LINDA WERTHEIMER, Host:
Mr. Wesley, welcome.
ANTHONY WESLEY: Good evening, Linda. Thank you.
WERTHEIMER: So what did it look like to you? What did you think you were looking at?
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?
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?
WESLEY: 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?
WESLEY: 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?
WESLEY: 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: Thanks very much.
WESLEY: Oh, my pleasure. Thank you.
WERTHEIMER: This is NPR News.
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