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From NPR News, this is SCIENCE FRIDAY. I'm Paul Raeburn.

Sometime last weekend, something happened out there. Jupiter was on its long, majestic arc around the sun when it was struck by something, maybe a space rock or a comet or an asteroid. Nobody knows yet. But this thing was roughly the size of planet Earth. And when it collided with Jupiter, it kicked up a shower of darkly colored debris near the planet's south pole, leaving what looked like a black scar on Jupiter's swirling colors.

Scientists around the world scrambled to turn their telescopes on Jupiter to try to figure out what had happened. And they're crunching the numbers and conferring to see what they might learn from this very rare event.

But the professionals were not the first to see that ominous (clears throat) - that honor goes to an amateur astronomer in Australia who spotted it with his backyard telescope. And we have him on the line. Anthony Wesley is an amateur astronomer in Murrumbateman, Australia, near Canberra.

Welcome to SCIENCE FRIDAY, Mr. Wesley. Thanks for getting up in the middle of the night to join us.

Mr. ANTHONY WESLEY (Amateur Astronomer): Hello, Paul. No, it's a pleasure.

RAEBURN: How is my Australian pronunciation there? Close?

Mr. WESLEY: You're doing very well.

(Soundbite of laughter)

RAEBURN: We have a second guest with us, who's been working day and night to do some of the number crunching and figure out what happened. That's Leigh Fletcher, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California. He joins us from a studio there.

Welcome to SCIENCE FRIDAY, Dr. Fletcher.

Dr. LEIGH FLETCHER (Jet Propulsion Laboratory): Hello. Good afternoon. Thanks for talking to us today.

RAEBURN: Nice to have both of you here.

So Anthony Wesley, how much time do you spend every day gazing at Jupiter?

Mr. WESLEY: Paul, the location of Jupiter changes throughout the year. Right at the moment, Jupiter is particularly well placed for amateur astronomers and professional astronomers to take images for many long hours every night. And Jupiter will reach the closest point to the Earth, and the point where it's opposite the sun in another couple of weeks. And at that point it's visible essentially all night and available for anyone who wants to look at it and look at its beautiful colors through a telescope or take any photographs of it.

So at the moment, at this time of year, especially, I - and in good weather conditions I can be out there for four or five hours.

RAEBURN: Now, I guess that the rock that hit Jupiter was not the size of the Earth. The scar is the size of the Earth. Was that clearly visible to you with your telescope in the backyard?

Mr. WESLEY: The location of the impact was clearly visible when I was looking at it last Sunday night. The impact site, as you say, has left a mark on Jupiter or a mark in the clouds or the cloud tops of Jupiter roughly the size of the Earth and slowly getting larger as the debris and the material from this explosion and impact slowly spreads out and starts to dissipate.

RAEBURN: Now, Leigh Fletcher, how did you and your professional colleagues let Anthony Wesley in Australia beat you to the discovery?

(Soundbite of laughter)

Dr. FLETCHER: That's an excellent question. The key advantage that amateur astronomers have over the professionals is that the benefit of time. These guys have their own equipment, extremely dedicated guys that are out there night in - day and night to observe these events.

Now, we were - it was just a normal Sunday afternoon here in California, and we'd, my colleague and I, Glenn Orton, had already got time scheduled on the NASA infrared telescope that's based out in Hawaii. And so when Anthony's news started to circulate that he'd made this incredible discovery of a dark, bruise-like smudge on the south pole of Jupiter, we certainly got extremely excited about it. And we were crowded around our computer screens here at the Jet Propulsion Laboratory waiting for those infrared images to come in.

And Anthony, I'm very happy to say, was able to participate in those first observations. And so we all got to celebrate together when we first saw what was happening.

RAEBURN: Now, Anthony Wesley, did you know you had something hot when you spotted this?

Mr. WESLEY: It did take a while for my eyes to convince my brain that I was seeing something very out of the ordinary.

It's not unusual to see, you know, dark shadows being cast on Jupiter from the - one of the four Jovian moons. That's a fairly regular occurrence. I hadn't seen, in the years that I've been watching, any of the shadows being cast way down near the south pole where this spot happened to be. So that looked a little bit unusual to me.

I could also see, I guess, as the spot rotated a little further around into view in the nighttime hours of Monday morning that it was not the circular shape that you would see or that you would expect to see from Earth if it was simply a shadow projected from one of the moons. The spot looked very much like it was circular on the surface of the planet, but we were seeing it at an oblique angle. So it was definitely something very unusual and very out of the ordinary.

