abundances found by the Interstellar Observatory will improve our understanding of how stars process matter and how the galaxy evolves, and it will improve our knowledge of the age of the universe.

Cosmic Rays and Modulation by the Solar Magnetic Field

In the heliosheath—the region beyond the termination shock, where the solar wind is heated and slowed—models show the formation of a large magnetic barrier that filters out the majority of the low-energy galactic cosmic rays (<100 MeV/nucleon) from the interstellar medium. In addition, the solar wind’s magnetic field—i.e., the interplanetary magnetic field—and its embedded large-scale magnetic disturbances exclude more galactic cosmic rays from the inner solar system over the ~2- to 4-year period when the Sun is most active during the 11-year solar cycle.

Highly penetrating galactic cosmic rays are one of the most serious hazards for astronauts on long-duration missions beyond the protection of Earth’s magnetic field (Figure B.6). By directly passing through and sampling the heliosheath, measuring both galactic cosmic rays and the heliosheath’s magnetic field, the Interstellar Observatory will study directly how the solar system is shielded from the majority of galactic cosmic rays.

Cosmic Rays and the Energy Density of the Galaxy

The spectrum of interstellar cosmic rays is not known because particles with energies <100 MeV/nucleon are excluded from the heliosphere. Only a spacecraft such as the Interstellar Observatory that travels to the interstellar medium can determine the full cosmic-ray energy spectrum and its contributions to the energy density and ionization state of the interstellar medium. These measurements will allow further study of astrophysical processes such as the acceleration of cosmic rays by supernova shocks, galactic radio and gamma-ray emissions, recent nucleosynthesis, and the heating of the interstellar medium. Finally, the Interstellar Observatory’s direct measurements of the cosmic-ray energy spectrum will determine how the cosmic-ray pressure in the local interstellar medium affects the size and shape of the solar system’s outer boundaries.


1. National Research Council, Exploration of the Outer Heliosphere and the Local Interstellar Medium, The National Academies Press, Washington, D.C., 2004.

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