The Explorer program continues and over more than five decades has launched 90+ missions into Space. It has matured into one of NASA's lower-cost mission programs, relative to its other programs. It began as a U.S. Army proposal to place a scientific satellite into orbit during the International Geophysical Year (1957–58); however, that proposal was rejected in favor of the U.S. Navy's Project Vanguard. The Explorer program was later reestablished to catch up with the Soviet Union after that nation's launch of Sputnik 1 on October, 1957. Explorer 1 was launched January 31, 1958; at this time the project still belonged to ABMA and JPL. Besides being the first U.S. satellite, it is known for discovering the Van Allen radiation belt.