NASA's Voyager 1 spacecraft has left the solar system — almost

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 the organization's 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."

"The Voyager team is aware of reports today that NASA's Voyager 1 has left the solar system," said Edward Stone, Voyager project scientist at Caltech and former chief of the Jet Propulsion Laboratory in La Cañada Flintridge, where Voyager was built. "It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space."

Though there is little doubt that the lonely probe will one day exit the solar system, scientists are discovering that the border is not as clearly defined as they expected it to be.

In the paper released Wednesday, lead author Bill Webber suggested that the probe had exited the heliosphere — that region of space dominated by solar winds and long considered to be the edge of the solar system — on Aug. 25, 2012.

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.

Together, 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, according to the paper.

"It appears that V1 has exited the main solar modulation region, revealing hydrogen and helium spectra characteristic of those to be expected in the local interstellar medium," wrote Webber, a professor emeritus of astronomy at New Mexico State University who is involved with cosmic ray experiments on Voyager.

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-- Monte Morin and Amina Khan, Los Angeles Times

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