Explorer 1 was the first satellite launched by the United States when it was sent into space on January 31, 1958. Following the launch of the Soviet Union's Sputnik 1 on October 4, 1957, the U.S. Army Ballistic Missile Agency was directed to launch a satellite using its Jupiter C rocket developed under the direction of Dr. Wernher von Braun. The Jet Propulsion Laboratory received the assignment to design, build and operate the artificial satellite that would serve as the rocket's payload. JPL completed this job in less than three months.
The primary science instrument on Explorer 1 was a cosmic ray detector designed to measure the radiation environment in Earth orbit. Once in space this experiment, provided by Dr. James Van Allen of the University of Iowa, revealed a much lower cosmic ray count than expected. Van Allen theorized that the instrument may have been saturated by very strong radiation from a belt of charged particles trapped in space by Earth's magnetic field. The existence of these radiation belts was confirmed by another U.S. satellite launched two months later, and they became known as the Van Allen Belts in honor of their discoverer.
Explorer 1 revolved around Earth in a looping orbit that took it as close as 354 kilometers (220 miles) to Earth and as far as 2,515 kilometers (1,563 miles). It made one orbit every 114.8 minutes, or a total of 12.54 orbits per day. The satellite itself was 203 centimeters (80 inches) long and 15.9 centimeters (6.25 inches) in diameter. Explorer 1 made its final transmission on May 23, 1958. It entered Earth's atmosphere and burned up on March 31, 1970, after more than 58,000 orbits. The satellite weighed 14 kilograms (30.8 pounds).
A launch attempt of a similar satellite, Explorer 2, was made on March 5, 1958, but the fourth stage of the Jupiter-C rocket failed to ignite. Explorer 3 was successfully launched on March 26, 1958, and operated until June 16 of that year. Explorer 4 was launched July 26, 1958, and operated until October 6 of that year. Launch of Explorer 5 on August 24, 1958, failed when the rocket's booster collided with its second stage after separation, causing the firing angle of the upper stage to be incorrect.
Ссылка на источник: https://www.nasa.gov/mission_pages/explorer/explorer-overview.html