Tractors, didgeridoos and laughter - that eclectic mix of sounds is from one gold record from the '70s that may never be played.

It is the Sounds of Earth, and it is hurtling through space at 60,000 kilometres per hour on each of the Voyager spacecraft in the hope of encountering aliens.

Music, images, and natural and man-made sounds are all on the playlist, along with greetings in 55 different languages.

The Voyager craft are "like little ambassadors heading out into the universe," said Mr Glen Nagle, public outreach manager at the Canberra complex of NASA's Deep Space Network.

"The whole idea was putting a message into a bottle and throwing it out into a very large ocean, just in case there's someone out there who's curious like us. It's a message of hope - here we are, come find us".

Interstellar diplomatic relations were not the only objectives of the Voyager mission.

Voyager 1 and 2 were launched in 1977 to explore the big planets in the outer half of the solar system.

Built to last five years, they have gone way beyond their original mission, and Voyager 1 broke through the boundary of our solar system and entered interstellar space in 2012. It is now four times further from the Sun than Pluto is.

Ever since their launch they have been sending radio signals back with data from their onboard experiments, including close-ups of Jupiter, Saturn, Neptune and Uranus and their moons.

The Voyagers are now focused on studying the balance of energy pushing outward from the Sun as solar wind, and that coming inward from all the other stars - the interstellar wind.

A golden record deserves some golden cover art Those squiggles and circles are enough for an alien physicist to work out how to play and interpret the sounds and images on Voyager's golden record, including what speed to play it at. The star-like pattern in the top right is a map of our Sun's location according to known pulsars, so if they like - or don't like - what they find, they'll be able to track us down.

The two 'winds' are made of high-speed charged particles, and they create a bow wave where they meet at the boundary of our solar system.

"When Voyager 1 pushed through that bough wave a few years ago, we learned that it's a lot messier and more varied than anyone had thought. It's more like white water than a bough wave ... with peaks and troughs, intense areas and peaceful voids," said Mr Nagle.

Almost 40 years since their launch the two Voyagers are still streaming their radio messages home.

Under their current rate of power loss and with the weakening of the radio signal over distance, Mr Nagle expects to continue to pick up the signal for another nine years.

"After that the signal will be drowned out by the background noise of the universe," he said.

So if we were to send another updated time capsule into space would Mr Nagle change anything?

"It was all a bit limited in its 1970s view of the Earth, there was a lot of inequality in our society back then. We've still got a long way to go, but I think it would be great to show how much we've grown as a civilisation," he said.

"And I'd get rid of the diagrams showing how to dissect a human being".