The term
skyhook is used to describe a class of 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 skyhook]
?s, 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.