Larry Niven and Jerry Pournelle teamed up to tell this apocalyptic tale in 1977, which was a time before such stories were a dime a dozen. This is a book composed of two distinct parts with the first half following various individuals as they go about their normal, and somewhat boring lives, as well as following an un-named comet across the eons as various cosmic events affect its path.  The comet is eventually named Hamner-Brown upon discovery, and it quickly becomes known as "The Hammer" as TV coverage ramps up post discovery. The odds are a billion to one that it will hit Earth, but of course that would make for a boring book, so the second part of the book starts upon "Hammer Fall" and covers the impact this devastating event has to the planet as well as the characters from the first half.