It was welcome news to Earthlings: The Voyager 1 spacecraft had seemingly crossed a momentous threshold and become the first man-made object to enter interstellar space.
"Voyager 1 has left the solar system," the American Geophysical Union declared Wednesday in a news release. An accompanying study published online in its journal, Geophysical Research Letters, also contained an unusually sentimental end note declaring that "we did it. Bon voyage!"
Alas, the elation that spread through news and social media was short-lived. Voyager 1 was still in the neighborhood, NASA said, even after traveling for more than 35 years. Then the American Geophysical Union press office issued a correction of its headline, omitting any reference to the spacecraft having departed "the solar system."
"It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space," responded Edward Stone, Voyager project scientist at the California Institute of Technology and former chief of the Jet Propulsion Laboratory in La Cañada Flintridge, where Voyager was built.
Though there is little doubt the lonely probe will one day exit the solar system, scientists are discovering that the border is not as clearly defined as expected.
In the paper released Wednesday, lead author Bill Webber suggested the probe had exited the heliosphere - that region dominated by solar winds and long considered to be the edge of the solar system - last Aug. 25.
It was on that day that Voyager's sensors registered drastic changes in radiation levels. There was a sharp drop in so-called anomalous cosmic rays - high-energy particles trapped within the "bubble" of the outer heliosphere - and a sudden surge in galactic cosmic rays from outside the solar system.
Those events seemed to indicate that Voyager had "crossed a well-defined boundary" and possibly entered interstellar space 11.3 billion miles from the sun, wrote Webber, astronomy professor emeritus at New Mexico State University who is involved with cosmic ray experiments on Voyager.
But scientists at NASA and elsewhere said Webber's report did not address one of the most unexpected elements of the boundary between the solar system and interstellar space - a mysterious region that Stone and others call a "magnetic highway."
In December, the Voyager science team reported that the spacecraft had reached a place where particles from the solar wind dropped off dramatically and cosmic rays from interstellar space increased. But they did not detect an anticipated change in the direction of the magnetic field emanating from the sun.
"If we had looked at particle data alone, we would have said, 'We're out! Goodbye, solar system!' " Stamatios Krimigis, a solar physicist at the Johns Hopkins Applied Physics Laboratory, said at the time.
Instead, the scientists surmised that Voyager had reached the magnetic highway, where interstellar particles can ride in and solar system particles can ride out. Only when the craft senses the magnetic field has changed direction will they declare that it has reached interstellar space. And that has not yet been observed, Stone said Wednesday.