Last August, Voyager 1 saw a sudden drop in solar radiation levels as it coasted along 11 billion miles away from the sun. At the same time, cosmic rays originating from outside the solar system spiked.
In a new study forthcoming in the journal Geophysical Research Letters, New Mexico State University astronomer Bill Webber and a University of Maryland scientist, the late Frank B. McDonald, say their findings indicate that Voyager has probably exited the heliosphere, a bubble of charged particles released from the sun, borne with intense velocity out into space. The end of the heliosphere is generally considered the boundary line for our solar system.
"Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere," Webber said in a statement.
The hydrogen and helium readings Voyager has received of late reflect what might be expected in interstellar space, according to Webber and McDonald.
But NASA begs to differ.
“The Voyager team is aware of reports today that NASA’s Voyager 1 has left the solar system,” Voyager scientist and Caltech researcher Edward Stone said in a statement Wednesday. But “it is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space. ... A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space, and that change of direction has not yet been observed.”
The crossing is expected in the near future, however; in December, NASA said Voyager 1 was likely nearing the limits of our solar system, based on the same observations Webber and McDonald cite in their paper. At the time, Voyager 1 was likely within a region dubbed "the magnetic highway," according to NASA.
"If we were judging by the charged particle data alone, I would have thought we were outside the heliosphere," Stamatios Krimigis, Johns Hopkins University researcher and Voyager scientist, said in a statement in December. "But we need to look at what all the instruments are telling us, and only time will tell whether our interpretations about this frontier are correct."
Whether Voyager has actually left the heliosphere or not, there’s also still some debate among scientists about just where the solar system ends. Voyager may have already exited the solar system, or it’s crossing through some new outer limit of it.
This uncertainty was reflected in the press release from the American Geophysical Union announcing the paper. The original version, issued Wednesday morning, bore the headline "Voyager 1 Has Left The Solar System, Sudden Changes In Cosmic Rays Indicate." A reissued press release in the afternoon was topped with the headline “Voyager 1 Has Entered A New Region Of Space, Sudden Changes In Cosmic Rays Indicate.”
"It's outside the normal heliosphere, I would say that," Webber said Wednesday. "We're in a new region. And everything we're measuring is different and exciting."
But we will only be able to keep tabs on Voyager’s journey for a few more years. Electrical limitations mean that the craft’s days as a working space probe are numbered. By 2015, Voyager’s Data Tape Recorder will shut down; in approximately 2016, the craft’s gyroscopes will turn off; in 2020, all science instruments, including Voyager’s magnetometer and cosmic ray detector, will shut down. Somewhere between 2025 and 2030, Voyager will go completely dark.
Even if it flies silent, Voyager will likely push towards even further frontiers for centuries – and millennia – to come. And who knows? Perhaps someday alien explorers will come across Voyager and its Golden Record, which contains music, sounds and images from Earth. To those spacefarers, it might be a message in a bottle sent a long, long time ago from a galaxy far, far away.
SOURCE: Webber et al. “Recent Voyager 1 data indicate that on August 25, 2012 at a distance of 121.7 AU from the Sun, sudden and unprecedented intensity changes were observed in anomalous and galactic cosmic rays.” Geophysical Research Letters published online 20 March 2013.