Comet ISON makes solar approach on Thursday, passing a mere 720,000 miles from the solar surface. It'll get hot. This could cause a break-up, with fragments then heading on as yet unpredictable trajectories. Could this pose a threat to Earth? Stuart Clark at the Guardian has the following obs:
ISON
© The GuardianComet ISON's projected position throughout December.
"If it survives an encounter with the sun this week, comet Ison will put on an impressive early morning display in the run-up to Christmas. But anyone hoping for a Bethlehem-style celestial sign on the big day will be disappointed. By then the comet will probably be too faint to see with a naked eye.

Ison is currently speeding towards a fiery encounter on Thursday, which could destroy it. It will pass 720,000 miles above the solar surface, 130 times closer than our planet ever reaches.

The intense sunlight will heat the comet to about 2,700C, speeding up its evaporation. In the past some comets have been seen to vaporise under such an onslaught.

Lovejoy skimmed 85,000 miles above the solar surface. It survived, but with very little of its 0.3-mile-wide nucleus left. Ison is estimated to be two miles wide, and its evaporating ices have already created a tail that stretches 8m miles through space.

"I'm not a gambling man but if I had to bet a fiver, I'd say Ison will survive,' said Brown.

Even if the majority of the comet emerges, fragments could still be blasted off. This would lead to a much more spectacular tail for skywatchers to see in December's sky. For viewers on Earth, the best time to start looking will be in the first and second weeks of December. By then the tail should be extremely well developed and Ison will appear as a ghostly fan shape in the pre-dawn sky.

The comet will also be visible in the western sky at sunset. It will be more difficult to spot at this time, however, because the tail is horizontal and immersed in the twilight. Better to set the alarm clock and rise early, when it will be visible in a truly dark sky."
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