Eichler thinks that such a sun-grazing comet may have triggered a large solar flare and cosmic ray-generating shockwaves as recently as 775 A.D.

FORBES: Sun-Grazing Comets As Triggers For Electromagnetic Armageddon

Or so says David Eichler, lead author of a forthcoming Astrophysical Journal Letters paper positing that a sun-grazing comet roughly the size of Hale-Bopp (with a nucleus some 30 kms in diameter), could trigger cosmic ray-generating shockwaves large enough to initiate a global electromagnetic Armageddon.

FORBES: Sun-Grazing Comets As Triggers For Electromagnetic Armageddon

Principal investigator Joe Veverka from Cornell University said the alterations were driven by the process material loss as the comet orbited the Sun.

BBC: Stardust probe finds 'subdued' comet crater

As a comet gets closer to the sun, the sun's heat causes these elements to melt, spewing out dust and gas in a brilliant tail.

CNN: Welcome to the year of the comet (we hope)

That would give it an orbital period of 341 years the longest yet measured and far longer than, for example, comet Halley, which goes around the sun in a sprightly 76 years.

ECONOMIST: Comet Ikeya-Zhang is enlivening the skies

This is not, it seems, comet Ikeya-Zhang's first visit to the sun.

ECONOMIST: Comet Ikeya-Zhang is enlivening the skies

Its path will put it spectacularly close to the Sun in November and if it does not burn up entirely, could be the "comet of the century".

BBC: Comet Ison caught on film by Deep Impact craft