COSMIC RAY ASTROPHYSICS

Lying at the intersection of particle physics, cosmology, and astronomy, particle astrophysics probes some of the most exotic objects in the universe. While much of traditional astronomy is concerned strictly with photons produced in fairly tranquil thermal processes, the cosmos is also filled with highly relativistic particles -- cosmic rays (CRs) -- producing non-thermal radiation over a vast range of wavelengths. Particle astrophysics concerns itself with the study of cosmic rays, the radiation they generate, and the extreme, non-equilibrium environments (e.g., as supernova explosions or active galactic nuclei) in which they are produced.

In many cases, the most profound and ground-breaking results of the relatively new scientific discipline of particle astrophysics have come via instrumentation on balloon and space-based platforms which avoid the obscuring and interfering effects of the atmosphere. Balloon and space-based missions have achieved unprecedented precision and accuracy in their CR measurements and are at the forefront of particle astrophysics. Two examples of many are the space mission Advanced Composition Explorer (ACE), and the balloon borne payload BESS, a superconducting solenoid studying antiprotons. Measuring CRs and connecting the properties of these particles, as observed at Earth, to the production and radiation of highly energetic particles in exotic environments is one of the vital keys to better understanding our local solar environment, and the high-energy universe in general, and lies at the leading edge of all astrophysical research. Despite the success of balloon and space-based CR work, CR-Astrophysics is relatively under-funded compared to disciplines of equal impact within NASA and the importance of a well-informed prioritization and planning process becomes clear.

The purpose of this assessment is to provide the scientific background and context required to perform this prioritization and planning. We will discuss the scope of science addressable with balloon and space-based observations, and the important connections that CR-Astrophysics has with the broader scientific community. In keeping with our charge, our focus is explicitly particle astrophysics research that can be performed at the edge of space and beyond and we concentrate our specific recommendations on missions concerned with the measurement and detection of cosmic rays. Other groups are addressing science based on photon observations. To the extent that neutrinos can be detected on balloon and space-based platforms, we will include them in this roadmap.