It was born four and a half billion years ago. It coalesced from dust and particles, reaching the size of a mountain. It settled into a regular, safe orbit with the aid of Jupiter. It remained in that orbit for billions of years, until an accident of interplanetary physics sent a cousin careening toward Earth. The dinosaur killer’s passing tugged it out of its own orbit and into a similar one. It did not crash into the devastated planet, however. Rather it became a far raging satellite of which the creatures living on Earth would take no notice for eons.