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Voyager Spacecraft Approaching Solar System's Final Frontier



 
 
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Old November 5th 03, 07:56 PM
Ron Baalke
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Default Voyager Spacecraft Approaching Solar System's Final Frontier

http://www.gsfc.nasa.gov/topstory/2003/1105voyager.html

VOYAGER SPACECRAFT APPROACHING SOLAR SYSTEM'S FINAL FRONTIER
Goddard Space Flight Center
November 5, 2003

NASA's Voyager 1 spacecraft is about to
make history again as 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 (Image 1).

Voyager's journey through this turbulent
zone will give scientists their first direct
measurements of our solar system's
unexplored final frontier, called the
heliosheath, and scientists are debating if this
passage has already begun. Two papers about
this research are being published in Nature on
November 5, 2003. The first paper, by Dr.
Stamatios M. Krimigis of the Johns Hopkins
University Applied Physics Laboratory,
Laurel, Md., and his team, gives evidence
supporting the claim that Voyager 1 passed
beyond the termination shock. The second paper, by Dr. Frank B. McDonald of the
University of Maryland, College Park, and his team, gives evidence against this claim. A
third paper, published October 30, 2003 in Geophysical Research Letters by Dr. Leonard F.
Burlaga of NASA's Goddard Space Flight Center, Greenbelt, Md., and collaborators, gives
evidence that Voyager 1 did not pass beyond the termination shock. (Refer to Image 2a for an
illustration of the termination shock and heliosheath).

"The Voyager 1 observations show we have
entered into a new part of the solar system.
Regardless of whether we crossed the
termination shock or not, the teams are
excited because this has never been seen before
- the observations are different here than in
the inner solar system," said Dr. Eric
Christian, Discipline Scientist for the Sun
Earth Connection research program at NASA
Headquarters, Washington, DC.

"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 Voyager 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.

At more than eight billion miles (13 billion
km) from the Sun, Voyager 1 is the most
distant object built by humanity. Launched on
September 5, 1977, it explored the giant
planets Jupiter and Saturn before being tossed
out toward deep space by Saturn's gravity. It
now approaches, and may have temporarily
entered, the region beyond termination shock.
(Refer to image 2a for a schematic illustrating
the current locations of the Voyager
spacecraft.)

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 300 - 700
km per second (700,000 - 1,500,000 mph).
(Refer to Movie 4 to see how this heats the
solar wind in the heliosheath).

The exact location of the termination shock is
unknown, and it originally was thought to be
closer to the Sun than Voyager 1 currently is.
As Voyager 1 cruised ever farther from the
Sun, it confirmed that all the planets were
inside an immense bubble blown by the solar
wind, and the termination shock was much
more distant (Animation 1).

Estimating the location of the termination
shock is hard because we don't know the
precise conditions in interstellar space, and
even what we do know, the speed and pressure
of the solar wind, changes which causes the
termination shock to expand, contract, and
ripple. You can see a similar effect every time
you wash dishes (Movie 3). If you place a
plate underneath a stream of water, you notice
the water spreads out over the plate in a
relatively smooth flow. The water flow has a
rough edge where the water slows down
abruptly and piles up. The edge is like the
termination shock, and as the water flow changes,
the shape and size of the rough edge change.

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 encountered
before. This led some to claim that Voyager
may have entered a transitory feature of the
termination shock. Just as small bumps and
"fingers" appear and disappear in the rough
edge of the water flow over a plate, Voyager
might have entered a temporary "finger" in the
edge of the termination shock.

The controversy would be resolved easily if
Voyager could still measure the speed of the
solar wind, because the solar wind slows
abruptly at the termination shock. However,
the instrument that measures solar wind speed
no longer functions on the venerable
spacecraft, so scientists must use data from
the instruments that are still working to infer
if Voyager pierced the termination shock.

Evidence for crossing the shock includes
Voyager's observation that high-velocity
electrically-charged particles (electrons and
ions) increased more than 100 times during the
August 1, 2002 to February 5, 2003 period.
This would be expected if Voyager passed the
termination shock, because the shock
naturally accelerates electrically charged
particles that bounce back and forth like ping
pong balls between the fast and slow winds
on the opposite sides of the shock.

Secondly, the particles were flowing
outward, past Voyager and away from the
Sun. This would be expected if Voyager
already crossed beyond the termination shock,
because the acceleration region in the
termination shock would now be behind the
spacecraft. Third, an indirect measure of the
solar wind speed indicated the solar wind was
slow during this period, as would be expected
if Voyager was beyond the shock.

"We have used an indirect technique to show that the solar wind slowed down from about
700,000 mph to much less than 100,000 mph. This same technique was used by us before,
when the instrument measuring the solar wind speed was still working, and the agreement
between the two measurements was better than 20% in most cases," said Krimigis.

Evidence against entry into the shock includes the observation that while there was a
dramatic increase in low-speed particles, they weren't seen at the somewhat higher speeds
scientists believe the termination shock generates.

However, the strongest evidence against entry is Voyager's observation that the magnetic
field did not increase during this period. According to theoretical models, this must happen
whenever the solar wind slows down. Imagine a highway with moderate traffic. If
something makes the drivers slow down, say a puddle of water, the cars pile up - their
density increases. In the same way, the density (intensity) of the magnetic field carried
by the solar wind will increase if the solar wind slows down.

"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," said Burlaga.

The teams agree that Voyager 1 has seen a new phenomenon: a six-month period when
low-energy particles were very abundant and flowing away from the Sun. When the unusual
period ended, both agree that Voyager 1 was back in the solar wind, so if this was a
temporary passage beyond the termination shock, the shock will be seen again, probably in
the next couple years. Finally, the observations indicate that the termination shock is a lot
more complicated than anyone thought.

For their original missions to Jupiter and Saturn, Voyager 1 and sister spacecraft Voyager 2
were destined to regions of space where solar panels would not be feasible, so each was
equipped with three radioisotope thermoelectric generators to produce electrical power for
the spacecraft systems and instruments. Still operating in remote, cold and dark conditions
26 years later, the Voyagers owe their longevity to these Department of Energy-provided
generators, which produce electricity from the heat generated by the natural decay of
plutonium dioxide.

The Voyagers were built by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.,
which continues to operate both spacecraft 26 years after their launch. The spacecraft are
controlled and their data returned through NASA's Deep Space Network (DSN), a global
spacecraft tracking system also operated by JPL. The Voyager Project Manager is Ed Massey
of JPL. The Voyager Project Scientist is Dr. Edward Stone of the California Institute of
Technology.


 




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