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Sunday, October 27, 2013
This November is a month of “what might be.” It might turn out to be a memorable time for skywatchers, or it might turn out to be largely forgettable.
On Sept. 21, 2012, Vitali Nevski and Artyom Novichonok of the International Scientific Optical Network (ISON) discovered a faint object lying 626 million miles from Earth, well outside the orbit of Jupiter. It was quickly determined to be a comet, one of exceptional brightness in light of its great distance.
Speculation ran rampant. It was cast in the media as being the “Comet of the Century,” possibly reaching the brightness of the full moon and possibly being visible during daylight hours. It would be the Thanksgiving Comet, best viewed at the end of November.
One year later, a different scenario is forecast. While ISON has brightened, it has not done so to the extent first predicted. This apparent capricious nature is due to an insufficient scientific understanding of its form and structure. Simply put, comets like ISON are hard to figure out. Comet seeker and astronomy popularizer David Levy has said, “Comets are like cats; they have tails, and they do precisely what they want.”
Astronomers realize that, as with most approaching comets, there are three big unknowns. In ISON’s case, there is also a fourth.
As it nears the sun, ISON’s 2-mile wide nucleus will begin to warm and to emit both water vapor and carbon dioxide. It also will begin to eject very fine grains of silicate and carbonaceous dust. If enough of this material is expelled, the comet will form a beautiful, sweeping tail that spans one million miles. That is the first big unknown.
On Thanksgiving Day, ISON flies around the sun, moving at a distance of about 700,000 miles from its surface, and receiving over 10,000 times more solar radiation than we do on Earth. ISON’s nucleus will certainly be ejecting large amounts of water, carbon dioxide and dust at this time and, consequently, it may fragment into several “mini comets” or it may disintegrate completely. Or it may not. That is the second big unknown.
If it survives its close passage reasonably intact, its surface should continue to heat, ejecting more material, allowing Earthbound observers to enjoy its hoped-for spectacular tail. That is the third big unknown.
There is a fourth and potentially fascinating unknown. On Jan. 14 or 15 the Earth crosses ISON’s orbital path. (There is no danger of Earth colliding with it, as on those dates ISON will be a comfortable 63 million miles away — about the same distance that Venus is from us now.) The dusty debris shed by ISON may enter our planet’s upper atmosphere creating a new meteor shower similar to the Geminids we see in mid December. Or it might not.
The key to finding Comet ISON before it dramatically brightens in our November morning sky is to find the bright star Spica. Locate the Big Dipper, which will be in the northeast before dawn begins. Trace the arc of the Dipper’s handle to the southeast first striking the bright star Arcturus, then reaching Spica. On Nov. 18, the comet’s head should glow immediately below Spica, with the star being caught in the much anticipated tail.
ISON’s nucleus with any tail jutting above it will be found on Nov. 23 and 24 just above the horizon, joining the celestial trio of Saturn, Mercury, and the double star Zubenelgenubi. Binoculars will help show all four objects, five if Zuben is counted as two.
If ISON acts true to its cat-like nature, all of this might come to pass. Or it might not.
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