Wow. I did not know this, and I've kept up with Voyager a bit since I watched it launch as a child...

Voyager 1 has no engine running. The 38,000-mile-per-hour speed it has held since its Saturn flyby in 1980 is borrowed momentum from planets it grazed more than four decades ago, and that free ride carries it to one light-day from Earth this November

The probe is moving because it was launched onto a carefully chosen path and then reshaped by planetary gravity. Its present outward motion is mostly the inheritance of encounters that happened when the spacecraft was still young: Jupiter in 1979, Saturn in 1980, and the rare alignment of the outer planets that made the Voyager mission possible in the first place.