The NASA website shows how far the space probes Voyager 1 and Voyager 2 have travelled from Earth since their launch in 1977. At the time of writing, Voyager 1 is 13,487,119,000 and Voyager 2 11,147,711,000 miles away.

Their primary purpose was to beam back data about the outer planets in our solar system, but as they head beyond into interstellar space both are still communicating with us. The signal from Voyager 1, travelling at the speed of light, takes almost a day to get here.

As far as we know, to date the flow of information has all been one-way. But that may not always be the case. The Voyagers carry a message for any alien civilisation that might recover them. The Golden Record, a twelve-inch gold-plated copper disc fixed to the side