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Starwatch: Comet Ison may put on a show next month. Or not.

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Starwatch diagram for 11/8/13

If you're a frequent flyer on the Internet, you may have come across news of a tremendous comet brighter than a full moon that will adorn our predawn morning skies next month.

You may have even heard that Comet Ison will be the "comet of the century!"

I don't need to tell you that you shouldn't believe everything you read on the internet — unless, of course, you're reading the online version of the Post-Bulletin! Unfortunately, stories about science aren't immune to media hype. Comet Ison is clearly an example of this. You can't put all of the blame on the media, though, because some overzealous astronomers also added to the hype.

Comets are basically mountain-sized frozen dirtballs made of ice, dust, and rocky rubble that are remnants from the formation of our solar system more than 4-1/2 billion years ago. Most comets spend their entire lives in the deeply cold outer reaches of the solar system beyond the planets.

Every so often, though, for a variety of reasons, one or more of these comets break loose and are drawn in toward the inner solar system by the sun's gravity. Depending on their trajectory, some comets get caught in very elongated orbits that take them close to the sun and then whip them back out to deep space only to return again. A good example of that is the famous Haley's comet, that runs on a 75-year orbital cycle. The last time it was by this part of the solar system was in 1986.

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Some comets, however, are "one-timers." They are drawn toward the sun, swing around it, and then head back to the celestial boonies never to be heard from again. Sometimes comets get too close to the sun and get completely obliterated by the intense radiation. Some even crash into the sun! What will happen to Comet Ison? No one really knows.

Dirty snowballs

As comets draw closer to the sun, these dirty snowballs start melting, vaporizing the ice and releasing debris. Comets then start developing a large cloud around the nucleus, called a coma, and tremendous solar winds push gas and debris back until the comet's tail is born and gets stretched out. In some cases, the tails of comets can stretch out over millions of miles.

As comets retreat from the inner solar system, their coma clouds and tails shrink. Comets also leave a trail of debris in their wake that becomes the fodder for meteor showers as Earth's orbit crosses into one of these trails. Debris gets gravitationally sucked into Earth's atmosphere and we can get quite a light show!

Ison was discovered in 2012. That in and of itself is not that unusual. New comets are discovered all the time. What made this comet unusual was that it started brightening way sooner than expected, when it was still in the far outer suburbs of our solar system. This got a lot of folks in astronomical circles very excited, as they prematurely concluded that this will be an extremely bright comet when it passes by the sun and our Earth. Spirits were dashed, though, as the pace of the brightening stalled out.

For now, Ison is about halfway between the orbits of Earth and Mars, and it's nowhere near visible to the naked eye. In fact, you definitely need a telescope to see it. Many astronomers believe that Comet Ison brightened earlier than expected because it's a brand new comet that's melting for the first time on its first trip toward the sun. Virgin comets like Ison have a coating of frozen carbon dioxide and monoxide that briefly flares up and can give a false indication of the comet's future brightness, thus setting off the hype.

Nevertheless, there's still a chance that Ison could wind up being a pretty decent comet that may be visible to the naked eye in December, but there's a lot of uncertainty. First off, Ison may not survive. On Nov. 28, the comet will reach perihelion, its closest point to the sun, and it's going to be a really close encounter, only a million miles away from our home star. The intense heat could completely disintegrate the dirty snowball.

But maybe...

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There's also a decent chance that Ison could hang together and the intense radiation from the sun could vaporize just enough of the ice and free just enough of the debris to make for a really nice comet. It certainly won't be the "comet of the century," but it could be bright enough to see with the naked eye and have a decent tail to it.

But again, no one knows for sure what will happen. Nov. 28 is the day of reckoning for Comet Ison.

As Comet Ison (assuming anything is left of it) pulls away from the sun on its way back out to deep space, we'll have a chance to see it in the early morning low southeastern twilight sky, just before sunrise. For the first few days of December, the comet should be at its brightest, but will rise only about 45 minutes before the sunrise so it could really be lost in the twilight glare.

As the month goes on, Ison will rise earlier and earlier in the southeast and may be easier to see. By about Christmas time, the show should be over with.

I know this column about Ison may seem as wishy-washy as a bad weather forecast, but the truth of the matter is that except for knowing that it won't the "comet of the century," the quality of the show Comet Ison puts on remains to be seen. I'll have more to say about the icy visitor from deep space as it gets closer to perihelion and its moment of cosmic truth.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul and is author of the book, "Stars, a Month by Month Tour of the Constellations" published by Adventure Publications.

The Rochester Astronomy Club welcomes new members and puts on public star parties. Their website is www.rochesterskies.org.

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