Tether propulsion systems are proposals to use long, very strong cables (known as tethers) to change the velocity of spacecraft. 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. 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 a theoretical tensile strength of at least 60 GPa.