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On 12th and 13th August 1883, an astronomer at a small observatory in 
Zacatecas in Mexico made an extraordinary observation. José Bonilla 
counted some 450 objects, each surrounded by a kind of mist, passing 
across the face of the Sun.

Hector Manterola at the National Autonomous University of Mexico in 
Mexico City has placed limits on how close the fragments must have been: 
between 600 km and 8000 km of Earth. What's more, Manterola and co 
estimate that these objects must have ranged in size from 50 to 800 
metres across and that the parent comet must originally have tipped the 
scales at a billion tons or more, that's huge, approaching the size of 
Halley's comet.

Each fragment was at least as big as the one thought to have hit 
Tunguska. Manterola and co end with this: "So if they had collided with 
Earth we would have had 3275 Tunguska events in two days, probably an 
extinction event."

 
http://www.technologyreview.com/view/425780/billion-ton-comet-may-have-missed-earth-by-a-few-hundred-kilometers-in-1883/

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