Opportunities for travel in space could be, like the Universe itself, vast beyond comprehension, if only we could get there quickly. The vastness itself is the problem. What we would find, though, is that there seems to be only one place that is suited to survival without very special protective kit. We are matched to the Earth, and its familiar ground is the one place where we are at home. That doesn’t stop us looking outward, at what seems to whirl over our heads, day or night. We use models—which might be just pictures in our heads, mechanical orreries, or patterns in the circuits of computers that mimic the wider world—so that we can not only begin to understand but can make predictions that allow us to send spacecraft out to where a myriad of objects move endlessly.