On January 31, 1958, Explorer 1 became the first U.S. satellite launched into orbit. It came four months after the launch of the world's first satellite, the former Soviet Union's Sputnik 1. While the Soviet satellite weighed 184 pounds (83.5 kilograms), the U.S. satellite weighed only 31 pounds (14-06 kilograms) and was nicknamed "the grapefruit."
Explorer 1 carried instrumentation that led to the discovery of the Van Allen belts (or zones), which are two rings of highly charged particles encircling the Earth. The belts are named for James Van Allen (1914-), the professor of physics who designed Explorer's instruments. (A physicist is a scientist specializing in the interaction between matter and energy.)
Sources: Braun, Wernher von, and Frederick 1. Ordway III. Space Travel: A History, pp. 170-73; Curtis. Anrhony R. Space Almanac, p. 13.