NASA's venerable Voyager I spacecraft has become the first man-made object to leave the solar system, astronomers report.
Scientists have been anticipating this moment for years but the boundary between the edge of the solar system and what's called interstellar space has been something of a moving target. Astronomers say it's the point where the stream of electrically changed particles from our sun, called the solar wind, peter out and in turn the cosmic rays from distant exploding stars coming from other directions become the dominant form of radiation.
The distances are somewhat mind-boggling. Pluto is about 3.7 billion miles from the sun. Voyager is now about 17 billion miles away, where any signal it sends takes 17 hours to reach Earth. For several years, the spacecraft, launched in 1977, has been crossing through the edge of the "heliosphere," the bubble of charged solar particles created by the solar wind that shields our solar system from these distant galactic rays.
The size of the heliosphere expands and contracts depending on the strength of the solar wind. On Aug. 25, 2012, astronomers report in the Geophysical Research Letters journal, galactic cosmic ray intensity suddenly doubled as measured by the spacecraft, indicating it had traveled beyond the protection of the solar wind. This boundary marks the edge of the sun's heliosphere.
"The cosmic ray intensity went up as you would expect if it exited the heliosphere," said astronomer Bill Webber of New Mexico State University in Las Cruces, in a statement. The observation had been presented in December a scientific meeting, but hadn't been accepted for publication into a scientific journal, until now.
Dan Vergano, USA TODAY