There is no way to answer that. We don't know
how many comets there are, how many there were, or
how many actually have hit the Earth in the past.
You also have to ask "what is the time
From looking at impact scars on the Moon, we
know that impacts were more common in the distant
past (before the evolution of multicellular life)
than in the more recent past, but we still are not
certain just how many objects there are that could
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