The Nature and Origin of Cosmic Rays

 Conventional wisdom holds that the majority of high energy atomic
 nuclei ("cosmic rays") that continually rain upon the Earth
 originate in galactic supernova shock waves. Despite many decades 
 of intensive research on the subject, indirect evidence for this
 scenario has been only slowly accumulating. This is largely due to the
 difficulty in measuring the spectrum and mass composition of 
 high-energy cosmic rays in the face of rapidly dwindling fluxes 
 with energy. There is an expectation that the cosmic 
 nuclei should have a progressively higher charge (and therefore 
 mass), on average, with increasing energy, up to the 
 astrophysical "knee" (spectral break) in the spectrum at around
 3x1015 eV, but this has never been conclusively observed. 
 Measurements of the details of the elemental particle spectra 
 at Earth, particularly for some of the rare nuclear species produced
 by spallation reactions during interstellar transport, can also 
 provide clues regarding the galactic propagation history of the 
 cosmic rays.

 The CREAM (Cosmic Ray Energetics And Mass) experiment is a
 complex particle detector flown by high altitude balloon to directly
 measure the charge and energy of the cosmic rays at energies near the
 spectral knee. It flew successfully in Antarctica in Dec 04 / Jan 05 for
 a record-breaking 42 days, and again in Dec 05 / Jan 06 for 28 more
 days. We will review the science and performance of the instrument in
 flight, and present preliminary results and discuss prospects for 
 additional CREAM missions.