Cassini Data Help Redraw Shape of Solar System

Feature Oct. 15, 2009

Cassini Data Help Redraw Shape of Solar System

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/features.cfm?feature=2337

Images from the Ion and Neutral Camera on NASA's Cassini spacecraft suggest that the
heliosphere, the region of the sun's influence, may not have the comet-like shape predicted by
existing models. In a paper published Oct. 15 in Science Express, researchers from the Johns
Hopkins Applied Physics Laboratory present a new view of the heliosphere, and the forces that
shape it.

"These images have revolutionized what we thought we knew for the past 50 years; the sun
travels through the galaxy not like a comet but more like a big, round bubble," said Stamatios
Krimigis of the Applied Physics Lab, in Laurel, Md., principal investigator for Cassini's
Magnetospheric Imaging Instrument which carries the Ion and Neutral Camera. "It's amazing
how a single new observation can change an entire concept that most scientists had taken as true
for nearly fifty years."

As the solar wind flows from the sun, it carves out a bubble in the interstellar medium. Models of
the boundary region between the heliosphere and interstellar medium have been based on the
assumption that the relative flow of the interstellar medium and its collision with the solar wind
dominate the interaction. This would create a foreshortened "nose" in the direction of the solar
system's motion, and an elongated "tail" in the opposite direction.

The Ion and Neutral Camera images suggest that the solar wind's interaction with the interstellar
medium is instead more significantly controlled by particle pressure and magnetic field energy
density.

"The map we've created from the images suggests that pressure from a hot population of charged
particles and interaction with the interstellar medium's magnetic field strongly influence the
shape of the heliosphere," says Don Mitchell, Magnetospheric Imaging Instrument/Ion and
Neutral Camera co-investigator at the Applied Physics Lab.

Since entering into orbit around Saturn in July of 2004, the Ion and Neutral Camera has been
mapping energetic neutral atoms near the planet, as well as their dispersal across the entire sky.
The energetic neutral atoms are produced by energetic protons, which are responsible for the
outward pressure of the heliosphere beyond the interface where the solar wind collides with the
interstellar medium, and which interact with the magnetic field of the interstellar medium.

"Energetic neutral atom imaging has demonstrated its power to reveal the distribution of
energetic ions, first in Earth's own magnetosphere, next in the giant magnetosphere of Saturn and
now throughout vast structures in space—out to the very edge of our sun's interaction with the
interstellar medium," says Edmond C. Roelof, Magnetospheric Imaging Instrument co-
investigator at the Applied Physics Lab.

The results from Cassini complement and extend findings from NASA's Interstellar Boundary
Explorer, or IBEX, spacecraft. Data from IBEX and Cassini have made it possible for scientists
to construct the first comprehensive sky map of our solar system and its location in the Milky
Way galaxy.

Researchers from University of Arizona, Tucson; Southwest Research Institute, San Antonio;
and University of Texas at San Antonio contributed to the article. The Cassini-Huygens mission
is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.
The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena,
manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington.
The Cassini orbiter was designed, developed and assembled at JPL. The Magnetospheric Imaging
Instrument was developed by the Applied Physics Laboratory.

More information on the Cassini mission is available at: http://www.nasa.gov/cassini,
http://saturn.jpl.nasa.gov and on the Magnetospheric Imaging Instrument Web site at
http://sd-www.jhuapl.edu/CASSINI/ .

More information on the Interstellar Boundary Explorer is available at: http://www.nasa.gov/ibex

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