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 Stamatios
Krimigis of the Johns Hopkins
Applied Physics
Laboratory 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 also is
considered the last stop before the invisible boundary of
the heliosphere, the bubblelike region of space under our
sun's energetic influence.
"Voyager 1 is giving us our first taste of
interstellar space," said Krimigis, principal investigator
for the 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 man-made 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; each team presents its
views in the Nov. 6 issue of the journal Nature.
One instrument team maintains that Voyager approached
but didn't cross the termination shock. Krimigis said 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 hundredfold 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 said, "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. Edmond Roelof, an LECP science team
co-investigator at APL who developed analysis tools for
just this type of data, said, "The solar wind had slowed
from 700,000 miles per hour to less than 100,000 miles per
hour."
"Flying a moving device on Voyager — in this
case an electric motor — was considered a risk," said
Robert Decker, an LECP science team co-investigator and the
instrument project manager at APL. "But that rotating
capability was key to collecting this data and [to] 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," said 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, the 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 said. "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 LECP 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;
George Gloecker, Douglas Hamilton and Hill, of the
University of Maryland, College Park; Thomas Armstrong, of
the University of Kansas, Lawrence; and Louis Lanzerotti,
of Bell Laboratories and New Jersey Institute of
Technology.
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