Many happy returns from Voyager 1 - some 18 billion miles away

As it marks its 35th birthday, the spacecraft Voyager 1 is leaving our Solar System about to enter interstellar space and to boldly go where no man-made object has gone before.

This image provided by Nasa shows the Earth as a dot in a solar system portrait taken by Voyager 1. Launched in 1977, the twin spacecraft are exploring the edge of the solar system. Voyager 1 is poised to cross into interstellar space. Nasa / AP Photo
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It will be a birthday party with a guest list of one. When you live 18 billion miles from Earth and are still travelling, company is hard to find.

Tomorrow the probe Voyager 1 marks its 35th birthday and a mission of trying to get as far away from home as possible. In this task it has succeeded beyond anyone's wildest imagination; now at the very edge of our Solar System, the craft is about to become the first man-made object to penetrate interstellar space.

The distances involved are almost beyond the capabilities of human imagination but these two photographs above give some idea of how far Voyager - and mankind - has journeyed.

One was taken by astronaut William A Anders on December 24, 1968 from Apollo 8 as it circled the Moon at a distance of around 380,000 kilometres.

As Anders captured the fragile blue and white sphere rising above the lunar surface, it was the first time we on Earth saw ourselves as an outsider might.

The second comes from Voyager 1 and was released by Nasa, the US space agency this week. At seven billion miles, this is the most distant photograph ever taken of Earth; the planet reduced to a blue speck to tiny that Nasa has helpfully ringed it for identification.

Voyager 1, and its sister probe Voyager 2, were part of programme to study the outer planets, including Jupiter, Saturn, Uranus and Neptune. Voyager 2 launched from the top of a Titan 3E Centaur rocket at Cape Canaveral in Florida on September 5, 1977, just two weeks after Voyager 1.

Both spacecraft carry a copy of the "golden record" a phonographic disc made of gold-plated copper that can playback images and sounds of life on Earth to any passing extraterrestrial being that might encounter them. The records come with instructions for replaying them using a stylus - they come from a time before the compact disc, let alone the MP3 file.

Voyager first passed Jupiter, between March and April 1979, photographing the planet's previously unseen rings and capturing the eruption of a volcano on the moon Lo.

A year later the probe had reached Saturn, using its scientific instruments to probe the planet's rings and swinging close by the giant moon Titan. It was here the primary mission ended, but not the voyage.

By 1990, Voyager 1 was moving out of the Solar System. On the orders of mission control, the craft swung its camera round for one last time, to record the image released by Nasa this week.

The craft was now beyond the orbit of Pluto, with Earth reduced to a blue dot, the result of polarisation. Transmitting the image at the speed of light, it took around five and a half hours to reach Earth.

Carl Sagan, the astronomer, dubbed the image the "pale blue dot" and used it as the title for his next book, in which he mused: "From this distant vantage point, the Earth might not seem of any particular interest. But for us, it's different. Consider again that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives."

Sagan died in 1996. Voyager 1 continued onwards.

Powered by electric generators that release heat from the decay of radioactive material, the craft is slowly shutting down to conserve energy. Four years ago it shut down the Planetrary Radio Astronomy Experiment, the planets now far behind. In 2010 the Ultraviolet spectrometer was turned off.

Scientists now believe Voyager is in the heliopause, the boundary, at least in theory, where the Sun's solar wind is no longer strong enough to push back the solar wind of other stars.

Beyond the heliopause is interstellar space. The question for Nasa scientists now is when Voyager 1, already the furthest man-made object, enters these uncharted waters. At the end of July, data from Voyager - which now takes 16 hours to reach Earth - showed a five per cent spike in cosmic rays, suggesting the craft had reached the heliosheath, the very edge of the protective bubble that surrounds our planetary system.

Three days later, the cosmic ray levels returned to normal. creating uncertainty. Later this month, Nasa hopes to product a report on the direction of the magnetic field around Voyager, which should confirm if it has finally left us behind.

We can expect to continue to communicate with the craft until 2025, at which point its power system will finally fail. But in the intense cold and utter silence, Voyager 1 will fly on at a steady 10 kilometres a second.

In 40,000 years it will finally reach a new solar system, still carrying its golden records with their message of life on Earth - and hoping to find someone or something to play them.