Voyager: 8 billion miles in the making

Of all the spectacular images sent back to Earth from the two Voyager spacecraft – which celebrate their 30th anniversaries this month – perhaps the most poignant was taken on 14 February 1990, when Voyager 1 turned its camera around to where it had come from and captured a picture of our home planet from a distance of more than four billion miles. The image became known as The Pale Blue Dot – the Earth floating as a single point in the enormity of space.

The American astronomer Carl Sagan wrote a book based on the inspiration of this, the most distant view of Earth. "That's here. That's home. That's us," Sagan wrote. "On it everyone you know, everyone you love, everyone you've ever heard of, every human being who ever was, lived out their lives... on a mote of dust suspended in a sunbeam."

Voyager 2 was launched on 20 August 1977, followed by its sister ship, Voyager 1, on 5 September. The primary mission, to explore Jupiter and Saturn, was intended to last five years. But between 1979 and 1989, the two probes together studied a total of 48 moons and four planets – more than any other spacecraft. Both have continued their journey of exploration, with Voyager 1 having now technically left the Solar System and become the furthest man-made object. Voyager 2 is expected to follow it and cross the heliosheath – the edge of the solar wind and the beginning of interstellar space – later this year.

The spacecraft are now about three times further away than Pluto. Both continue to maintain contact with their mission controllers, thanks to small nuclear energy sources, each of which provides about 300 watts.

The first big milestone of the Voyager mission came in 1979 when they each made their closest approach to Jupiter. They took 52,000 pictures of the planet. Scientists were able to see the giant storm known as the Great Red Spot, and one of the greatest surprises was the discovery of active vulcanism on Io, one of Jupiter's moons. Next came the visit to Saturn, in November 1980 for Voyager 1 and August 1981 for Voyager 2. Dramatic images of Saturn's famous rings revealed their level of intricacy.

Voyager 2 then went on to explore Uranus and Neptune. In addition to studying the five largest moons of Uranus, it also detected 10 previously unseen moons, as well as the planet's unusual magnetic field. The encounter with Neptune was the closest approach it had made to any planet, passing 3,000 miles over the planet's north pole. It also passed close to Triton, Neptune's largest moon.

Now both are heading for the even emptier voids of deep interstellar space. Each carries a 12-inch, gold-plated copper disc – a phonographic record designed to be played in 1970s style with a needle and turntable. They carry visual and audio images of Earth, such as the sound of surf, wind and thunder, of birds, whales and other animals. Sagan was on the committee that decided what messages should be on the record. It included greetings in 55 languages and one from Sagan's six-year-old son, who wished any life forms out there well, from the children of the world.

It will take 40,000 years before the two space probes skim past the nearest interplanetary system, so it is unlikely that the messages will be read for millennia, if at all. But, as Sagan said: "The launching of this bottle into the cosmic ocean says something very hopeful about life in this planet."