NASA's long-running Voyager 1 spacecraft is nearing the edge of our solar system after 33 years. Now hurtling toward interstellar space some 10.8 billion miles from the Sun, Voyager 1 has crossed into an area where the velocity of the hot ionized gas, or plasma, emanating directly outward from the Sun has slowed to zero.
Scientists have suspected the solar wind has been turned sideways by the pressure from the interstellar wind in the region between stars.
The event is a major milestone in Voyager 1's passage through the heliosheath, the turbulent outer shell of the Sun's sphere of influence, and the spacecraft''s upcoming departure from our solar system.
"The solar wind has turned the corner," said Ed Stone, Voyager project scientist based at the California Institute of Technology in Pasadena.
Launched on Sept. 5, 1977, Voyager 1 crossed the termination shock in December 2004 into the heliosheath. Scientists have used data from Voyager 1's Low-Energy Charged Particle Instrument to deduce the solar wind''s velocity.
When the speed of the charged particles hitting the outward face of Voyager 1 matched the spacecraft''s speed, researchers knew that the net outward speed of the solar wind was zero. This occurred in June, when Voyager 1 was about 10.6 billion miles from the Sun.
Because the velocities can fluctuate, scientists watched four more monthly readings before they were convinced the solar wind''s outward speed actually had slowed to zero. Analysis of the data shows the velocity of the solar wind has steadily slowed at a rate of about 45,000 mph each year since August 2007, when the solar wind was speeding outward at about 130,000 mph. The outward speed has remained at zero since June.
"When I realized that we were getting solid zeroes, I was amazed. Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again," said Rob Decker, a Voyager Low-Energy Charged Particle Instrument co-investigator.
Scientists believe Voyager 1 has not crossed the heliosheath into interstellar space. Crossing into interstellar space would mean a sudden drop in the density of hot particles and an increase in the density of cold particles. Scientists are putting the data into their models of the heliosphere''s structure and should be able to better estimate when Voyager 1 will reach interstellar space.
The results were presented at the American Geophysical Union meeting in San Francisco.