Voyager 1 and the solar system

Credit: Jen Christiansen

Tracking the location of the Voyager 1 spacecraft can be exhausting for a science journalist, and I can only imagine how confusing it gets for the interested reader. The relevant question pertaining to Voyager 1’s location is this: Has the venerable NASA spacecraft exited the heliosphere, the sun’s plasma cocoon in space, and crossed into the interstellar medium? The answer has swung for years between variations on “no” and “maybe,” most recently landing on “not yet” when mission scientists unveiled the latest data from the probe in a December teleconference.

So a press release this morning from the American Geophysical Union, proclaiming that “Voyager 1 has left the solar system,” caught my attention. (The headline was later revised to state that “Voyager 1 has entered a new region of space.”) The gist of the release: a new study in Geophysical Research Letters, analyzing cosmic ray data from Voyager 1, concludes that the probe “appears to have travelled beyond the influence of the Sun and exited the heliosphere.” On August 25, 2012, Voyager 1 registered a precipitous drop in local cosmic rays (charged particles accelerated to high energies) and an increase in cosmic rays from interstellar space.

The data are intriguing, but the interpretation is problematic. Voyager project scientists have been aware of the August changeover for months and have already offered more mundane explanations for the cosmic-ray data. “Voyager has discovered a new region of the heliosphere that we had not realized was there,” Voyager project scientist Ed Stone of the California Institute of Technology said in December. “We’re still inside, apparently.”

Voyager 1 spacecraft

Credit: NASA

At that time, the researchers noted that the particle data were indeed compelling but that magnetic-field data from Voyager 1 had not yet registered the kind of change expected at the boundary of the heliosphere. “If we had only looked at particle data alone, we would have said, ‘Well, we are out. Goodbye to the solar system,’” said Tom Krimigis of the Johns Hopkins University Applied Physics Laboratory.

The Geophysical Research Letters study does just that—it looked at the particle data alone—so its conclusions should be taken with an interstellar grain of salt. And it is worth noting that the paper is somewhat more cautiously worded than its accompanying press release, claiming only that the boundary Voyager 1 crossed in August is “possibly related to the heliopause,” or the edge of the heliosphere.

In any case, a more comprehensive look at the boundary is forthcoming—studies from three instruments on Voyager 1, including the magnetometer, have been submitted to the journal Science. A Voyager scientist contacted today said that his team had not been informed about the preemptive Geophysical Research Letters paper.

Voyager 1 has ventured farther from Earth in its 35 years than any other man-made object, and when it does leave the heliosphere it will offer humankind our first taste of interstellar space. Given the importance of that accomplishment, I will await official word on Voyager 1’s location—as exhausting and confusing as waiting can be.