In order to leave orbit, a spacecraft needs to be going fast enough to break free of gravity. A huge push is needed to do that. Either that push was given to a ship as it was launched or it is given to a ship already in orbit. To push a ship that is already in orbit farther from the planet, thrusters should be fired at the highest point in the orbit. Generally, a ship will go into higher and higher orbits until it intersects with its destination.

Spacecraft can go from planet to planet that way.

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