SKY HOOK, is also known as a ‘Momentum Exchange Tether’. But I prefer to call them giant space catapults because they literally pick-up a craft flying at the edge of the planet's atmosphere and hurl them out into space at breakneck speed!
Despite sounding like a ride at Disney World this technology exists and it could dethrone SpaceX as the poster child for our cosmic future.

A Skyhook is simply a rotating satellite with a very long tether(proposed to be around1000km long) on one-side and a smaller tether with a counterweight on-the-other. A spacecraft attaches to the long tether side and gets flung out into space by the rotation.

The diagram below is by far the easiest way to explain.
The blue circle shows the Skyhook’s orbit. The blue Satellite is a Skyhook with a square counterweight and on-the-other-side of the rotating satellite is the long attachment tether. The black line shows the path taken by the spaceship.

When the spaceship attaches to the Skyhook the velocity of the tether-end of the satellite is in the opposite direction to its orbit. These two velocities work against each other which means that the end of the tether is traveling at its slowest when the tether is facing the earth. Then when the tether-end has rotated to the point where it is facing away from the Earth its velocity is in the same direction as the orbit. This makes the velocities work together, which means the tether-end is traveling at its fastest.

By the spacecraft attaching onto the tether at its slowest point and releasing it at its fastest, this basically turns the rotational momentum of the satellite into speed for our craft! It is literally a giant space catapult.

We could have several Skyhooks orbiting Earth enabling us to cheaply, easily and regularly send spacecraft to the inner and out Solar System. But once we have mastered this technology on Earth we can replicate it all over the Solar System. With this we could colonize it’s planets!

Source: https://medium.com/predict/skyhook-the-technology-that-could-challenge-spacex-3ed5fa84696f