The world's most far-flung spacecraft is marking 40 years of service from the icy depths of interstellar space.

Despite its age, Voyager 1 travels around one million miles per day, following its launch by NASA on September 5, 1977, to study the outer solar system.

An image of Jupiter's Great Red Spot. Credit: NASA

Launched from Cape Canaveral, Florida, 16 days after its twin, Voyager 2, the pair were intended to complete a "grand tour of the solar system", helping to unveil clues at to how Earth formed more than 4.5 billion years ago.

Both spacecraft have long-lasting nuclear power plants and continue to communicate with the US space agency across billions of miles.

The pair revealed 23 new planetary moons at Jupiter, Saturn, Uranus and Neptune and returned the first detailed pictures of the four giant planets in the outer solar system.

An image of Saturn taken by Voyager 2. Credit: NASA

The images revealed that Jupiter's moon, Io, is the most volcanically active body in the solar system, and gave the first hints that Jupiter's moon, Europa, contains a global subsurface ocean that could contain life.

They also showed an Earth-like atmosphere on Saturn's moon, Titan, icy geysers on Neptune's moon Triton, Jupiter's ring system, and the intertwined structure of Saturn's rings.

Neptune's moon, Triton. Credit: NASA

Seeing the moons of Jupiter and Saturn "was the most remarkable thing I've ever seen in my life", recalled Charlie Hord, a former planetary scientist who worked with Voyager as a doctoral student, adding that "all of the scientists were dazzled by the pictures".

The photos were "a really big deal," added Professor Fran Bagenal of CU Boulder's Laboratory for Atmospheric and Space Physics (LASP), a mission scientist who began working with Voyager data as a doctoral student at MIT.

"The outer solar system went from being fuzzy planets with dots for moons to a whole set of new and amazing worlds."

Uranus' moon, Miranda. Credit: NASA

Voyager 1 is now almost 13 billion miles from Earth, travelling northwards in relation to the planet at 30,000mph, while Voyager 2 is only around 11 billion miles away and travelling in the opposite direction.

Should it continue on this trajectory, Voyager 1 will fly past the star AC+79 3888 in the constellation Ophiuchus in around 40,000 years.

Five years ago it became the only spacecraft to have ever ventured beyond the solar system, revealing the harshness of the interstellar environment where cosmic radiation levels are four times higher than they are around the Earth.

A false-colour image of Uranus. Credit: NASA

"I believe that few missions can ever match the achievements of the Voyager spacecraft during their four decades of exploration," said Thomas Zurbuchen, associate administrator at NASA's Science Mission Directorate.

He continued: "They have educated us to the unknown wonders of the universe and truly inspired humanity to continue to explore our solar system and beyond."

Neptune Credit: NASA

Should either of the Voyager spacecraft encounter a civilisation, each are carrying what are known as the "Golden Records" - gold-plated copper phonograph records with greetings in 54 languages; the sounds of surf, wind, thunder, birds and whales; analog photos of people and places on Earth; diagrams of DNA; and snippets of music ranging from Bach and Beethoven to Chuck Berry.

Should this newfound civilisation now have the correct technology, both spacecraft also carry a stylus set up in the correct position so that aliens could immediately play the record, named "Murmurs from Earth" by the late Carl Sagan, who conceived the Golden Record effort.

Mr Sagen argued that any extraterrestrials who came across the Golden Records would recognise them as a way of humans trying to say hello.

The Golden Records are created. Credit: NASA

The Golden Records are etched. Credit: NASA

When the Voyager spacecraft turn 53 in 2030, it is expected that controllers will turn them off, however, they cannot be returned and so will continue to travel forever, completing an orbit of the Milky Way galaxy every 225 million years.

Voyager project scientist Ed Stone, based at the California Institute of Technology, said: "None of us knew, when we launched 40 years ago, that anything would still be working, and continuing on this pioneering journey.

"The most exciting thing they find in the next five years is likely to be something that we didn't know was out there to be discovered."