Reports of naked-eye sightings of Comet ISON are coming in from around the world. Experienced observers put the comet's magnitude at +5.5 on Nov. 16th. This means it is now fully 10 times brighter than it was only three days ago before the outburst. To the naked eye, ISON appears as a faint smudge of pale green light low in the pre-dawn sky. The view through a telescope
is more dramatic. The comet's tail has become a riotous crowd of gaseous streamers stretching more than 3.5 degrees across the sky. Amateur astronomer Waldemar Skorupa sends this picture from Kahler Asten, Germany:
© Waldemar Skorupa
The tail is so long, he couldn't fit the whole thing in the field of view. How long is it? Comet ISON's tail extends more than 8 million kilometers behind the comet's nucleus
. For comparison, that's 21 times the distance between Earth and the Moon.
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.
© Waldemar Skorupa
"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."
In short, no one knows for sure what is happening
to Comet ISON. This could be the comet's death throes--or just the first of many brightening events the comet experiences as it plunges toward the sun for a close encounter on Thanksgiving Day (Nov. 28th).
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.
Sky maps: Nov. 15