NASA Probe Sees Solar Wind Decline En Route To Interstellar Space

PASADENA, Calif. -- The 33-year odyssey of NASA's Voyager 1 spacecraft has reached a distant point at the edge of our solar system where there is no outward motion of solar wind.

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 suspect 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, Calif. "Voyager 1 is getting close to interstellar space."

Our sun gives off a stream of charged particles that form a bubble
known as the heliosphere around our solar system. The solar wind
travels at supersonic speed until it crosses a shockwave called the
termination shock. At this point, the solar wind dramatically slows
down and heats up in the heliosheath.

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.

The results were presented at the American Geophysical Union meeting
in San Francisco.

"When I realized that we were getting solid zeroes, I was amazed,"
said Rob Decker, a Voyager Low-Energy Charged Particle Instrument
co-investigator and senior staff scientist at the Johns Hopkins
University Applied Physics Laboratory in Laurel, Md. "Here was
Voyager, a spacecraft that has been a workhorse for 33 years, showing
us something completely new again."

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. Researchers
currently estimate Voyager 1 will cross that frontier in about four years.

"In science, there is nothing like a reality check to shake things up,
and Voyager 1 provided that with hard facts," said Tom Krimigis,
principal investigator on the Low-Energy Charged Particle Instrument,
who is based at the Applied Physics Laboratory and the Academy of
Athens, Greece. "Once again, we face the predicament of redoing our models."

A sister spacecraft, Voyager 2, was launched in Aug. 20, 1977 and has
reached a position 8.8 billion miles from the sun. Both 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, scientists expect Voyager 2 to encounter the same kind of
phenomenon as Voyager 1.

The Voyagers were built by NASA's Jet Propulsion Laboratory in
Pasadena, Calif., which continues to operate both spacecraft. For
more information about the Voyager spacecraft, visit:


Source: NASA

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