About the Voyager 1 spacecraft, this much is clear: the NASA probe has traveled farther than any other. Voyager 1 is now more than 18.5 billion kilometers from the sun--almost 125 times the distance between Earth and the sun. The spacecraft, one of two Voyagers launched by NASA in 1977, is truly in unexplored territory--so much so that defining its current whereabouts poses a bit of a challenge.
Someday soon, it is expected, Voyager 1 will become the first spacecraft to exit the heliosphere--the bubble of solar plasma encasing the sun and planets--and enter the interstellar medium. Right now, however, Voyager appears to be crossing through one of several puzzling new regions of the heliosphere that mission planners had not anticipated.
In a trio of studies published online June 27 in Science, Voyager scientists describe the latest heliospheric wrinkle discovered by the probe en route to interstellar space. Voyager 1, they report, appears to have crossed last August into what is now being called the "heliosheath depletion region." The researchers described some characteristics of the new region in a December 2012 teleconference with reporters, but the new studies go into far more detail about Voyager 1's environs.
One major change signaling that Voyager had entered new territory was a sudden decrease in the number of particles from the sun hitting the spacecraft's Low Energy Charged Particle (LECP) instrument. On August 25, those solar particles dropped to less than one-thousandth their prior levels. Simultaneously, cosmic rays emanating from sources elsewhere in the galaxy began striking the LECP at a rate nearly 10 percent above the previous clip.
A second particle detector known as the Voyager Cosmic Ray Subsystem noted a similar change: charged atomic nuclei from the sun disappeared as nuclei from outside the solar system surged.
Such changes might seem a clear marker that Voyager 1 has completed its ultimate mission by breaking free of the sun's plasma cocoon and reporting back from the unexplored realm of interstellar space. But crucial measurements from a third Voyager instrument contradict that conclusion.
The heliopause--the boundary between the local heliosphere and the interstellar medium beyond--is a boundary not only of particles but of magnetic fields as well. And although the magnetometer on Voyager 1 did register an increase in magnetic field strength as the spacecraft crossed over into the new realm, the direction of the magnetic field did not change. So either Voyager 1 has measured the interstellar magnetic field and found that it somehow perfectly aligns with the solar system (a possibility that the Voyager researchers consider "highly improbable") or, more likely, the spacecraft remains within the sun's magnetic domain.
Taken together, the evidence suggests that the heliosheath depletion region now being explored by Voyager is a new kind of solar system environment, one where the sun's influence wanes and interstellar particles can enter more easily. And although the new region appears not to qualify as interstellar, the researchers report, it "may form part of the interface between solar plasma and the galaxy."
So godspeed, Voyager. May you soon reach interstellar space--a place humankind has never before accessed. And may we know it when you see it.