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.