Flying By A Small, Hyperactive Comet When it comes to comets, gassy is good, or at least informative, says astronomer Michael A'Hearn. NASA's Deep Impact probe has been snapping pictures of Hartley 2 -- a small comet that is spewing a lot of gas and dust for its size. What do researchers hope to learn from the comet?

Flying By A Small, Hyperactive Comet

Flying By A Small, Hyperactive Comet

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When it comes to comets, gassy is good, or at least informative, says astronomer Michael A'Hearn. NASA's Deep Impact probe has been snapping pictures of Hartley 2 — a small comet that is spewing a lot of gas and dust for its size. What do researchers hope to learn from the comet?


You're listening to SCIENCE FRIDAY. I'm Ira Flatow.

NASA's Deep Impact probe has been snapping pictures of a celestial body that's small, curvy and spewing a lot of gas. I'm talking about a comet called Hartley 2, and it's the gassy part that has my next guest over the moon.

Michael A'Hearn is the principal investigator of NASA's EPOXI Mission and professor of astronomy at the University of Maryland. If you check out Hartley 2 for yourself, you can go to our website,, to see pictures taken from a few hundred miles from the comet's nucleus. Let me bring Michael A'Hearn into the picture. He's the principal investigator of this mission, and a professor of astronomy at the University of Maryland, as I said before. So Michael, welcome to SCIENCE FRIDAY.

Professor MICHAEL A'HEARN (University of Maryland): Thank you, Ira. I'm glad to be back with you.

FLATOW: Thank you. What's so gassy about this comet? What makes it so gassy?

(Soundbite of laughter)

FLATOW: And why are you interested in that(ph)?

Prof. A'HEARN: Well, we knew going in that it was very gassy.

(Soundbite of laughter)

Prof. A'HEARN: But we didn't know why, and that was one of the things we wanted to investigate, and how is this comet very different from Temple 1. One key thing is that it has a much smaller nucleus than comet Temple 1, where we delivered our Impact there five years ago. And despite being much smaller, it puts out much more gas per second than does Temple 1. And one of the things that we discovered in the approach images coming in close is that there appears to be a significant variation in the amount of dry ice that is evaporating as the comet rotates. And it appears that it is the dry ice that may be driving the activity. We normally think of comets as being driven primarily by water ice.

FLATOW: A-ha. So this has a combination of water and dry ice.

Prof. A'HEARN: Correct.

FLATOW: Unexpected? Unexpectedly high?

Prof. A'HEARN: Well, higher than we have measured before, is what I should say.


Prof. A'HEARN: We don't have good numbers yet, but there does appear to be much more carbon dioxide relative to water than in the other - few other comets we've measured. It's very hard to measure from Earth.

FLATOW: And so it's a pretty small comet giving up a lot of gas, basically.

Prof. A'HEARN: Right.

FLATOW: Yeah. How small is it?

Prof. A'HEARN: It's about two kilometers, a little over three-quarters of a mile long, and maybe half a mile wide, and thick.

FLATOW: And...

Prof. A'HEARN: It's a long skinny thing.

FLATOW: And is that why it was chosen, because of all of the gas being given off?

Prof. A'HEARN: Well, largely it was because it was very small, and that coupled with the large amount of gas were the two dominant criteria. The first one, though, was that it was small and therefore different from Temple 1.

FLATOW: And what do you hope to learn from this? You know the source, you say. You know where the gas is coming from. What does a mission to a comet like this really tell you in the long term?

Prof. A'HEARN: What we really want to know is how we got here, how the planets formed four and a half billion years ago. And the comets preserve a much better record of those conditions than the planets do. So we want to use them to study how the planets formed. The problem is, comets are influenced by their evolution every time they come very close to the sun, and what we're trying to do is separate out the parts of the comet that are due to evolution...

FLATOW: Right.

Prof. A'HEARN: ...from those that have been there since four and a half billion years ago. And looking at comets of very different sizes changes the importance of different processes relative to the primordial characteristics.

FLATOW: So these comets really are left over from the time the Earth formed, our solar system formed then(ph)?

Prof. A'HEARN: That's correct. Everything formed together four and a half billion years ago.

FLATOW: Wow. 1-800-989-8255 if you'd like to talk about the comets. 1-800-989-TALK. Also you can tweet us @scifri, @-S-C-I-F-R-I. Where did you find this comet? Where is it living?

Prof. A'HEARN: Well, this is one of a large group of comets called Jupiter family comets. They - their orbits take them out a little bit beyond the orbit of Jupiter and bring them in somewhere in the inner solar system from a little bit inside the Earth's orbit. Some of them only come in to the orbit of Mars. But Jupiter's gravity is what is dominating the perturbation(ph) - the changes in their orbit over time.

FLATOW: If you want to see this comet, you can go to our website at, where we have some of the photos that were sent back from there. Are there any other plans to go to other comets? Are they - why would you pick a comet over, let's say, an asteroid? Or basically are they all the same sorts of things, comets having more dust than - and water and CO2, and asteroids being more hunky pieces of metal?

Prof. A'HEARN: There's probably a continuum from asteroids that are really chunks of metal and hard rock, through asteroids that are damp or wet and on out to comets.

The comets are more interesting because the comets really are the cores of the giant planets: Jupiter, Saturn, Uranus and Neptune. They all have large cores, much bigger than the Earth, which basically are made up of comets.

The asteroids have been somewhat mixed up. And in fact recent theoretical calculations say they've been mixed up a lot, much to my own surprise.


