The US space probe Voyager 1 has crossed a new frontier, becoming humanity's first spacecraft ever to leave the solar system, NASA has said.
Thirty-six years after it was launched from Earth on a tour of the outer planets, the plutonium-powered spacecraft is more than 11 billion miles from the Sun in interstellar space - the vast, cold emptiness between the stars, the US space agency said on Thursday.
Voyager 1 actually made its exit more than a year ago, according to NASA, but the evidence has only just come to light. "It's a milestone and the beginning of a new journey,'' said mission chief scientist Ed Stone at the agency's Jet Propulsion Laboratory.
Voyager 1 will now study exotic particles and other phenomena in a never-before-explored part of the universe and radio the data back to Earth, where the Voyager team awaits the starship's discoveries.
The interstellar ambassador also carries a gold-plated disc containing multicultural greetings, songs and photos, just in case it reaches an intelligent species.
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Voyager 1's odyssey began in 1977 when the spacecraft and its twin, Voyager 2, were launched on a tour of the gas giant planets of the solar system. After beaming back photographs of Jupiter's giant red spot and Saturn's shimmering rings, Voyager 2 moved on to Uranus and Neptune.
Last year, scientists monitoring Voyager 1 noticed charged particles streaming from the sun suddenly vanished from its readings. At the same time, there was a spike in galactic cosmic rays - suggesting it was beyond the heliosphere.
A chance solar eruption caused the space around Voyager 1 to echo like a bell last spring and provided the scientists with the information they needed, convincing them the boundary had been crossed in August of last year.
"It took us 10 seconds to realise we were in interstellar space,'' said Don Gurnett, a Voyager scientist at the University of Iowa who led the new research, published online in the journal Science.
Voyager 2 trails behind its sister ship by about two billion miles. Both spacecraft will run out of nuclear fuel and will have to power down their instruments, perhaps by 2025.
At current speed, it would take Voyager 1 more than 70,000 years to reach the nearest extra-terrestrial star, Proxima Centauri, were it travelling in that direction.