Explorer 1 – the first U.S satellite

Artist's impression of Explorer 1. Credit: NASA/JPL

On February 1, 1958, Explorer 1 was launched on an adapted Jupiter C (also called Juno I) rocket from LC-26A at the Cape Canaveral Missile Test Center, becoming the first U.S. satellite. It carried instrumentation for the study of cosmic rays, micrometeorites, and for monitoring of the satellite's temperature. Explorer 1 was the first spacecraft to detect the Van Allen radiation belt.

Explorer I satellite installed atop the Jupiter-C launch vehicle. Credit: NASA

   Launched late on January 31, 1958 (10:48 p.m. EST, or 03:48 UTC on February 1), Explorer 1 was the first satellite launched by the United States. 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 (1912–1977). 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 Jupiter-C launch vehicle consisted of four propulsive stages. The first stage was an upgraded Redstone liquid-fueled rocket. The second, third, and fourth stage rockets consisted of eleven, three, and one (respectively) Sergeant motors. The satellite itself was the fourth stage of the Jupiter-C rocket. It was cylindrical, 2.03 m long and 0.152 m in diameter. Four whip antennas were mounted symmetrically about the mid-section of the rocket. The spacecraft was spin stabilized. It had a mass of 13.97 kg.

   The 4.82 kg instrumentation package was mounted inside of the forward section of the rocket body. A single Geiger-Mueller detector was used for the detection of cosmic rays. Micrometeorite detection was accomplished using both a wire grid (arrayed around the aft section of the rocket body) and an acoustic detector (placed in contact with the midsection). Data from the instruments were transmitted continuously, but acquisition was limited to those times when the spacecraft passed over appropriately equipped ground receiving stations. Assembly of data proceeded slowly also due to the fact that the satellite's spin-stabilized attitude transitioned into a minimum kinetic energy state, that of a flat spin about its transverse axis. This was deduced from the modulation of the received signal, which produced periodic fade-outs of the signal. 

An exploded view of the Explorer 1. Credit: NASA/MSFC

   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 (1914–2006) 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 to Earth and as far as 2,515 kilometers. It made one orbit every 114.8 minutes. 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.

Liftoff of the Jupiter-C launch vehicle with Explorer 1. Credit: NASA

The three men responsible for the success of Explorer 1, America’s first Earth satellite which was launched January 31, 1958. At left is Dr. William H. Pickering, former director of JPL, which built and operated the satellite. Dr. James A. van Allen, center, of the State University of Iowa, designed and built the instrument on Explorer that discovered the radiation belts which circle the Earth. At right is Dr. Wernher von Braun, leader of the Army’s Redstone Arsenal team which built the first stage Redstone rocket that launched Explorer 1. Credit: NASA

© 2026, Andrew Mirecki