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The Sciences

A Novice Observer's Guide to Comet ISON

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If you enjoy a dramatic spectacle in the sky, you have probably heard about Comet ISON, currently streaking toward the sun. (If you haven’t heard about it, bear with me—I’ll try to make it worth your while.) But the media coverage has been downright confusing. The first reports described it as a “comet of the century,” possibly as bright as the full moon. Then came some whipsaw downbeat news stories suggesting that the comet was fizzling and might have already begun to disintegrate.

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Comet ISON as seen by the Hubble telescope on October 9, when it was 177 million miles from Earth. (Credit: NASA/ ESA/Hubble Heritage Team) No wonder DISCOVER readers have been sending in a steady stream of inquiries: What will Comet ISON really look like? When will it be visible? Where can I see it? Good questions. Time for some answers—and I’ll have a lot more to say about the science of the comet itself in an upcoming blog post.What it will look like Almost all of the forecasts about Comet ISON contain huge uncertainties. As comet hunter David H. Levy is fond of saying, “Comets are like cats; they have tails and they do precisely what they want.” The Comet ISON Observing Campaign has put together a set of scenarios that explore the staggering range of possibilities. It now seems almost certain that Comet ISON will fall short of the early, optimistic assessments (truth be told, it was never going to light up the night like the full moon, since peak brightness will happen while it is less than one degree from the sun...in the middle of the day). The dire predictions that the comet has fizzled are simply wrong, however. Recent Hubble images show it beautifully intact. A recent study by Jian-Yang Li of the Planetary Science Institute gives additional reason for hope. Comet ISON is oriented such that one side has remained shadowed so far. When the comet whips around the sun on November 28, the comet’s night side will suddenly emerge into daylight and be hit with an intense blast of solar heat. That could potentially lead to a spectacular eruption of gas and dust. The upshot is that Comet ISON is sure to be a pretty sight through binoculars or a wide-field telescope, and it still has plenty of potential to be an exciting addition to the visible sky after Thanksgiving. When and where to look I created a quick reference guide about when and where to look for Comet ISON. It appears in my Urban Skygazer column in the November issue of DISCOVER, and you can read it here. A viewing skychart is available from my colleagues at Astronomy magazine. There are two crucial things to know about looking for Comet ISON.

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Projected brightness of Comet ISON, with actual measurements in orange, shows how rapidly it will peak at closest approach to the sun on November 3. (Credit: NASA CIOC/Matthew Knight) First, it is a peaky comet (full details here if you want to take a deep dive). It will brighten rapidly in the days just before Thanksgiving, so don’t even try looking for it with your naked eyes right now—it’s far too faint. Probably it will become visible from dark skies by mid-November, but may be tricky to see on its way in. The best viewing should come in the couple weeks after November 28, when the comet is heading away from the sun but toward the Earth. Because of the shifting geometry, Comet ISON will probably fade quite a bit more slowly than it brightened. Even so it may be lost to the naked eye well before it makes its closest approach to Earth on Christmas day. Second, you will need dark skies and an unobstructed horizon to get the best view. At its brightest, Comet ISON will be close to the sun and hence close to the horizon just before sunrise. In the first half of December the comet will get higher in the sky and (depending on how it behaves) may have a ghostly tail stretching up to a quarter the way across the sky. To appreciate its full extent, though, you will need to get far away from urban light pollution. And as you may have already picked up, you need to wake up early in the morning, just as dawn is breaking. OK, so will it be worth the effort? Nature doesn't always make things easy or convenient. The payoff is that you will get to see something rare and wonderful: A primeval ball of ices and dust grains, frozen since the time of Earth's formation 4.5 billion years ago, vaporizing and spreading out into space. It's also amazing to recall how small the actual comet is. Its tail may stretch millions or even tens of millions of miles across, but the solid body creating that whole spectacle is just a couple miles across! If you get an exceptionally good look at Comet ISON's tail you may be able to see that comets actually sprout two kinds of tail--an ion tail (composed of individual, electrically charged gas atoms) and a dust tail (composed of larger particles). Both tails are affected by both the solar wind and by light pressure from the sun, but they react in different ways. The ion tail always points straight away from the sun. The dust tail lags behind a bit as the comet orbits around the sun. The result is that the two tails typically show up separately, often with distinctive structures, shapes, and colors. So when you view a comet you get to see the invisible interplanetary dynamics of the solar system, as well as a relic from a time when Earth was just a lifeless ball of molten rock. Worth getting up early for, I think. Follow me on Twitter: @coreyspowell

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