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The largest OBJECT ever to hit the Earth was probably the proto-planet Theia that we believe collided with the proto-Earth, the debris from which formed the Moon. That probably was not a comet. Theia may have been as large as the planet Mars.

Beyond that, we really don't know. There isn't any way to determine, 65 million years after the fact, whether the impactor that probably caused the K-T extinctions and killed off the dinosaurs was really a comet, or an asteroid; perhaps it doesn't really matter. That object may have been as large as 10 miles in diameter. And we don't really KNOW the cause of the Great Permian Extinction 252 million years ago; it may have been an impact event, or there may have been other causes for the destruction of perhaps 95% of all life on Earth.

We believe that during the first couple of million years of Earth's existence, it probably suffered a great number of quite massive impacts, but we do not (as yet) have any detailed information about the early history of our planet.

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15y ago

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