RAEBURN: Well, when you realized that, how did you let the rest of the world know?

Mr. WESLEY: I was up here at the house, starting to send emails at about 2:00 Monday morning, and really, at that stage, was shaking with excitement and hoping desperately that Leigh or Glenn or any of their colleagues were close to their computer and were able to pick up my email. I didn't know at that stage that Glenn and Leigh had time already organized for imaging Jupiter that same day. I was hoping that they would pick up on the emails and be able to organize time.

RAEBURN: Did you know these folks or did you just meet them as a result of this discovery?

Mr. WESLEY: I have corresponded with Glenn many times over the last few months, or last couple of years. It's becoming more common these days for the professional astronomers to take images from amateurs on a semi-regular basis just to keep an eye on Jupiter and see if anything unusual is happening.

RAEBURN: Who is Glenn?

Mr. WESLEY: Okay. Glenn is Dr. Glenn Orton, that is Leigh's colleague. He's a - you have to correct me here if I'm wrong, Leigh - he's a planetary scientist at the Jet Propulsion Laboratory.

Dr. FLETCHER: Yeah. That's absolutely correct. Glenn Orton has been at the Jet Propulsion Lab for over 30 years now and has been involved in outer planet gas giants and dynamics for all of that time.

RAEBURN: Now, Leigh Fletcher, Anthony gave you this image - gave you this heads-up. It's up to you, I guess, not so much him, to figure out what happened. So, what do we know so far?

Dr. FLETCHER: Well, a lot of the things that we're finding out about this incredible collision are based on comparisons to a similar event back in July of 1994. In fact, as the media have been reporting, the event was almost 15 years ago to the day for Shoemaker-Levy 9. Now, when we look at the impact scar itself, we're seeing that there's a small, dark localized feature and a region of debris that's extending out to the northwest. Well, that already tells us something about the impact. What happens is you have this huge amount of energy and momentum of the impact as it plummets in toward its fate in the Jovian atmosphere. And it creates, when it first hits, a kind of splash back along the direction that the impact came from, so in this case from the northwest.

And that splash back of material pulls a lot of ammonia gas - there is aerosols, particulates, dust and some really exotic molecules and particles high up into the Jovian atmosphere. And what we can do with the infrared telescopes here on Earth is sort of determine the three-dimensional structure of that splash back. And it's that splash that we've been now tracking over the past five days since this impact occurred.

There's a huge amount of work to do. And I have to say that the professional astronomers - there's an international team now involved in trying to determine the aftermath of this event. Just to go back to your original question, we believe, by comparison to Shoemaker-Levy 9, that the impact itself was of the order of a few hundred meters across. And given the prevalence of large, icy material in the outer solar system, it seems likely that this was an icy body that collided with Jupiter.

RAEBURN: So let's take a question. I have Alister(ph) in Oregon. Are you with us?

ALISTER (Caller): Yes, I am.

RAEBURN: Go ahead.

ALISTER: The gentleman already answered a portion of my question. But I was just trying to visualize as far as what it would have looked like with this impact on the surface of Jupiter. I've heard two conflicting accounts, that the surface of Jupiter is almost kind of like a semi-liquid hydrogen type of mass there below the gaseous outer atmosphere or is just a solid mass of swirling gas. So what exactly did this rock or icy body hit, and what is the effect of the impact on the actual surface of Jupiter?

RAEBURN: Leigh Fletcher, I had exactly the same question. When you talk about debris, what was that stuff?

Dr. FLETCHER: That's a really great question. And what you have to visualize here is a rocky body that's plummeting down through the various gassy layers of Jupiter's outer atmosphere. Now, Jupiter is a big ball of hydrogen and helium gas. That's the most dominant gasses that are there. But there's also a lot of trace compounds, things like ammonia and methane and various other heavier molecules, hydrocarbons, for example.

Now, these - ammonia in particular, at a particular altitude in Jupiter's atmosphere, starts to condense. And if you imagine an image of Jupiter in your mind's eye and you see those incredible banded structures and the Great Red Spot, all of these different colors, what you're seeing there is the cloud tops, these condensed molecules in the atmosphere of Jupiter.

Now, as the rock came in plummeting towards these cloud tops, it would have been disrupted, it would have started to vaporize and it would deposit all of its kinetic energy into the surrounding atmosphere. That initiates this huge splash back and plume of material. And what we're seeing in the (unintelligible) and what Anthony was seeing in the visible, that dark patch, is a sort of dusty aerosol deposits as an aftermath of that collision.

But you're right. This is a giant fluid ball of gas, where the pressure increases as you go down towards the center of the planet.

