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GUY RAZ, host:

Welcome back to ALL THINGS CONSIDERED from NPR News. I'm Guy Raz.

Now, just about any amateur astronomer can tell you the basics about Jupiter. It's the fifth planet from the sun. It's got a giant red spot on its lower half. And wrapped around it are two thick brown stripes, each one about as wide as planet Earth. Well, check your telescope tonight and you'll find one of those stripes, the southern one, has gone missing, and scientists aren't entirely sure why.

Kelly Beatty is a senior contributing editor for Sky and Telescope magazine, and he joins me now from member station WBUR in Boston to tell us a bit more about what's going on.

Kelly Beatty, welcome.

Mr. KELLY BEATTY (Senior Contributing Editor, Sky and Telescope): Thank you, Guy.

RAZ: So when did you first notice this stripe or band in the southern half of Jupiter was gone?

Mr. BEATTY: Amateur astronomers around the world started noticing this actually last fall. And then Jupiter ducked behind the sun for a few months, and only about April did it come back into the morning sky, where it could be viewed again. And when astronomers looked again, this belt, this dark band, had completely disappeared.

RAZ: So when they first saw it, it wasn't completely gone, it was sort of fading?

Mr. BEATTY: Right. It tends to fade out. This is not the first time this has happened. It happens every 10 years or so, 18 times since the turn of the 19th century. So we know that it does happen and we have some confidence that the belt will come back, but we just don't know why.

RAZ: Can you sort of describe what these bands are made up of?

Mr. BEATTY: Sure. Jupiter is a very big place. It's about 11 times the diameter of the Earth, and unlike the Earth, it doesn't have a solid surface. What we see when we look through a telescope is a planet-wide cloud deck surrounding the entire place. So these two bands, which are kind of like racing stripes around the midsection of Jupiter, are dark bands that have a different composition than the other clouds around them.

What scientists think has happened is that some kind of disturbance has taken over in the Southern Hemisphere and kind of created cirrus clouds, maybe, that has completely enveloped the planet and covered this band with a high, thin blanket that will eventually go away.

RAZ: So all that's really happened, they think, is that it's there, but it's just being obscured?

Mr. BEATTY: That's right. That's the theory, anyway.

RAZ: So when is it expected to return?

Mr. BEATTY: Well, I can tell you, Guy, that there are amateur astronomers around the world with their eyes glued to their telescopes in the hope that they will be the first to be able to see the beginning of the return of the South Equatorial Belt.

RAZ: But nobody knows? Nobody knows.

Mr. BEATTY: No one knows. It could be six months from now. It could be two years from now.

RAZ: Now, obviously, Jupiter is a popular planet for amateur stargazers. When is a good time to see Jupiter if you have a telescope?

Mr. BEATTY: First of all, if you're just eyeballing the sky, Jupiter is easy to spot in the morning sky before dawn. It's a bright star in the Eastern sky. It's the only star that bright anywhere nearby. It's very obvious.

And if you have a pair of binoculars, you can look at it. You'll see hold them steady and you'll see that Jupiter's actually a little disk. If you have a small telescope, you'll be able to see not only this disk, but the two stripes across it or what were the two stripes. You'll only see one, and that will be an indication that something is very wrong.

RAZ: Now, we probably wouldn't have found out about this as quickly had it not been for amateur astronomers, right?

Mr. BEATTY: You raise a good point. In astronomy, amateur sky watchers and professionals have a very close relationship. There aren't enough professionals to keep track of everything going on in the universe all the time. So in a sense, they rely on amateur astronomers who have very good equipment, by the way to actually keep an eye on things; look for supernovas or watch for stars that pulsate, and in this case, watch for changes on the planets in our solar system. And when they see something, they notify the professionals and the big guns get swung over to take a better look.

RAZ: All right. Kelly Beatty, one last question for you. When you look at a photograph of Jupiter, it is actually quite a beautiful planet, probably second only in beauty to Saturn, I would say, right?

Mr. BEATTY: Yes.

RAZ: Do you kind of miss it? I mean, do you kind of miss that second stripe?

Mr. BEATTY: I do. I kind of miss the symmetry of it because it tells me that there is order on Jupiter. And right now, the fact that that one belt is missing, it's like a missing tooth. There is disorder on Jupiter and we just don't know why.

RAZ: That's Kelly Beatty. He's a senior contributing editor for Sky and Telescope magazine. He joined me from WBUR in Boston.

Kelly, thank you so much.

Mr. BEATTY: Oh, a pleasure, Guy. Thanks.

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