Interstellar Hydrogen Shadow Observed by Cassini

Interstellar hydrogen shadow observed
Southwest Research Institute News Release
December 8, 2003

More than a year before the Cassini spacecraft arrives at Saturn, the Cassini
Plasma Spectrometer (CAPS) has made the first in situ observations of
interstellar pickup ions beyond the orbit of Jupiter. This is the first major
discovery using data gathered by CAPS, destined to reach Saturn in July 2004.

Pickup ions are neutral particles in the solar system that become ionized near
the Sun and join the solar wind, the supersonic stream of charged particles
flowing out from the Sun. By observing these pickup ions, researchers can
better understand the interstellar medium, the low-density gas and dust that
fill the space between stars.

Astronomers have observed interstellar pickup ions as early as 1985 from a
distance of 1 astronomical unit (AU, the distance from the Earth to the Sun), but
never before have they seen pickup ions beyond 5 AU -- Jupiter's orbit. The
CAPS team uploaded software that allowed the instrument to collect and
transmit detections of the relatively rare pickup ions it encounters on its journey
to Saturn.

During the observation period of October 2001 to February 2003 at distances of
6.4 to 8.2 AU, the instrument collected 2,627 samples. Analyses revealed that
there is a strong depletion of hydrogen pickup ions compared to helium pickup
ions in the region behind the Sun. The team determined that this newly
observed depletion, or "interstellar hydrogen shadow," is produced by radiation
pressure and ionization of the neutrals. Most hydrogen atoms cannot penetrate
into the downstream shadow region because they must pass near the Sun
where they have a high probability of being ionized and swept out with the solar
wind.

"These are very hard particles to measure because there are so few of them,"
says Dr. David J. McComas, senior executive director of the SwRI Space
Science and Engineering Division. "Previous models have included something
like this interstellar hydrogen shadow, but these are the first direct
measurements of it."

Institute Scientist Dr. David T. Young is principal investigator of the CAPS
instrument, the largest, most complex space plasma instrument flown to date,
which will detect and analyze plasma (electrons and ions) found throughout the
solar system. The overall mission of the Cassini spacecraft is to image the
Saturn system at infrared, ultraviolet and visible wavelengths and to directly
sample the dust, neutral and charged particle environment. Cassini also carries
the Huygens probe, built by the European Space Agency, to study Saturn's
moon, Titan.

"This is certainly the first of many new discoveries to come by the Cassini
spacecraft, and the Cassini Plasma Spectrometer in particular," says McComas.
"To have been able to make such an important contribution to heliospheric
phenomenon on the way out to Saturn has been a great treat."

SwRI also leads a feasibility study for the proposed Interstellar Boundary
Explorer (IBEX) program, one of five candidates vying to fill two NASA
mission slots. If selected, the program would launch a pair of energetic neutral
atom cameras to directly image the interaction between the solar system and
the interstellar medium - the region that the interstellar neutrals must flow
through to enter the heliosphere.

The paper "The Interstellar Hydrogen Shadow: Observations of Interstellar
Pickup Ions Beyond Jupiter," is being presented December 9 at the American
Geophysical Union (AGU) meeting in San Francisco and is in press in the
Journal of Geophysical Research.