Skyhooks are a theoretical class of cable based techniques intended to lift payloads to high altitudes and speeds. The name skyhook is a reference to an imaginary hook that hangs from the sky.
Plausible near-term proposals for skyhooks include designs that employ tethers spinning at hypersonic speeds for catching payloads from high altitude aircraft and placing them in orbit.
There are also hypothetical skyhooks that are intended be used for non-rocket spacelaunch into orbit, for example, a space elevator.
Another concept is orbital rings with geostationary 'spokes'.
See tether propulsion for more details on various types of skyhooks.
Tether propulsion systems are proposals to use long, very strong cables (known as tethers) to change the velocity of spacecraft and payloads. The tethers may be used to initiate launch, complete launch, or alter the orbit of a spacecraft. Spaceflight using this form of spacecraft propulsion may be significantly less expensive than spaceflight using rocket engines.
Tethers are kept straight by either rotating end for end, with very high tips speeds (several km/s), or by the difference in the strength of gravity over their length (tidal stabilisation). Tethers require strong, light materials. Some current tether designs use crystalline plastics such as ultra high molecular weight polyethylene, aramid or carbon fiber. A possible future material would be carbon nanotubes, which have an estimated tensile strength between 140 and 177 GPa (20.3-25.6 million psi), and a proven tensile strength in the range 50-60 GPa.
A momentum exchange tether is a rotating tether that would grab a spacecraft and then release it at later time. Doing this can transfer momentum and energy from the tether to and from the spacecraft with very little loss; this can be used for orbital manoeuvring. A rotating momentum exchange tether is known as a bolo.
Another type of tether is an electrodynamic tether, this is a conductive tether that carries a current that can generate thrust or drag from a planetary magnetic field, in much the same way as an electric motor.
To exceed the self-support length the tether material can be tapered so that the cross-sectional area varies with the total load at each point along the length of the cable. Correct tapering ensures that the tensile stress at every point in the cable is exactly the same. For very demanding applications, such as an Earth Space Elevator, the tapering can result in excessive ratios of cable weight to payload weight.