From: baalke@zagami.jpl.nasa.gov   
      
   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.   
      
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