NEVER before has a human-built spacecraft traveled so far. NASA's Voyager 1 probe has left the solar system and is wandering the galaxy, US scientists have said.The spacecraft, which looks like a combination of a satellite dish and an old television set with rabbit ear antennas, was launched in 1977 on a mission to explore planets in our solar system.
Against all odds, Voyager kept on moving and now is about 19 billion kilometres from our Sun in a cold, dark part of space that is between the stars, said Ed Stone, Voyager project scientist.
"We are indeed in interstellar space for the first time," said Stone, who is based at the California Institute of Technology in Pasadena.
"We got there. This is something we all hoped when we started on this 40 years ago," Stone said. "None of us knew anything could last as long as the two Voyager spacecraft."
They discovered new details about the nature of Saturn's rings and found volcanoes on Jupiter's moon Io.
Voyager 2 traveled on to Uranus and Neptune, before the duo's mission was extended to explore the outer limits of the Sun's influence.
The precise position of Voyager has been fiercely debated in the past year, because scientists have not known exactly what it would look like when the spacecraft crossed the boundary of the solar system -- and the tool on board that was meant to detect the change broke long ago.
However, US space agency scientists now agree that Voyager is officially outside the protective bubble known as the heliosphere that extends beyond all the planets in our solar system.
Their findings -- which describe the conditions that show Voyager actually left the solar system in August 2012 -- are published in the US journal Science.
NASA said Voyager 1 "is in a transitional region immediately outside the solar bubble, where some effects from our Sun are still evident."
Voyager 1 -- with Voyager 2 a few years behind in its travels -- sent back data to scientists on Earth on August 25 last year, showing an abrupt drop in energetic charged particles, or cosmic rays, that are produced inside the heliosphere.
Scientists expected that the direction of the magnetic field in space would reverse at the barrier known as the heliopause.
The Voyager 1 magnetometer did not show this change, leading scientists to be extra cautious about declaring whether or not the spacecraft had left the solar system.
However, an analysis of data from Voyager's plasma wave science instrument between April 9 and May 22 this year showed the spacecraft was in a region with an electron density of about 0.08 per cubic centimeter.
Astrophysicists have projected that the density of electrons in interstellar space would be between 0.05 and 0.22 per cubic centimetre, placing Voyager squarely in that range.
"This historic step is even more exciting because it marks the beginning of a new era of exploration for Voyager, the exploration of the space between the stars," said Stone.
While the Voyager team has reached a consensus, not all are convinced.
"I don't think it's a certainty Voyager is outside now," space physicist David McComas of the Southwest Research Institute in San Antonio, Texas told Science magazine.
"It may well have crossed," he said. "But without a magnetic field direction change, I don't know what to make of it."
The spacecraft is expected to keep cruising, though the radioisotope thermo-electric generators that power it are beginning to run down.
Voyager's instruments will have to shut down permanently in 2025, Science reported. However, experts say the spacecraft may keep traveling indefinitely.
NASA said the total cost of the twin Voyager missions has been $US988 million dollars, including launch, mission operations and the spacecraft's nuclear batteries.
"Even though it took 36 years, it's just an amazing thing to me," said co-author Bill Kurth, of the University of Iowa.
"I think the Voyager mission is a much grander voyage of humankind than anyone had dreamed."
With many thanks to The Australian.
Below: this 1977 artists' impression shows what was expected to be Voyager 1's four-year mission to Saturn.
Thirty-six years and 19 billion km later, it has left the solar system - and is still going. Image: NASA
Top Voyager picture: thanks to NPR