Voyager 1 Approaches Solar System's Outer Limits

http://www.jhuapl.edu/newscenter/pre...003/031105.htm

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November 5, 2003

FOR IMMEDIATE RELEASE

VOYAGER 1 APPROACHES SOLAR SYSTEM'S OUTER LIMITS

NASA Spacecraft Offers First Direct Look at Dynamic Region Before
Interstellar Space

More than 25 years after leaving home, NASA's Voyager 1 spacecraft reached
a key checkpoint on its historic journey toward interstellar space.

Analyzing six months of data from Voyager's Low-Energy Charged Particle
instrument, a team led by Dr. Stamatios Krimigis of the Johns Hopkins
University Applied Physics Laboratory (APL), Laurel, Md., determined that
the spacecraft, while nearly 8 billion miles from Earth, passed through and
later returned behind the turbulent zone known as the solar termination shock.

At the termination shock, streams of electrically charged gas blown from
the Sun -- called the solar wind -- slow down rapidly after colliding with
gas and magnetic pressure from between the stars. The shock is also
considered the last stop before the invisible boundary of the heliosphere,
the bubble-like region of space under our Sun's energetic influence.

"Voyager 1 is giving us our first taste of interstellar space," says
Krimigis, principal investigator for the Low-Energy Charged Particle (LECP)
instrument, which was designed and built at APL. "This is our first direct
look at the incredibly dynamic activity in the solar system's outer limits."

Voyager 1 is the farthest manmade object in space, and from about Aug. 1,
2002, to Feb. 5, 2003, scientists noticed unusual readings from several
instruments on the spacecraft indicating it had entered part of the solar
system unlike any encountered before. Science team members' views vary on
what the data means; one instrument team maintains that Voyager approached,
but didn't cross, the termination shock. (Each team presents its views in
the Nov. 6 issue of the journal Nature.)

Krimigis says his team, however, found compelling evidence of a shock
crossing in data from the LECP. The instrument, mounted on a motorized,
rotating platform that allows it to scan the sky in all directions,
determines the composition, charge and direction of certain energized
particles as they zip through space.

First, the team noticed a hundred-fold increase in the intensity of these
charged particles, and that they were streaming by the spacecraft mostly
along the magnetic field perpendicular to Voyager's path. "This was
remarkable," Krimigis says, "because for 25 years, particles from the Sun
were flowing straight out. We knew something strange must have happened to
the solar wind that helps push these particles out."

At a termination shock, the solar wind would brake abruptly from supersonic
to subsonic speed. The instrument on Voyager 1 that could measure solar
wind speed no longer operates; however, the LECP detector can measure it
indirectly from the speed and direction of the ions riding with the solar
wind. "The solar wind had slowed from 700,000 miles per hour to less than
100,000 miles per hour," says Dr. Edmond Roelof, an LECP science team
co-investigator at APL who developed analysis tools for just this type of
data.

"Flying a moving device on Voyager -- in this case an electric motor -- was
considered a risk," says Dr. Robert Decker, an LECP science team
co-investigator and the instrument project manager at the Applied Physics
Laboratory. "But that rotating capability was key to collecting this data,
and helping us figure out that the solar wind had virtually stopped."

The team also found a third crucial clue: by measuring the composition of
particles in the area, the instrument detected signatures of interstellar
materials -- the atoms and other particles from explosions of dying stars.
"That tells us materials originally from outside the solar system are
becoming accelerated near the spacecraft -- again, something you expect to
happen at the termination shock," says Dr. Matthew Hill, a science team
member from the University of Maryland, College Park.

Estimating the shock's exact location has been hard since no one knows the
precise conditions of interstellar space, though scientists do believe the
constantly changing speed and pressure of the solar wind causes the shock's
boundary to expand and contract. In this case, LECP readings indicate
Voyager 1 crossed the shock at about 85 times the Earth-Sun distance,
before the shock moved past the spacecraft at 87 times this distance.

Such movement also makes it difficult to predict when the spacecraft will
again encounter that boundary. Until then, LECP team is correlating its
results with those from other instrument teams, hoping to get a clearer
picture of the interplay between the solar wind and interstellar medium,
and matching that information to long-held models of the outer solar
system. Already, there are some differences.

"We saw the right mix of interstellar materials where we thought we would,
but overall, things didn't behave the way we expected from models,"
Krimigis says. "It was strange, but just another indication that nature
behaves the way it wants, not according to what our theories predict."

Voyager 1 launched on Sept. 5, 1977, and flew past Jupiter and Saturn
before heading northward out of the planets' orbital plane. Voyager 2,
which launched on Aug. 20, 1977, and explored Jupiter, Saturn, Uranus and
Neptune, is also moving out but in a southward direction and hasn't
traveled as far. An APL-built Low-Energy Charged Particle detector flies on
each; the Laboratory later developed similar instruments for the Galileo
spacecraft, which recently ended its mission at Jupiter, and the Cassini
spacecraft, which will begin orbiting Saturn in July 2004.

LECP team members presenting their results in the Nature article are
Krimigis, Decker and Roelof of APL; Dr. George Gloecker, Dr. Douglas
Hamilton and Hill of the University of Maryland, College Park; Dr. Thomas
Armstrong of the University of Kansas, Lawrence; and Dr. Louis Lanzerotti,
Bell Laboratories, Murray Hill, N.J. and New Jersey Institute of
Technology, Newark. For more information on the articles, visit
www.nature.com/nature.

On the Web:

NASA news release and images:
http://www.gsfc.nasa.gov/topstory/2003/1105voyager.html

Animation of the Low-Energy Charged Particle instrument:
http://www.jhuapl.edu/newscenter/pre...003/031105.htm

Bell Laboratories news release and contacts:
http://www.bell-labs.com

New Jersey Institute of Technology news release and contacts:
http://www.njit.edu/publicinfo/newsroom/index.php

The Voyager Mission:
http://voyager.jpl.nasa.gov/

Low-Energy Charged Particle instrument and science mission:
http://hurlbut.jhuapl.edu/VOYAGER/

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