Opportunities in Cosmic-Ray Physics and Astrophysics

The Board on Physics and Astronomy of the National Research Council established the Committee on Cosmic-Ray Physics to prepare a review of the field that addresses both experimental and theoretical aspects of the origin of cosmic radiation from outside the heliosphere. This action was initially motivated by a request from the Space Physics Division of the National Aeronautics and Space Administration (NASA) to consider the program of research in this discipline in light of new constraints on the scope of missions at NASA, which made previously planned cosmic-ray missions on a large space station and on the Space Shuttle seem difficult to realize at that time. In the meantime, it has become apparent that exciting new opportunities in cosmic-ray physics are ripe for significant progress with ground-based detectors as well as with observations from spacecraft or balloons. Accordingly, the committee was charged to provide a balanced assessment of the entire field at this point and to consider the experiments needed to take advantage of current scientific opportunities.

Another reason for undertaking a balanced assessment of the field is that cosmic-ray physics is an intrinsically interdisciplinary subject. It is a part both of physics and of astrophysics. Its support, moreover, is drawn from several different sources, including NASA, the National Science Foundation (NSF), and the Department of Energy (DOE). The scientific rationale for the field becomes fully apparent only when all aspects of the subject are seen together. Thus, for example, measurements of positrons and antiprotons are relevant both to models of cosmic-ray propagation (space astrophysics) and to searches for dark matter in the universe (particle physics and cosmology). A direct measurement of the composition of high-energy cosmic rays above the atmosphere (supported by NASA) will not only clear up an important question about the efficiency of supernovas as cosmic accelerators, but also calibrate ground-based experiments (supported by NSF and DOE) that can extend the measurements to still higher energies.

The work of the committee began in late 1993, at which time there was a call for comments from the community by electronic mail and through the Division of Astrophysics of the American Physical Society. An interim report entitled Cosmic Rays: Physics and Astrophysics, A Research Briefing (National Academy Press, Washington, D.C.) was issued in mid-1994. The committee met again during the Snowmass Summer Study, "Particle and Nuclear Astrophysics and Cosmology in the Next Millennium." The present report reflects interactions of the individual committee members with many working groups and individual scientists during the summer study and afterward, as well as earlier discussions.

Thomas K. Gaisser, Chair
Committee on Cosmic-Ray Physics

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