Voyager Makes an Interstellar Discovery

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http://science.nasa.gov/headlines/y2009/23dec_voyager.htm

Voyager Makes an Interstellar Discovery
NASA Science News
12.23.2009

December 23, 2009 - The solar system is passing through an interstellar
cloud that physics says should not exist. In the Dec. 24th issue of
Nature, a team of scientists reveal how NASA's Voyager spacecraft have
solved the mystery.

"Using data from Voyager, we have discovered a strong
magnetic field just outside the solar system," explains lead author
Merav Opher, a NASA Heliophysics Guest Investigator from George Mason
University. "This magnetic field holds the interstellar cloud together
and solves the long-standing puzzle of how it can exist at all."

The discovery has implications for the future when the solar system will
eventually bump into other, similar clouds in our arm of the Milky Way
galaxy.

Astronomers call the cloud we're running into now the Local Interstellar
Cloud or "Local Fluff" for short. It's about 30 light years wide and
contains a wispy mixture of hydrogen and helium atoms at a temperature
of 6000 C. The existential mystery of the Fluff has to do with its
surroundings. About 10 million years ago, a cluster of supernovas
exploded nearby, creating a giant bubble of million-degree gas. The
Fluff is completely surrounded by this high-pressure supernova exhaust
and should be crushed or dispersed by it.

"The observed temperature and density of the local cloud do not provide
enough pressure to resist the 'crushing action' of the hot gas around
it," says Opher.

So how does the Fluff survive? The Voyagers have found an answer.

"Voyager data show that the Fluff is much more strongly magnetized than
anyone had previously suspected - between 4 and 5 microgauss," says
Opher. "This magnetic field can provide the extra pressure required to
resist destruction."

NASA's two Voyager probes have been racing out of the solar system for
more than 30 years. They are now beyond the orbit of Pluto and on the
verge of entering interstellar space - but they are not there yet.

"The Voyagers are not actually inside the Local Fluff," says Opher. "But
they are getting close and can sense what the cloud is like as they
approach it."

The Fluff is held at bay just beyond the edge of the solar system by the
sun's magnetic field, which is inflated by solar wind into a magnetic
bubble more than 10 billion km wide. Called the "heliosphere," this
bubble acts as a shield that helps protect the inner solar system from
galactic cosmic rays and interstellar clouds. The two Voyagers are
located in the outermost layer of the heliosphere, or "heliosheath,"
where the solar wind is slowed by the pressure of interstellar gas.

Voyager 1 entered the heliosheath in Dec. 2004; Voyager 2 followed
almost 3 years later in Aug. 2007. These crossings were key to Opher et
al's discovery.

The size of the heliosphere is determined by a balance of forces: Solar
wind inflates the bubble from the inside while the Local Fluff
compresses it from the outside. Voyager's crossings into the heliosheath
revealed the approximate size of the heliosphere and, thus, how much
pressure the Local Fluff exerts. A portion of that pressure is magnetic
and corresponds to the ~5 microgauss Opher's team has reported in Nature.

The fact that the Fluff is strongly magnetized means that other clouds
in the galactic neighborhood could be, too. Eventually, the solar system
will run into some of them, and their strong magnetic fields could
compress the heliosphere even more than it is compressed now. Additional
compression could allow more cosmic rays to reach the inner solar
system, possibly affecting terrestrial climate and the ability of
astronauts to travel safely through space. On the other hand, astronauts
wouldn't have to travel so far because interstellar space would be
closer than ever. These events would play out on time scales of tens to
hundreds of thousands of years, which is how long it takes for the solar
system to move from one cloud to the next.

"There could be interesting times ahead!" says Opher.

To read the original research, look in the Dec. 24, 2009, issue of
Nature for Opher et al's article, "A strong, highly-tilted interstellar
magnetic field near the Solar System."

Author: Dr. Tony Phillips <mailto:james.a....@earthlink.net> |
Credit: Science@NASA <http://science.nasa.gov>