My physics professor told us something about interstellar travel, which I have a question on. He told us that scientists figured out a way to send objects into space and frequently increase velocities. He said they do this by sending probes and aiming them at planets and large bodies in space thus sending the probes to orbit around the body and eventually they will escape the gravitational orbit of the planet after they reach the escape velocity when they will head towards another body and continue the same process until they reach a fast velocity. He said scientits can direct the probe's launch so they can plan it to go to a certain place through these collisions. The question I have is how do the probes increase their velocities and climb up to the escape velocities? I would ask him the question now, but the quarter ended. I tried searching this up on google, but couldnt find anything related to this. I was wondering if anyone can help me out.