Comet ISON's recent outburst of activity has done more than simply brighten the comet. Whatever exploded from the comet's core also created a spectacularly-long tail, more than 16 million kilometers from end to end. Pn the left is Comet ISON as photographed on Nov. 17th by Michael Jäger of Ebenwaldhöhe, Austria.
"The tail of the comet stretches more than 7° across the sky," says Jäger. It's almost as wide as the bowl of the Big Dipper.
Physically, ISON's tail is about 12 times wider than the sun. So, when the head of ISON plunges into the sun's atmosphere on Nov. 28th, 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 see clearly what caused Comet ISON's outburst on Nov. 13-14. One possibility is that fresh veins of ice are opening up in the comet's nucleus, vaporizing furiously as ISON approaches the sun. Another possibility is that the nucleus has completely fragmented.
"If so, it will still be several days before we know for sure," says Karl Battams, an astronomer with NASA's Comet ISON Observing Campaign. "When comet nuclei fall apart, it’s not like a shrapnel-laden explosion. Instead, the chunks slowly drift apart at slightly different speeds. Given that ISON’s nucleus is shrouded in such a tremendous volume of light-scattering dust and gas right now, it will be almost impossible to determine this for at least a few days and perhaps not until the comet reaches the field of view of NASA's STEREO HI-1A instrument on November 21, 2013. We will have to wait for the chunks to drift apart a sufficient distance, assuming they don't crumble first."
Monitoring is encouraged. Comet ISON rises in the east just before the sun. Amateur astronomers, if you have a GOTO telescope, enter these coordinates. Dates of special interest include Nov. 17th and 18th when the comet will pass the bright star Spica, making ISON extra-easy to find.