Comet ISON is nearing its closest approach to the sun and it surprised many astronomers by suddenly brightening quite significantly late last week. If that trend continues, it could make for a sight visible with the naked eye in the low east-to-southeast horizon in the pre-dawn sky very soon, and it could last through January. If the comet is torn apart; however, there might only be a bright flash visible to Earth and then nothing more.
The comet’s recent outburst of activity has done more than simply brighten the comet. Whatever exploded from the comet’s core created a spectacularly long tail that is estimated to be more than 16 million kilometers from end-to-end. The tail of the comet now stretches more than 7 degrees across the sky which is almost as wide as the bowl of the Big Dipper (image courtesy of spaceweather.com). Physically, ISON’s tail is about 12 times wider than the sun so when the head of the comet plunges into the sun’s atmosphere on November 28th, Thanksgiving Day, more than 15 million kilometers of the comet’s tail will still be jutting into space behind it.
Because so much gas and dust is spewing from the comet’s core, it is impossible to tell what caused the outburst in activity late last week. Is this a short-lived event or the beginning of a more active phase? It is possible that the nucleus has become completely fragmented and, if so, it will be several days before we know for sure. Stay tuned.