旅行者1号已离开太阳系

Thirty-five years after its launch, Voyager 1 appears to have travelled beyond the influence of the Sun and exited the heliosphere（太阳风层）, according to a new study appearing online today. The heliosphere is a region of space dominated by the Sun and its wind of energetic particles, and which is thought to be enclosed, bubble-like, in the surrounding interstellar（星际的） medium of gas and dust that pervades¹（遍及，弥漫） the Milky² Way galaxy³.   On August 25, 2012, NASA's Voyager 1 spacecraft measured drastic（激烈的） changes in radiation levels, more than 11 billion miles from the Sun. Anomalous⁴ cosmic rays, which are cosmic rays trapped in the outer heliosphere, all but vanished, dropping to less than 1 percent of previous amounts. At the same time, galactic cosmic rays -- cosmic radiation from outside of the solar system -- spiked⁵ to levels not seen since Voyager's launch, with intensities⁶ as much as twice previous levels.   The findings have been accepted for publication in Geophysical Research Letters, a journal of the American Geophysical Union.   "Within just a few days, the heliospheric intensity⁷ of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere," said Bill Webber, professor emeritus⁸（退休的） of astronomy at New Mexico State University in Las Cruces. He calls this transition boundary the "heliocliff."   In the GRL article, the authors state: "It appears that [Voyager 1] has exited the main solar modulation⁹ region, revealing [hydrogen] and [helium] spectra¹⁰ characteristic of those to be expected in the local interstellar medium."   However, Webber notes, scientists are continuing to debate whether Voyager 1 has reached interstellar space or entered a separate, undefined region beyond the solar system.   "It's outside the normal heliosphere, I would say that," Webber said. "We're in a new region. And everything we're measuring is different and exciting."   The work was funded by NASA's Jet Propulsion Laboratory in Pasadena, Calif.