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Comet ISON: Coming to a sky near you

From now through mid-December of 2013, all over the world, a brand-new comet will become visible to the naked eye in our morning skies before dawn. This morning comet is named ISON, and this is its first trip through our inner solar system.

Discovered Sept. 21, 2012, by two astronomers working near Kislovodsk, Russia, Vitali Nevski and Artyom Novichonok of the International Scientific Optical Network (ISON), this comet was given the formal scientific designation C/2012 S1. Commonly, comets also are named after their discoverers, but in this case, it was named after the ISON network instead. This network normally tracks asteroids and space debris, so comet ISON was quite a bonus find.

When it arrives, comet ISON may put on an exceptional show because it is following an extraordinary orbit around the sun. This comet is headed almost directly at the sun, which puts it in the category of sun grazers, comets which pass extremely close to the sun’s surface or which even plunge into the sun to their death. This is a very dangerous path for any comet to follow for two reasons.

First, comet ISON will pass the sun less than 750,000 miles above the sun’s blazing, incredibly hot surface. This near-miss will roast comet ISON at temperatures above 3,500 degrees Fahrenheit, which can boil off many layers of the comet’s surface, including rock. By way of comparison, Earth is about 93 million miles from the sun.

Since comet bodies can be many miles in diameter, comet ISON may not evaporate completely even in such heat. But as it evaporates, astronomers will study emissions radiating from its coma, the cloud of particles and gas around the comet’s nucleus, also called the head or body, and from its tail, the long, most visible feature of comets, which is pushed by the out-flowing solar wind into a bright plume that always points away from the sun. Thus, astronomers hope to gather knowledge about comet ISON’s composition.

Second, as comet ISON passes close to the sun, the sun’s powerful gravity will induce strong tidal forces within the comet. Comets are loose compositions of rock, dust, ice and other volatiles, so these forces may tear apart comet ISON into many fragments, which then would form a cluster or long string of mini-comets, each with its own minitail.

Thus, comet ISON has a very uncertain future as it rounds our hot sun. Will it evaporate entirely, will it break into fragments, or will it hold together as it emerges from behind the sun? How bright will the comet become as it gets close to the sun? This celestial drama will become a very interesting comet watch.

For planet Earth, this will be a very safe watch, as this comet will not collide with Earth. But next Jan. 12, Earth will pass through the edge of comet ISON’s trail, so we may see a brand-new meteor shower added to our annual schedule of these recurring showers.

To see comet ISON, watch the dark southeast sky over Mesa Verde before dawn, starting mid-November. Comet ISON will rise over the horizon before the sun does, but as the comet gets closer to the sun each day, it will rise closer and closer to sunrise, so it will get harder to see through emerging morning daylight in late November.

On Nov. 28, comet ISON will pass close to the sun and be invisible behind the sun from Earth. As it emerges from behind the sun, it will become visible again to the southeast a few days later in early December. As it continues to move away from the sun, it will rise earlier and earlier before the sun and become steadily more visible in pre-dawn skies.

What kind of comet will we see then, if any? Watch the skies to find out.

© 2013 James F. Andrus.

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