After completing its primary mission of outer solar system exploration many years ago, Voyager 1 (and its twin probe Voyager 2) has been ploughing through the outermost reaches of the heliosphere - called the heliopause.
Although data collected by the aging Voyager 1 have been showing strong signs of flying beyond the heliopause, mission scientists at the AGU conference in San Francisco announced on Monday that hopes of an interstellar Voyager 1 are premature.
However, the mission is beginning to "taste" the interstellar shores.
It appears that the probe has flown into a new and unexpected region of the heliosphere where the rush of particles generated by the sun - that, in turn, form the solar wind - are carried by the weakening solar magnetic field, pushing against the interstellar medium. The outside (interstellar medium) pressure sweeps back the sun's magnetic field, channeling solar particles into a high-speed "magnetic highway." But high-energy particles from outside the heliopause are leaking into the highway and washing over the spacecraft's instruments.
"Although Voyager 1 still is inside the sun's environment, we now can taste what it's like on the outside because the particles are zipping in and out on this magnetic highway," Edward Stone, Voyager project scientist based at Caltech in Pasadena said in a NASA press release. "We believe this is the last leg of our journey to interstellar space. Our best guess is it's likely just a few months to a couple years away. The new region isn't what we expected, but we've come to expect the unexpected from Voyager."
Voyager 1 has been experiencing the outermost regions of the solar system for several years, hitting the solar system's "termination shock" in 2004. It then blasted into the "helioshieth", a region where the solar wind slowed rapidly and succumbed to turbulence. Most recently, Voyager 1 detected the solar wind particles slow to zero signifying that the probe must be approaching the outermost boundary before interstellar space. At the same time, a strengthening of the magnetic field was detected.
Most recently, a rapid drop in lower-energy particles (originating from the sun) coincided with a noticeable increase in high-energy particles (originating from interstellar space) leading many to speculate that Voyager 1 had officially left the solar system's heliopause. However, NASA scientists aren't yet ready to call Voyager 1 an "interstellar mission."
"If we were judging by the charged particle data alone, I would have thought we were outside the heliosphere," said Stamatios Krimigis, principal investigator of the low-energy charged particle instrument, based at the Johns Hopkins Applied Physics Laboratory, Laurel, Md. "But we need to look at what all the instruments are telling us and only time will tell whether our interpretations about this frontier are correct."
"We are in a magnetic region unlike any we've been in before - about 10 times more intense than before the termination shock - but the magnetic field data show no indication we're in interstellar space," said Leonard Burlaga, a scientist on the Voyager magnetometer team at NASA's Goddard Space Flight Center in Greenbelt, Md. "The magnetic field data turned out to be the key to pinpointing when we crossed the termination shock. And we expect these data will tell us when we first reach interstellar space."