It's 36 years since Voyager 1 was dispatched in 1977 on a mission to send back images of Jupiter's turbulent atmosphere and volcanic eruptions on one of its moons, Io. Then it was due to travel on to Saturn to examine that planet's intricate system of rings and moons. But after traveling more than 18 billion km, where is Voyager now? No one, it seems, knows for sure.