Space weather activity is one factor, but ISON will also dive much deeper into the corona than Encke did -- some 30 times closer, with a close approach (perihelion) of a little over a million kilometers from the sun's 'surface' (photosphere) on Nov. 28.
"The CME that ran over Comet Encke back in 2007 was slow, barely creating a pressure pulse by compressing the solar wind ahead of it," said Angelos Vourlidas also of the Naval Research Lab and CIOC. "It was this compression which caused the Encke's tail to fly off."
In ISON's case, a CME impact will likely be far more dramatic. "Any CME that hits Comet ISON close to the sun would very likely be faster, driving a shock wave with a much stronger magnetic field," added Vourlidas. "Frankly, we can't predict what would happen."
Comet ISON: 5 Things You Should Know
Coincidentally, Comet ISON has the veteran Comet Encke for company on its first dive into the corona and a fleet of solar observatories will be watching the pair's tails being buffeted by the solar wind.