Skyhook is a momentum exchange tether designed to reduce the cost of placing payloads in low Earth orbit. A heavy orbital station is connected to a cable that extends down to the upper atmosphere. The payload, which is much lighter than the space station, is suspended at the end of the cable as it passes through the cable, and then thrown into the orbit by rotating the cable around the center of mass.
The space station can then orbit another object by electromagnetic propulsion, rocket propulsion, or transfer of the same kinetic energy to the payload to return it to its original height.