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LINDA WERTHEIMER, host:

Anthony Wesley wasn't expecting much the other night. He's an amateur astronomer. He was looking at Jupiter and the weather predictions weren't great for observation. He'd just about decided to call it quits when he went on just a little bit longer and saw a black spot on Jupiter as big as the Pacific Ocean.

It turns out he was the first person to see what is likely a massive impact site on Jupiter. He joins us now from Murrumbateman in New South Wales, Australia.

Mr. Wesley, welcome.

Mr. ANTHONY WESLEY (Amateur Astronomer): Good evening, Linda. Thank you.

WERTHEIMER: So what did it look like to you? What did you think you were looking at?

Mr. WESLEY: When I first saw this black spot near Jupiter's south pole, the first thing that comes to mind is that it's probably the shadow being cast by one of Jupiter's moons. However, after a few minutes of looking at this on Sunday night it became clear that this shadow was the wrong shape and it was in the wrong place to be one of the moons.

WERTHEIMER: What do you think caused it?

Mr. WESLEY: Well, the most likely cause was a comet. Probably a comet no more than one kilometer across traveling at something like 50 to 100 kilometers per second in order to leave a mark which is now spread out to an area approximately the size of the Earth on Jupiter's clouds.

WERTHEIMER: What is it about Jupiter that a comet could leave a big, big impact mark like that?

Mr. WESLEY: Jupiter is all clouds, at least at the upper layer that we see. We can't see any solid surface. Jupiter probably has a solid surface, but it's a long, long way down inside.

So a comet or an asteroid coming in from above moving at incredibly high speed will plow into this atmosphere and explode with great force, much more force than if it had hit a solid body where a lot of the energy would just be dissipated. The black material that we're seeing is a combination of the leftover debris from this explosion and also dark cloud and dark gas that's been brought up from inside Jupiter's atmosphere.

WERTHEIMER: This is not the only comet to hit Jupiter. I understand there was one just 15 years ago. Why do comets slam into Jupiter but they haven't hit the Earth anything like as recently as 15 years ago?

Mr. WESLEY: Comets are generally few and far between in the current environment in the solar system. The amount of gravity in the outer solar system is dominated by Jupiter. And so Jupiter really is the most likely target to be hit by any comets that are going to be coming in our direction.

The Earth is really a very, very small target by comparison. And it would be a very infrequent event, perhaps a hundred to millions or billions of years apart for the Earth to be hit by a comet.

WERTHEIMER: Mr. Wesley, what's been the reaction to the fact that an amateur astronomer saw something that all the big telescopes and observatories and whatnot did not see?

Mr. WESLEY: The reaction has been very positive from the professional community. In the last few years it's becoming more and more evident that amateur astronomers in the modern world with current telescopes and current computer technology can really produce images that rival or in many cases exceed what was possible even by the best professional observatories only 20 to 25 years ago.

I think it's a great boon for the professional organizations such as NASA that they know that on any given night there's an army of amateur astronomers with telescopes and cameras monitoring not just Jupiter but other planets as well, such as Venus and Saturn.

WERTHEIMER: We've been speaking with Anthony Wesley. He's an amateur astronomer from New South Wales, Australia.

Thanks very much.

Mr. WESLEY: Oh, my pleasure. Thank you.

WERTHEIMER: This is NPR News.

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