This line in particular caught my eye:
If it struck, it would have stuck. And we wouldn't be here today. Planets, even dwarf planets, have too much mass and gravity to carom off each other like billiard balls. They would have collapsed into each other, releasing a lot of energy in the process, making one big mushy ball of molten goo.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort