SkyhookA skyhook is a hypothetical structure used for transporting material to and from a planet's surface into orbit. The largest and perhaps simplest of these is the space elevator, a cable that runs all the way from the planet's surface to beyond synchronous orbit. Smaller skyhooks include hypersonic skyhooks, rotating cables in lower orbits whose ends dip repeatedly down close to the planet's surface to snag payloads and lift them up. Large rotating tethers can also be used far from a planet's surface to transfer momentum to and from payloads, changing their orbits without the expenditure of reaction mass.
See tether propulsion for more details on various types of skyhooks.
The term skyhook was also used by Daniel Dennett in the book Darwin's Dangerous Idea to describe a source of design complexity that did not build on lower, simpler layers -- loosely, a miracle. Dennett contrasts theories of complexity which require such miracles with those based on "cranes" -- structures which permit greater complexity but are founded solidly "on the ground" of that which has gone before.