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ROBERT SIEGEL, host:

Now, the comet in the sky. When we turn to astronomers to sort through news of visible heavenly bodies, we usually hear about predictable phenomena, eclipses, for example. So it's unusual for us to hear about something so unusual that it's really surprising to the experts. But that is certainly the case with Comet Holmes, which had a huge outburst in late October, and which has gotten bigger and dimmer since.

Alan MacRobert is a senior editor at Sky & Telescope magazine in Cambridge, Massachusetts. He joins us now. Are you surprised about Comet Holmes?

Mr. ALAN MacROBERT (Senior editor, Sky & Telescope magazine): Well, we certainly were. This thing brightened by a factor of a million way out beyond the orbit of Mars for no known reason.

SIEGEL: And how visible is it in the sky?

MR. MacROBERT: A big part of the significance of this is that anybody can go out and see it and enjoy it. You don't have to be a trained astronomer. You don't have to have a Ph.D. in physics. Just go out, look northeast, find the constellation Perseus, and there it is. It's a big fuzzy puff ball of a thing among the ten point stars. You don't even need a dark sky for this.

SIEGEL: Well, why would a comet undergo a great outburst?

MR. MacROBERT: Well, one theory that astronomers are pretty sure is wrong is that it got hit by a meteorite. The thing is, comets do go through these sorts of inexplicable outbursts fairly often, and there just aren't enough meteorites out there to account for it. Space is just very, very empty. Another possibility is that the comet has a hard, frozen crust, sort of like a snow bank at the end of February, when it's black with dirt, but it's still snowy on the inside, and that pressure builds up inside it and the crust all of a sudden broke.

SIEGEL: Well, considering how Comet Holmes took us by surprise, do we have any idea how long it's likely to remain visible?

MR. MacROBERT: Nobody knows. Eventually, it is going to fade out. That much we are sure of. However, this comet did it once before in 1892, when it was first discovered. And that year, it did it twice. After it faded out, two months later it erupted again.

SIEGEL: This strikes me as the kind of celestial event that a few millennia ago, this could really throw people off looking at something like this up in the sky and make them think things about what was going on up there.

MR. MacROBERT: Well, people thought the universe revolved them. And that if there was some kind of a sign going on up in the heavens, it must mean that the king was going to be overthrown or that he was going to overthrow somebody else or that bad news was coming our way. And, in fact, this was a frozen dirt ball out in space that went through an eruption. And it really doesn't care about us.

SIEGEL: But a sitting king might be careful just the same with this happening right now.

MR. MacROBERT: Always a good idea.

SIEGEL: Always a good idea. Well, Alan MacRobert, thank you very much for talking with us.

MR. MACROBERT: Thank you.

SIEGEL: Mr. MacRobert is a senior editor at "Sky & Telescope" magazine. He spoke to us from Cambridge, Massachusetts.

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