Prof. A'HEARN: But because the outer parts of the solar nebula were very cold, figuring out what the conditions were, what gasses would condense into ice, is a key question in understanding how the disk that the planets form from was made and how materials moved from place to place.

FLATOW: Mm-hmm. Is it still thought that the water from all these untold number of comets sitting there in its early life was the source of the oceans and the moisture on Earth?

Prof. A'HEARN: Yeah. The source of the Earth's water, and of its organics, for that matter, is still controversial. A large part of the community thinks that it did come from comets, or mostly come from comets. Another large segment of the community thinks it came from those very wet asteroids that were right next to the comets in the early formations scenario. And a smaller part of the community argues that the Earth collected the water and organics as it formed.

FLATOW: What camp are you in?

Prof. A'HEARN: Well, I'm with the comet camp.

(Soundbite of laughter)

Prof. A'HEARN: I recognize there are problems with the model. We don't have we have too much heavy water compared to light water, heavy water being what you need in nuclear reactors.

FLATOW: Right.

Prof. A'HEARN: Compared to what the Earth has - there's a little bit too much in the comets we've studied. But we haven't studied very many comets.

FLATOW: Mm-hmm. Let's go to Juan...

Prof. A'HEARN: (Unintelligible) differences.

FLATOW: ...Juan in Fort Lauderdale. Hi. Welcome to SCIENCE FRIDAY. Hi, Juan. Are you there?

JEFF (Caller): Hello?

FLATOW: Hi, there. Go ahead.

JEFF: Yeah. This is Jeff, actually, from Michigan.

FLATOW: Oh, I'm sorry.

JEFF: I've got two I have two questions about the comets. Number one, since they've been around for billions of years, why don't they just boil away in the vacuum of space? And number two, when they go around the sun, if they're so small and the ice is being melted and the rocks are being vaporized or whatever, why don't they just slowly get so small that they just disappear? Why do they stick around for millions of years billions?

FLATOW: Yeah. If this comet is tiny now, how big could it have been originally?

Prof. A'HEARN: Unfortunately we can't calculate backwards to figure out how big it was originally. But we can certainly calculate forward and indeed it should disappear.


Prof. A'HEARN: It just probably loses, oh, a tenth of a percent of its radius every time it comes by the sun. So another thousand times by the sun and it would all be gone. And that's characteristic of comets, depending on how close to the sun the comet, when they're closest, they change varies the amount of material that they lose. But they all will lose material until they run out.

FLATOW: Does this have a big tail, this comet?

Prof. A'HEARN: This does not have a particularly prominent tail. It has well, you can easily see it in telescopic images. But it's not a dramatic one like some of the really bright comets that you see.

FLATOW: Will it get close enough to the Earth for us to see it and its tail?

Prof. A'HEARN: Not anymore. Not now. It came closest to the Earth about the middle of October.

FLATOW: Mm-hmm.

Prof. A'HEARN: And it is one of the comets that comes closest to the Earth. But because it's a very small comet, you don't it was not an easy one to see. It was an easy object in binoculars for people who knew where to look. By now it's moved to the Southern Hemisphere and is moving away from us.

FLATOW: Mm-hmm. One last question. Brian in Tipton, Michigan. Hi, Brian.

BRIAN (Caller): Wow. I'm glad you took my call because I got a really good question. I want to know why does all of this stuff have to have an explanation of billions and trillions and gazillion of years ago? Why can't we just accept the fact - I mean I know there's a lot of people that don't believe in God and believe in creation, but what if God created them and there's really no explanation and nobody's really ever going to know why we got so much water, why we got trees? I mean, maybe nobody will ever know for sure. You know?

Prof. A'HEARN: Well, we may never know we may never know exactly how things came to pass. But God presumably created the universe to work in certain ways. And...

BRIAN: Right.

Prof. A'HEARN:'s a little puzzling to understand why he would create it to work in ways that would deliberately mislead us about how it works. There are lots of natural processes that suggest that the entire solar system formed four and half billions years ago. Now, indeed, we may be misled, but there are so many different pieces of evidence that that seems very unlikely.

BRIAN: Right.

FLATOW: Okay? Okay, Brian?

BRIAN: Thank you.

FLATOW: Thanks. Thanks for your call. 1-800-989-8255. But you're asking, you know, these very simple questions - you know, where - the origins of the universe, origins of our solar system, our galaxy. And you and there are these little clues left behind, right? Like these comets.

Prof. A'HEARN: Right. We have these clues. And I don't think I like to think that I have been deliberately misled.

(Soundbite of laughter)

FLATOW: Do you think it's a it's within our grasp to learn, to understand the origin, I mean if we study it long enough?

Prof. A'HEARN: It is I certainly can believe that we will get to a point where we can no longer understand the physical processes. The classical theory of the universe, that the Big Bang theory, had a beginning. And that would have been the point at which God created the universe. But he could've created it in many ways. And the way in which he created it is up to us to find out.

FLATOW: Mm-hmm. So you don't have a conflict in your own mind about believing in God and believing in science at the same time.

Prof. A'HEARN: No, I don't.

FLATOW: All right. That's a good way to end. Thank you for taking time to be with us today. And good luck, good luck on your mission.

Prof. A'HEARN: Thank you, Ira.

FLATOW: You're welcome. Michael A'Hearn is the principal investigator of NASA's EPOXI mission. They're gluing the comet back together. Professor of astronomy, sorry...

(Soundbite of laughter)

FLATOW: the University of Maryland.

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