Elvis had been dead just days, and the Iron Curtain still divided east from west. Jimmy Carter was in the White House, and Malcolm Fraser was in his first term in the Lodge. Australians were awed by colour television, and Sony was marketing a revolutionary video recorder called Betamax. Home computers were a distant dream.

Launched on September 5, 1977, Voyager 1 zoomed past Jupiter in 1979, and Saturn in 1980, heading on a path clear out of the solar system. Its identical twin, sent aloft two weeks earlier on August 20, also took in the sights of Neptune in 1986 and Uranus in 1989. Today Voyager 1 is humanity's most remote object, 15.5 billion kilometres from the sun. Voyager 2 is 12.5 billion kilometres from it. Both continue beaming home reports, but now they are space-age antiques.

"The Voyager technology is so outmoded," said Tidbinbilla's spokesman, Glen Nagle, "we have had to maintain heritage equipment to talk to them." That is because the ageing probes can only chat at a sluggish 32 bits a second, far too slow for modern computers.

"The computers look like the stuff out of the old sci-fi movies, with blinky lights and big colourful buttons," Mr Nagle said. To keep them humming, Tidbinbilla relies on its most experienced engineers, including John Murray, who will have been working there for 40 years on Monday. His colleague Ian Warren has knotched up 42 years in the space business. With the nuclear-powered Voyagers tipped to keep transmitting until at least 2020, Mr Murray has to show younger staff how to maintain the vital hardware. "I find it funny that I send people I am teaching to go and get parts for these machines that are older than them," he said.

It is not just the age of the Voyagers that makes communicating difficult. "When the signal leaves the spacecraft it's about 19 watts, about the power of a light bulb in a fridge." Mr Nagle said. "When it reaches us it's 20 billion times weaker than a watch battery. It is a whisper from space." Voyager 2 phones home to Tidbinbilla every day. Travelling at the speed of light, the signals take 11.5 hours to arrive. Voyager 1 calls once a month, its signals taking almost 14 hours. A two-way exchange takes more than a day. "It's slow conversation, but its a great conversation," Mr Nagle said.

Travelling at 34 kilometres a second, the Voyagers are seeking the point where the wind of particles blowing off the sun is overwhelmed by those streaming in from other stars. Beyond that is interstellar space. Both carry disks loaded with pictures and sounds from Earth. "It's like a message in a bottle, a message from humanity for whoever might find it," Mr Nagle said. Assuming, of course, that aliens can play such outmoded technology.

Tidbinbilla, open daily, is staging a display of Voyager pictures and movies until Wednesday.