NASA's aging Voyager 1 probe, 35 years and 11 billion miles outbound from Earth, has crossed into an unexpected, exceedingly remote region of the solar system that may represent the spacecraft's final step before leaving the sun's influence and moving into the vast realm of interstellar space.
The region is believed to be a sort of "magnetic highway" allowing high-energy charged particles from ancient supernova explosions to move into the sun's sphere of influence and for lower-energy particles to move out into deep space.
"This is really another exciting step in the Voyager journey of exploration," Project Scientist Ed Stone told reporters Monday. "Voyager's discovered a new region of the heliosphere that we had not realized was there. It's a magnetic highway where the magnetic field of the sun -- we're still inside apparently -- but the magnetic field is connected to the outside. So it's like a highway, letting particles in and out."
The sun's magnetic field and the outward flow of electrically charged particles blasted away from the star -- the solar wind -- define a gigantic region in space known as the heliosphere. It is shaped somewhat like a teardrop because of the sun's motion, due to the rotation of the Milky Way, combined with the effects of the sun's passage through a cloud of interstellar debris produced by ancient supernova explosions. The tail of the teardrop stretches away in the opposite direction of travel.
The outward flow of the solar wind and the shepherding magnetic field act as a shield of sorts for the inner solar system, affecting the passage of high-energy cosmic rays from deep space.
In recent years, Voyager 1 and its sister ship, Voyager 2, have been moving through a region known as the heliosheath, plowing through a realm where the outward velocity of the solar wind has dropped to near zero.
Just in front of the heliosheath is the heliopause, the boundary between the heliosphere and the interstellar wind the sun and its solar system are moving through. Just in front of the heliopause is yet another zone, the so-called bow shock, where the interstellar wind crashes into the heliosphere.
Now, it appears there is yet another layer in the complex boundary between the heliosphere and interstellar space.
"We believe, from the magnetic field data, that the layer we're in actually came from the southern hemisphere of the sun," Stone said. Even though Voyager 1 is heading outward on a trajectory tilted 35 degrees above the plane of the solar system, "the pressure on the outside of the heliosphere from the outside magnetic field has forced some of the southern solar wind to flow northward and it creates a layer just on the inside of the heliopause."
"That, we believe, is the highway we're on now," Stone said. "We don't know exactly how long it will take [to cross it and move out of the solar system]. It may take several more months, it may take several more years. But we do believe this may be the very last layer between us and interstellar space."
He said three lines of evidence should let scientists know when that long-awaited milestone has occurred.
Once outside the heliosphere, low-energy cosmic rays should be detected that were unable to make it into the solar system due to the shielding effect of the sun's magnetic field and solar wind. At the same time, slower particles from inside the heliosphere should drop off to extremely low levels. Finally, Voyager's magnetometer should detect a north-south field instead of the east-west orientation of the sun's field.
In recent months, Voyager 1's particle observations have seemed consistent with flying through interstellar space.
"If we had only looked at the particle data alone, we would have said, 'well, we're out, goodbye solar system,'" said Stamatios Krimigis, principal investigator for Voyager's low-energy charged particle instrument. "But nature is very imaginative, and Lucy pulled out the football again."
That's because the magnetic field direction has not yet changed to the expected north-south orientation of interstellar space.
"We're quite confident that there's really no reason to believe we're outside the heliosphere," said Leonard Burlaga, a magnetometer team scientist the Goddard Space Flight Center. "There's no evidence that we have entered the interstellar magnetic field."
Not yet, maybe. But soon.
Voyager 2 was launched on August 20, 1977, followed 16 days later by Voyager 1 on September 5, 1977. Voyager 1 flew past Jupiter on March 5, 1979, and then made a dramatic flyby of Saturn on November 12, 1980. After beaming back spectacular views of Saturn and its enigmatic moon, Titan, the spacecraft headed out of the solar system on a trajectory above the plane of the planets.
Voyager 2 flew past Jupiter on July 9, 1979, and then past Saturn on August 25, 1981. The spacecraft then continued on to Uranus for a January 24, 1986, flyby and then Neptune and one of its moons, Triton, on August 25, 1989. The Neptune flyby put Voyager 2 on a trajectory at a steep angle below the plane of the planets.
The Voyagers have been beaming back a steady stream of data ever since their planetary flybys, measuring the fields and particles present in the far reaches of the solar system as they moved toward the limits of the sun's influence.
As of this writing, Voyager 1 is about 11.3 billion miles from Earth and Voyager 2 is roughly 9.2 billion miles out. At those distances, it takes more than 16 hours for 20-watt radio signals, moving at 186,000 miles per second, to cross the vast gulf between the spacecraft and the giant antennas on Earth that are needed to collect the data.
On its current trajectory, Voyager 1 will pass within about 1.6 lightyears of a star in the constellation Camelopardalis in about 40,000 years. It will take even longer for Voyager 2 to pass within several light years of another star.
As a NASA mission overview concludes, "the Voyagers are destined -- perhaps eternally -- to wander the Milky Way."