There are some old threads about these, so I thought I would mention them now since they are both swinging by this year. (One wonders how soon the
threads will appear about them actually colliding with earth or they are harboring alien motherships)
Source
First is Comet PANSTARRS - which is due to pass by in March. Comet PANSTARRS is considered a non-periodic comet. It probably took millions of years
to come from the great Oort comet cloud surrounding our solar system. This one, they aren't sure will be visible to the naked eye, but are
hoping.
By October 2012, its surrounding coma was estimated to be about 75,000 miles (120,000 kilometers) wide. Comet PANSTARRS will pass closest to the
Earth on March 5, and it’ll be closest to the sun on March 10. By some estimates, it should get as bright as Venus, but do remember that comets are
notoriously difficult to predict.
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Next we have ISON - which is supposed to outshine the moon - IF it survives it's close encounter with the sun. They are calling this a sun-diving
comet. And the mars rover may get a preview of it for us!
A NASA astronomer says ISON's fiery tail may be visible to those watching the night sky from October 2013 through January 2014.
That amazes me! To potentially watch a comet for 3 months