Nasa’s long-lived Voyager 1 spacecraft, which is heading out of the solar system, has reached a “magnetic highway” leading to interstellar space, scientists said.
The probe, launched 35 years ago to study the outer planets, is now about 18 billion kilometres from Earth.
At that distance, it takes radio signals travelling at the speed of light 17 hours to reach Earth. (Light moves at 300 000 km per second).
Voyager 1 will be the first manmade object to leave the solar system.
Scientists believe Voyager 1 is in an area where the magnetic field lines from the sun are connecting with magnetic field lines from interstellar space. The phenomenon is causing highly energetic particles from distant supernova explosions and other cosmic events to zoom inside the solar system, while less-energetic solar particles exit.
“It’s like a highway, letting particles in and out,” lead Voyager scientist Ed Stone told reporters at an American Geophysical Union conference in San Francisco.
Scientists don’t know how long it will take for the probe to cross the so-called “magnetic highway,” but they believe it is the last layer of a complex boundary between the region of space under the sun’s influence and interstellar space.
“Our best guess is it’s likely just a few months to a couple years away,” Stone said.
Voyager 1 hit the outer sphere of the solar system, a region called the heliosphere, in 2004 and passed into the heliosheath, where the supersonic stream of particles from the sun — the so-called “solar wind” — slowed down and became turbulent.
That phase of the journey lasted for 5.5 years. Then the solar wind stopped moving and the magnetic field strengthened.
Based on an instrument that measures charged particles, Voyager entered the magnetic highway on July 28, 2012. The region was in flux for about a month and stabilised on August 25.
The number of particles from the outside jumped sharply and the number of particles from the inside fell by a factor of 1 000.
"It is as if someone opened the floodgates and they were all moved down the river, also some boaters powered up stream with close to the speed of light have been able to get in at last," said Stamatios Krimigis, Voyager's principal investigator of low-energy charged particles.
While the magnetic field is exciting, Krimigis sounded somewhat disappointed that Voyager had not yet escaped the solar system.
"Nature is very imaginative and Lucy pulled up the football again," he said, making reference to the classic comic strip Peanuts in a conference call with reporters.
Each time Voyager re-entered the highway, the magnetic field strengthened, but its direction remained unchanged. Scientists believe the direction of the magnetic field lines will shift when the probe finally enters interstellar space.
Other clues that Voyager has reached interstellar space could be the detection of low-energy cosmic rays and a dramatic tapering of the number of solar particles, Stone said.
Voyager 1 and a sister spacecraft, Voyager 2, were launched 16 days apart in 1977 for the first flybys of Jupiter, Saturn, Uranus and Neptune.
Voyager 2, travelling on a different path out of the solar system, has experienced similar, though more gradual changes in its environment than Voyager 1.
Scientists do not believe Voyager 2, which is about 14.5 billion kilometres from Earth, has reached the magnetic highway.
The scientists controlling Voyager 1 - whose 1970s technology gives it just a 100 000th of the computer memory of an eight-gigabyte iPod Nano - decided to turn off its cameras after it passed Neptune in 1989 to preserve power.
Assuming the craft continues to function normally, they will have to start turning off other on-board instruments from 2020, and it is expected to run out of power completely in 2025.
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