JOHNS HOPKINS U. (US) — By the end of next year, NASA’s Voyager 1 spacecraft may after 34 years finally go where no man-made object has gone before—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 the solar wind may have been stopped by pressure from the interstellar magnetic field in the region between stars.
Since December 2004, when it entered the heliosheath, Voyager 1 has been monitoring the solar wind speed there. The wind’s 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—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,” says Stamatios Krimigis of the Applied Physics Laboratory at Johns Hopkins University.
“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.
Published in the journal Nature, the location of the heliopause is estimated by combining the Voyager 1 observations and energetic neutral atom images of the heliosheath from NASA’s Cassini probe near Saturn. Researchers concluded that the heliopause may be as close as 11.3 billion miles from the sun, meaning that Voyager 1 could exit the transition layer and enter the galactic medium by the end of 2012.
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.
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.
When the speed of the charged particles hitting the outward face of Voyager 1 matched the spacecraft’s own outward speed in April 2010, researchers knew the net outward speed of the solar wind was zero. The spacecraft at the time was about 10.6 billion miles from the sun.
Because the velocities can fluctuate, scientists watched several 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 very nearly zero (or sometimes even slightly inward) for the past year.
A sister spacecraft, Voyager 2, was launched in Aug. 20, 1977, and is 8.8 billion miles from the sun. The two spacecraft have been traveling along different trajectories and at different speeds.
Voyager 1 is traveling faster, at a speed of about 38,000 mph, compared to Voyager 2’s velocity of 35,000 mph. In the next few years, however, Voyager 2 may encounter the same kind of extended transition layer as Voyager 1 found.
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