NASA’s Voyager 1 spacecraft is about to make history
again. It is the first spacecraft to enter the solar system’s
final frontier, a vast expanse where wind from the sun blows
hot against thin gas between the stars: interstellar space.
However, before it reaches this region, Voyager 1 must pass
through the termination shock, a violent zone that is the
source of beams of high-energy particles. Voyager’s journey
through this turbulent zone will give scientists the first
direct measurements of our solar system’s unexplored final
frontier, the heliosheath. Scientists are debating if this
passage has already begun. Two papers about this research are
being published in Nature today.
shock, Nature (subscription)
The first paper, by Dr. Stamatios Krimigis of the Johns
Hopkins University Applied Physics Laboratory, Laurel, Md.,
and his team, supports the claim Voyager 1 passed beyond the
termination shock. The second paper, by Dr. Frank McDonald of
the University of Maryland, College Park, Md., and his team,
disputes the claim. A third paper, published October 30 in
Geophysical Research Letters by GSFC’s Dr. Leonard Burlaga,
and collaborators, states Voyager 1 did not pass beyond the
termination shock.
“Voyager 1 has seen striking signs of the region deep in
space where a giant shock wave forms, as the wind from the
sun abruptly slows and presses outward against the
interstellar wind. The observations surprised and puzzled us,
so there is much to be discovered as it begins exploring this
new region at the outer edge of the solar system,” said Dr.
Edward Stone, Voyager Project Scientist, California Institute
of Technology, Pasadena, Calif.
Launched on September 5, 1977, Voyager explored the giant
planets Jupiter and Saturn before being tossed out toward
deep space by Saturn’s gravity. It is approaching, and may
have temporarily entered, the region beyond termination
shock. At more than eight billion miles from the sun, Voyager
1 is the most distant object from Earth built by humanity.
The termination shock is where the solar wind, a thin stream
of electrically charged gas blown constantly from the sun, is
slowed by pressure from gas between the stars. At the
termination shock, the solar wind slows abruptly from its
average speed of 700,000 to 1,500,000 mph.
Estimating the location of the termination shock is hard,
because we don’t know the precise conditions in interstellar
space. We do know speed and pressure of the solar wind
changes, which causes the termination shock to expand,
contract and ripple.
> From about August 1, 2002, to February 5, 2003, scientists
noticed unusual readings from the two energetic-particle
instruments on Voyager 1, indicating it had entered a region
of the solar system unlike any previously encountered. This
led some to claim Voyager 1 may have entered a transitory
feature of the termination shock.
The controversy would be resolved if Voyager could measure
the speed of the solar wind, because the solar wind slows
abruptly at the termination shock. However, the instrument
that measured solar wind speed no longer functions on the
venerable spacecraft. Scientists must use data from
instruments still working to infer if Voyager pierced the
termination shock.
“We have used an indirect technique to show the solar wind
slowed down from about 700,000 mph to much less than 100,000
mph. We used this same technique when the instrument
measuring the solar wind speed was still working. The
agreement between the two measurements was better than 20
percent in most cases,” Krimigis said.
“The analysis of the Voyager 1 magnetic field observations in
late 2002 indicate that it did not enter a new region of the
distant heliosphere by having crossed the termination shock.
Rather, the magnetic field data had the characteristics to be
expected based upon many years of previous observations,
although the intensity of energetic particles observed is
unusually high,” Burlaga said.
The Voyager spacecraft were built and are operated by JPL.
The Voyagers were equipped with three radioisotope
thermoelectric generators to produce electrical power for the
spacecrafts’ systems and instruments. Steadily operating for
26 years, the Voyagers owe their longevity to these
generators, which produce electricity from the heat generated
by the natural decay of plutonium dioxide. For images,
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