Voyager 1, a space probe launched in 1977, successfully performed fly-by's of Jupiter and Saturn before exiting the confines the heliosphere and based on Cosmic Ray data, has entered a region regarded as Interstellar Space. It is currently the farthest human-made object launched from earth, and continues its voyage into deeper space at a velocity of approximately 38,000 mph. It's counterpart, Voyager 2 is the only probe to have soared by both Uranus and Neptune, and is now also in interstellar space travelling at a velocity of approximately 34,500 mph.

Neither of these space probes are equipped with engines, except for tiny jets that allow for a correction in rotation. How is it possible that these probes are able to continue their journeys at these velocities without the benefit of any propulsion?