RAEBURN: Thanks, Alister.

ALISTER: Thank you.

RAEBURN: Don(ph) in Michigan. Welcome to SCIENCE FRIDAY.

DON (Caller): Thank you very much. I have million questions but…

(Soundbite of laughter)

DON: …my…

RAEBURN: Pick one.

DON: My first question is, I've heard different accounts as far as just what the size might be. One of them is as big as the Pacific Ocean. If that is the case, how was this missed? I mean, this thing has got - must have been huge. If we would not have had Jupiter on the way to act as a magnet to suck this thing in, how soon would we have noticed this thing bearing - I mean, if it was going in our general direction, how soon would we have noticed this thing traveling at the speed it is or the size it is? And two, is there any evidence to suggest that it might be part of a field that was included in the Levy comet?

RAEBURN: Leigh Fletcher?

Dr. FLETCHER: Okay. Let's start with the beginning of that question. Now, the - we've got to distinguish here between the size of the impact field, the ejector and the debris that we're seeing, and the size of the body that caused the impact itself. Now, what happens is this plume of material we've been talking about, right, is so high in the atmosphere, that it's easily spread over an area that's - yes, you're absolutely right, the size of the Pacific, comparable in scale to the size of the Earth.

But the impact itself was much, much, much smaller than that. When we look to the - at least, 16 fragments of Shoemaker-Levy 9 that collided with Jupiter in 1994, they were, the absolute maximum, a kilometer across. And it's more reasonable to suppose that this particular object was only a few tens to maybe a few hundreds of meters across.

Now, if you imagine that the way that we spot this features is by the reflected light from the Sun, and out in the orbit of Jupiter, there's not a lot of that light available for us to spot these things. In fact, Shoemaker-Levy 9 was only observed by Gene Shoemaker and his team because they were doing a very deep sky survey at the time.

Imagine as well - another great example is the fact that Saturn's rings contains material that can be of these meters to tens of meter sizes, and we have no hope of resolving those from planet Earth. So I find the fact that we didn't spot it not unsurprising.

RAEBURN: Thanks, Don. Let me remind listeners, I'm Paul Raeburn. And this is SCIENCE FRIDAY from NPR News.

Anthony Wesley, what type of setup do you have in your backyard? Are we talking hundreds of thousands of dollars worth of equipment and your family's disserted you because of your obsession?

(Soundbite of laughter)

Mr. WESLEY: No. Not quite that radical. No. In fact, all I have is what would be considered a relatively average - maybe slightly more than average - setup for a backyard amateur astronomer these days. It would have been the case, I suppose, that if we look back 10 to 20 years ago, perhaps more like 20 years ago, your average home astronomer would have a telescope, somewhere in the six to eight-inch diameter mirror. And as time has gone by now with the increase in technology and the ability for manufacturers to make these optical components with great precision at ever larger sizes now - really, the 14 1/2-inch telescope that I have here would really be considered a midrange or a mid - or middle of the class in terms of size.

RAEBURN: What does it take to get into this? Can somebody get into it for a couple of hundred bucks or does it take a little more than that?

Mr. WESLEY: Look, people can get into it for almost nothing. Astronomy, like many of these hobbies, has a whole range of different areas. And really, people I know can get into astronomy with nothing more than a good pair of binoculars. Probably, the most important thing that you need to think of if you're considering getting into astronomy or if you have some interest in it is to contact some other people in your area that are already into the hobby and maybe are members of a local club or society, and just take the opportunity to go talk with them and maybe look at what they've got, look through their telescopes to get a bit of an idea of the slippery slope before you start going down that path, because this hobby is like cameras and it's like many other hobbies: Once you start, it's a great way to spend a lot of money.

RAEBURN: So what do you plan to discover next?

Mr. WESLEY: Look, I don't know.

(Soundbite of laughter)

Mr. WESLEY: I'm just going to keep on taking photographs, I think.

RAEBURN: Yeah. I'd like to thank you both for being with us. My guests this hour have been Anthony Wesley, an amateur astronomer in Murrumbateman, Australia near Cambra, and Leigh Fletcher, a planetary scientist at the Jet Propulsion Laboratory in Pasadena, California. Thanks to both of you for joining me.

Mr. WESLEY: Thank you.

Dr. FLETCHER: Thank you so much.

Mr. WESLEY: It was nice to meet you, Leigh. Thank you.

Dr. FLETCHER: And you too, Anthony.

RAEBURN: We're glad that we've hooked the two of you up. For NPR News in New York, I'm Paul Raeburn.

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