When Facebook was growing, one of its infrastructure goals was to always be able to support 10 times the site’s then-current usage. There’s nothing special about the number 10, apart from the evolutionary accident that we have 10 fingers, but this heuristic turned a vague goal (“Always be ready for growth”) into a highly specific one (“If we’re using 11 percent of our capacity, we need to buy more servers now”). And for Apollo, it meant that instead of getting to the Moon “soon” or “as quickly as possible,” there was a date after which it would be too late.
