  Rossi's career paralleled the evolution of cosmic-ray physics. Starting from
the early 1930s his pioneering work on the nature and behavior of cosmic rays
led to fundamental contributions in the field of experimental cosmic-ray
physics and laid the foundation for high-energy particle physics. After the
war, under his leadership the Cosmic Ray group at MIT investigated the
properties of the primary cosmic rays elucidating the processes involved in
their propagation through the atmosphere, and measuring the unstable particles
generated in the interactions with matter. When accelerators came to dominate
particle physics, Rossi's attention focused on the new opportunities for
exploratory investigations made possible by the availability of space vehicles.
He initiated a research program which led to the first in situ measurements of
the density, speed and direction of the solar wind at the boundary of Earth's
magnetosphere and inspired the search for extra-solar X-ray sources resulting
in the detection of what revealed to be the most powerful X-ray source in
Earth's skies. The discovery of Scorpius X-1 marked the beginning of X-ray
astronomy, which soon became a principal tool of astrophysics research.
