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.)

  • 5 November 2003: Voyager 1 exited the solar wind at a distance of ~85 AU from the Sun, Nature (subscription)

  • 5 November 2003: Enhancements of energetic particles near the heliospheric termination
    shock    , Nature (subscription)

    • 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.

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