taking it seriously. We're expecting another response to the UK Near Earth Object task force report later on this year. It isn't something to panic about, you're quite right. No we don't, we shouldn't walk around with a metal umbrella, but it is a significant hazard when you compare it to other things, which we do take very seriously. The good news is, it is something we can do something about. This mission is a start towards this kind of idea of, as you put it, of being able to divert one of these things if we did need to.

RA: I'm sorry about being an under-alarmist. It goes against all of my media training. This is a randomly picked comet? There are some people out there who think this is an act of cosmic vandalism. Do we have to go and put our mark on the galaxy in this way by blowing things up? I ask that to both of you really. You first, Duncan.

DS: Well, no we don't have to. But this is science. This is taking new ground. This is exploration. The Antarctic is quite pristine, and if nobody ever went there it would stay that way. But actually there are scientists that go down there to take a look, and to study the penguins, study other things which live there, study all sorts of things like the climate there. There are also tourists that go down to take a look. If you're going to try stop people from investigating comets then there're lots of other things on the earth I think we should stop first. I don't' think we should stop. We should be exploring. We should be traveling. We should be taking a look and seeing our context in the overall scheme of the cosmos, this is one part of it.

RA: Mike...

MA: If you think of the Moon and the amount of litter that has been left behind on the Moon would draw a tremendous fine - if there were fines for littering on the Moon.

RA: Mike, now we've established this is a serious mission with meaningful scientific goals, but my final question for you, and I want you, even though you're on a telephone line, I want you to look me in the eyes as you answer this one. Isn't there a bit of you, the small boy about this... You're going out into space, and you're zapping an alien object. That's got to be appealing, hasn't it?

MA: Sure. Of course. There is a little bit of that in all of science. Perhaps more so in this mission than in most parts of science, but that in essence is what drives us to understand how things work. A child-like innocence in trying to understand what's going on can be very valuable to a scientist.

RA: Mike A'Hearn, deep thanks for being candidly child-like and Duncan Steel - well, just thanks.

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CREDIT: Material World, BBC Radio 4
Transcribed: Jessica Laus

(Note: The Deep Impact launch period has since been changed to begin in January 2005).

Biographical details contained on these pages were correct during the Deep Impact mission which ended in 2006. Several scientists from Deep Impact are now working on related missions such as EPOXI and Stardust-NExT.