Of course, you can’t actually see it, but you feel it every day. It has no physical walls. And yet it is very real; fail to decelerate as you approach the bottom, and your craft will be crushed along with all its occupants. From an engineering standpoint, the Well de nes a terrain of space—a steep wall requiring enormous force to climb.

Here at the bottom of the Well, we have already worked wonders. In little more than a century, we built vehicles that left the surface and occupied the space just above it. We built robots and sensing equipment to look down on our planet and allow global communication and navigation. We even set foot on the Moon. More recently, we have begun to create an economy in space, with ever-cheaper satellites and more efficient vehicles. We have sent astronauts to live for months in space.

And yet, that space we are making our own still lies near the bottom of Earth’s Gravity Well. Most of today’s space economy occupies low Earth orbit, a 1,200-mile-wide band that’s a slice of less than one one-thousandth of the Gravity Well.