The premise of Neal Stephenson’s new novel Seveneves is that the human race of 2020 or so will have just enough of a toehold in space to give us—a few of us—some chance of surviving the destruction of all life on Earth, if allowed a couple of years’ lead time. We don’t have the technology to colonize other planets, but we might survive in Earth orbit. Stephenson can therefore destroy the planet without having recourse to warp drives or multigeneration starships.

So the dread event arrives …