San Francisco, CA. - NASA's Voyager 2
spacecraft has followed its twin Voyager 1 into the solar system's
final frontier, a vast region at the edge of our solar system where
the solar wind runs up against the thin gas between the stars.
However, Voyager 2 took a different
path, entering this region, called the heliosheath, on August 30,
2007. Because Voyager 2 crossed the heliosheath boundary, called the
solar wind termination shock, about 10 billion miles away from
Voyager 1 and almost a billion miles closer to the sun, it confirmed
that our solar system is "
squashed" or "
dented"- that the bubble carved into interstellar space by the solar
wind is not perfectly round. Where Voyager 2 made its crossing, the
bubble is pushed in closer to the sun by the local interstellar
magnetic field.
"Voyager
2 continues its journey of discovery, crossing the termination shock
multiple times as it entered the outermost layer of the giant
heliospheric bubble surrounding the Sun and joined Voyager 1 in the
last leg of the race to interstellar space." said Voyager Project
Scientist Dr. Edward Stone of the California Institute of Technology,
Pasadena, Calif.
The solar wind is a thin gas of electrically charged particles
(plasma) blown into space by the sun. The solar wind blows in all
directions, carving a bubble into interstellar space that extends
past the orbit of Pluto. This bubble is called the heliosphere, and
Voyager 1 was the first spacecraft to explore its outer layer, when
it crossed into the heliosheath in December 2004. As Voyager 1 made
this historic passage, it encountered the shock wave that surrounds
our solar system called the solar wind termination shock, where the
solar wind is abruptly slowed by pressure from the gas and magnetic
field in interstellar space.
Even though Voyager 2 is the second spacecraft to cross the shock, it
is scientifically exciting for a couple of reasons. The Voyager 2
spacecraft has a working Plasma Science instrument that can directly
measure the velocity, density and temperature of the solar wind.
This instrument is no longer working on Voyager 1 and estimates of
the solar wind speed had to be made indirectly. Secondly, Voyager 1
may have had only a single shock crossing and it happened during a
data gap. But Voyager 2 had at least five shock crossings over a
couple of days (the shock "
sloshes" back and forth like surf on a beach, allowing multiple
crossings) and three of them are clearly in the data. They show us
an unusual shock.
In a normal shock wave, fast-moving material slows down and forms a
denser, hotter region as it encounters an obstacle. However, Voyager
2 found a much lower temperature beyond the shock than was predicted.
This probably indicates that the energy is being transferred to
cosmic ray particles that were accelerated to high speeds at the
shock.
"The
important new data describing the termination shock are still being
pondered, but it is clear that Voyager has once again surprised us,"
said Dr. Eric Christian, Voyager Program Scientist at NASA
Headquarters, Washington.
The two Voyager spacecraft will be the only source of local
observations of this distant but highly interesting region for years
to come. But in the summer of 2008, NASA will be launching a mission
specifically designed to globally image the termination shock and
heliosheath remotely from Earth orbit. The Interstellar Boundary
Explorer (IBEX), led by Dr. David McComas of the Southwest Research
Institute in San Antonio, Texas, will use energetic neutral atoms
(ENAs) to create all-sky maps at various energies of the interaction
of the heliosphere with interstellar space. ENAs are formed when
energetic electrically-charged particles "
steal" an electron from another particle. Once neutral, they travel
straight, unaffected by the solar magnetic field. IBEX will detect
some of the particles that happen to be headed towards the Earth, and
the number and energy of the particles coming from all different
directions will tell us much more about the overall structure of the
interaction between the heliosphere and interstellar space.
Results on the Voyager 2 shock crossing from the entire Voyager
science team are being presented at the Fall 2007 meeting of the
American Geophysical Union in San Francisco. The Voyagers were built
by NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., which
continues to operate both spacecraft.
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