Cassini's Big Sky - The View from the Center of Our Solar System

JPL/NASA News

Feature Nov 19, 2009

Cassini's Big Sky - The View from the Center of Our Solar System

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/features.cfm?feature=2370
http://www.kintera.org/TR.asp?a=lkIUK7PPLdKUKlL&s=fpKSISPtHbJEIROzFpF&m=koIOJPNrEbJQG

When NASA's Cassini spacecraft began orbiting Saturn five years ago, a dozen
highly-tuned science instruments set to work surveying, sniffing, analyzing and
scrutinizing the Saturnian system.

But Cassini recently revealed new data that appeared to overturn the decades-old
belief that our solar system resembled a comet in shape as it moves through the
interstellar medium (the matter between stars in our corner of the Milky Way
galaxy).

Instead, the new results suggest our heliosphere more closely resembles a bubble
– or a rat – being eaten by a boa constrictor: as the solar system passes through
the "belly" of the snake, the ribs, which mimic the local interstellar magnetic field,
expand and contract as the rat passes. An animation is available here
http://photojournal.jpl.nasa.gov/catalog/PIA12310.

"At first I was incredulous," said Tom Krimigis, principal investigator of the
Magnetospheric Imaging Instrument (MIMI) at Johns Hopkins University's Applied
Physics Laboratory in Laurel, Md. "The first thing I thought was, 'What's wrong
with our data?'"

Krimigis and his colleagues on the instrument team published the Cassini findings
in the Nov. 13 issue of the journal Science, which featured complementary results
from NASA's Interstellar Boundary Explorer (IBEX). Together, the results create
the first map of the heliosphere and its thick outer layer known as the heliosheath,
where solar wind streaming out from the sun gets heated and slowed as it interacts
with the interstellar medium.

The Cassini data also provide a much more direct indication of the thickness of the
heliosheath, whereas scientists previously had to rely on calculations from models.
The new results from Cassini show that the heliosheath is about 40 to 50
astronomical units (3.7 billion to 4.7 billion miles) thick and that NASA's twin
Voyager spacecraft, which are traveling through the heliosheath now, will cross
into true interstellar space well before the year 2020. Estimates as far out as 2030
had been suggested.

"These new data from Cassini really redefine our sense of our home in the galaxy,
and we can now do better studies of whether our solar system resembles those
elsewhere," Krimigis said.

The Voyagers have sent back rich data on the heliosphere and heliosheath, but just
at two locations. Scientists want more context. One way to learn about the region is
to track energetic neutral atoms streaming back toward the sun from the
heliosheath.

Energetic neutral atoms form when cold, neutral gas collides with electrically-
charged particles in a cloud of plasma, which is a gas-like state of matter so hot that
the atoms split into an ion and an electron. The positively-charged ions in plasma
can't reclaim their own electrons, which are moving too fast, but they can steal an
electron from the cold gas atoms. Since the resulting particles are neutrally
charged, they are able to escape magnetic fields and zoom off into space. The
emission of these particles often occurs in the magnetic fields surrounding planets,
but also happens when the solar wind mingles with the interstellar medium.

How did Cassini, with 22,000 wire connections and 14 kilometers (8.7 miles) of
cabling specifically tweaked to get the most out of its investigation of the solar
system's second largest gas bag, recently end up helping to redefine how we look
at our entire solar system?

Krimigis and his Cassini colleagues working with MIMI weren't sure their
instrument could pick up emissions from far-out, exotic locations, such as from the
boundary of our heliosphere, the region of our sun's influence.

Last year, after spending four years focused on the energetic electrons and ions
trapped in the magnetic field that surrounds Saturn, as well as the offspring of
these particles known as energetic neutral atoms, the team started combing
through the data from the instrument's Ion and Neutral Camera, looking for
particles arriving from far beyond Saturn.

"We thought we could get some hits from energetic neutral atoms from the
heliosheath because Cassini has really been in an excellent position to detect these
particles," said Don Mitchell, MIMI instrument scientist and a researcher at the
Applied Physics Laboratory.

Cassini was farther away from the sun than previous spacecraft trying to image the
heliosphere and even swung very far away from Saturn on some of its orbits,
Mitchell said. The data would likely be free of much of the interference that
hampered other efforts.

Mitchell, Krimigis and their team were able to stitch together data from late 2003 to
the summer of 2009. They created a color-coded map of the intensity of the
energetic neutral atoms and discovered a belt of hot, high-pressure particles where
the interstellar wind flowed by our heliosheath bubble.

The data matched up nicely with the IBEX images of lower-energy particles and
connected that data set to the Voyager data on higher-energy particles.

"I was initially skeptical because the instrument was designed for Saturn's
magnetosphere," Mitchell said, "But our camera had long exposures of months to
years, so we could accumulate and map each particle that streamed through the
tiny aperture from the far reaches of the heliosphere. It was luck, but also a lot of
hard work."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space
Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division
of the California Institute of Technology in Pasadena, Calif. manages the mission for
NASA's Science Mission Directorate, Washington, D.C.

For more information about the Cassini-Huygens mission visit
http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.

-end-

To remove yourself from this mailing, please go to http://www.kintera.org/TR.asp?a=edJGLMMnF9JHIYK&s=fpKSISPtHbJEIROzFpF&m=koIOJPNrEbJQG

To remove yourself from all mailings from NASA Jet Propulsion Laboratory, please go to http://www.kintera.org/TR.asp?a=hqLMIVNzHcIOK8J&s=fpKSISPtHbJEIROzFpF&m=koIOJPNrEbJQG