The countdown clock is ticking to comet ISON's hair-raising passage around the sun on Nov. 28, as it grazes the sun at a distance of only 1.5 times the sun's radius. Astronomers continue reworking estimates of how much of an impression the far visitor will make to Earthbound observers.
The prognostication is so chancy that I couldn't even get comet-hunting veteran Mike A' Hearn of the University of Maryland to give me an estimate of how bright it might get. When pressed for a guess, A'Hearn waved his arms and said ‘perhaps moderately bright,' which would make the nucleus dimmer than the stars in the Big Dipper.
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This led A'Hearn to dub ISON the "comet of the year" - not the century as earlier Internet chatter had suggested. It's hard to believe that at the beginning of this year some astronomers were humming with speculation that the comet might become as bright as the full moon.
The excitement was built around the fact the comet is a first-time visitor to the solar system, fresh with virgin ices. Comet ISON was discovered in September 2012 by the International Scientific Optical Network located near Kislovodsk, Russia. But a search of archival data from the Pan-STARRS sky survey showed that the comet was bright enough to be photographed as early as 2011, but overlooked.
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This means that it was detectable at a whopping distance of nearly 1 billion miles from the sun. When the rate of brightening was extrapolated from early 2013 to today, it showed the comet could get quite bright. But a few months ago ISON's brightness rise flat-lined, and has now dimmed by one stellar magnitude.