Voyager 1 was slung out of our solar system using the gravitation of the planets within our solar system, like a sling shot. Once the voyager 1 is far enough away from our solar system there are no longer any heavenly bodies or planets to act on the body of the craft to speed it up or slow it down. In theory it will keep drifting at this speed till another larger gravitational body acts upon it. That bodies force will either speed up the craft or slow it down. It is the hope of our science that by freak chance that another intelligent group of beings will discover the Voyager and by dissecting it they will learn what we are about The sad part is, we will never, ever be around when this happens as it could take as much as millions of years to drift into a solar system that has intelligent life. But it is fun to think of the possibilities.
it was acheived by being shot into space by a rocket then it got even faster by many gravity assists from the gas giants so it was in a way ;shot into interstellar space