NASA's Voyager 1 spacecraft has reached a new and possibly last leg in a 35-year journey that is taking it out of the solar system, scientists said Wednesday.
The evidence comes from a sudden change in radiation levels measured by Voyager 1 on Aug. 25, 2012. On that day, particles caused by cosmic rays trapped in the envelope of space under the sun's influence, the heliosphere, virtually vanished. At the same time, measurements of galactic cosmic rays from outside the solar system spiked to levels not seen since Voyager's launch.
"Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere," astronomer Bill Webber, a professor emeritus at New Mexico State University in Las Cruces, said in a statement.
Still to be determined is if Voyager 1 is in interstellar space or a new, previously unknown region between the solar system and interstellar space.
"It's outside the normal heliosphere, I would say that," Webber said. "Everything we're measuring is different and exciting."
In December, Voyager 1 reached what scientists called a "magnetic highway," where magnetic field lines from the sun connect with magnetic field lines from interstellar space. The phenomenon causes highly energetic particles from distant supernova explosions and other cosmic events to zoom inside the solar system, while less-energetic solar particles exit.
Scientists said they don't know how long it will take for Voyager 1 to cross the boundary, but they believed it was the last leg in a complex zone lying between the heliosphere and interstellar space.
"Our best guess is it's likely just a few months to a couple years away," Stone told Discovery News in December.
Stone was traveling and could not be reached for comment on Wednesday.
Voyager 1 hit the outer sphere of the solar system 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 stabilized on Aug. 25.
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.
Stone said that another clue 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.
In a statement, Stone said he didn’t believe Voyager 1 had yet passed into interstellar space because the probe has not yet detected a change in the direction of the magnetic field.
"This is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed,” 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, traveling 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 9 billion miles from Earth, has reached the magnetic highway.
The research is being published in an upcoming edition of Geophysical Research Letters.