Conventional wisdom holds that most of the cosmic rays
that continually rain upon the Earth originate in
galactic supernova shock waves, although some extragalactic
origin must be invoked to explain the highest-energy particles.
Changes are expected with increasing energy in the charge and
mass composition of cosmic rays, induced by a combination of
origin and propagation effects. Because of rapidly dwindling
fluxes with energy, despite many decades of intensive
research, the evidence in support of these ideas is still scant.
Improved measurements of cosmic rays are needed to elucidate
their nature and origin.
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 up
to about 10^15 eV. It was flown successfully in Antarctica three
times since 2004, for an unprecedented cumulative exposure of
nearly 100 days. We will present preliminary measurements of the
spectrum and composition of the cosmic-ray flux, at the limit of
sensitivity possible with instruments at the edges of the atmosphere.
