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On the 1st of July 1770 Lexell's Comet (Wikipedia, Cometography) passed around 0.0146-0.0152au behind the Earth in it's orbit.
The challenge I am proposing is that instead the comet strikes the Earth on this date. Anyone who takes this challenge has to describe both the short and long term consequences (likely to be extreme) to the various major powers of this event.
The first condition I am laying is that whoever takes up this challenge works out, where the impact would have taken place, whether over land or sea and in which hemisphere.
The second condition is that for the purposes of using the impact effects calculator (Link), the size, density and mass of comet 81P/Wild (Wikipedia) are used.