By the time 2012 comes to a close, NASA’s Voyager 1 spacecraft may finally reach humanity’s last frontier: interstellar space. Cruising through space some 10.8 billion miles from the sun, Voyager 1 has crossed into an area where the velocity of the solar wind – the flow of hot ionized gas emanating directly from the sun – has slowed from 150,000 miles an hour down to near zero. The spacecraft has entered a transitional “layer” within the heliosheath, one between the solar wind and the interstellar medium. Data shows that the solar wind may have been stopped by pressure from the interstellar magnetic field in the region between stars. "There is one time we are going to cross that frontier, and this is the first sign it is upon us," said Stamatios “Tom” Krimigis, principal investigator for Voyager’s low-energy charged particle instrument, based at the Johns Hopkins University Applied Physics Laboratory. An artist's concept shows NASA's two Voyager spacecraft exploring a turbulent region of space known as the heliosheath, the outer shell of the bubble of charged particles around our sun. Voyager 1 now is expected to reach interstellar space by the end of next year. It will be the first manmade object to leave the solar system. (Image: NASA/JPL-Caltech) Analysis of data from the Voyager 1 and Cassini spacecraft indicates that the boundary between interstellar space and the bubble of charged particles the sun blows around itself is likely between 16 billion and 23 billion kilometers from the sun, with a best estimate of approximately 18 billion kilometers. Since Voyager 1 is already nearly 18 billion kilometers out, it could cross into interstellar space at any time. Launched on Sept. 5, 1977, Voyager 1 crossed the termination shock in December 2004 and moved into the heliosheath. Scientists have used directional flow data from the probe’s Low-Energy Charged Particle instrument to deduce the solar wind’s velocity. The velocity outward from the sun has been decreasing steadily over the past three years, from more than 40 miles per second to zero. The transverse component – wind flowing sideways relative to the sun – is also trending toward zero. “This tells us that Voyager 1 may be close to the heliopause, or the boundary at which the interstellar medium basically stops the outflow of solar wind,” Krimigis said. “The extended transition layer of near-zero outflow contradicts theories that predict a sharp transition to the interstellar flow at the heliopause – and means, once again, we will need to rework our models.” When it crosses into interstellar space, Voyager will experience a sudden drop in the density of hot particles it has found in the heliosheath and an increase in the density of cold particles from the interstellar plasma. Solar wind travels at supersonic speed until it crosses a shockwave called the termination shock, at which point it dramatically slows down and heats up in the heliosheath. The recent data were published in the journal Nature. For more information, visit: www.nasa.gov/voyager