NASA Probe Sees Solar Wind Decline

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov

Jia-Rui Cook 818-359-3241
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov

Dwayne Brown 202-358-1726
Headquarters, Washington
dwayne.c.brown@nasa.gov

News release: 2010-415 Dec. 13, 2010

NASA Probe Sees Solar Wind Decline

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-415&cid=release_2010-415

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 17.4 billion kilometers (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 17 billion kilometers (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 20 kilometers per second each year (45,000 mph each year) since August 2007,
when the solar wind was speeding outward at about 60 kilometers per second (130,000
mph). The outward speed has remained at zero since June.

The results were presented today 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
14.2 billion kilometers (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 17 kilometers per second (38,000 mph), compared to Voyager 2's
velocity of 15 kilometers per second (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: http://www.nasa.gov/voyager . JPL is a division of the California Institute of
Technology in Pasadena.

-end-

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