Probably the most-loved survivor of 1970s space optimism, Voyager, has sent back signals indicating that it's left the heliosphere.
Scientists are now discussing whether they should consider the 35-year-old probe to be in interstellar space, or to have entered a new region of space that hasn't been previously described.
The signals analysed by scientists, to be published by the American Geophysical Union, showed a precipitous fall in solar radiation detected by Voyager in August last year, when it was more than 11 billion miles – nearly 18 billion kilometres – from the Sun.
“Anomalous 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 – spiked to levels not seen since Voyager's launch, with intensities as much as twice previous levels,” the AGU announcement states.
The transition from radiation trapped in the heliosphere (as the AGU says, rather like a bubble) to galactic radiation was so sudden, New Mexico State University emeritus professor of astronomy Bill Webber calls the transition boundary the “heliocliff”.
Whether Voyager is truly in interstellar space or in a new, undefined region, Webber says, “everything we're measuring is different and exciting.”
The research paper describing Voyager's exit from the heliopause is here. ®