# Explorer 1
**Explorer 1** (also designated [Satellite](https://www.edgepedia.org/satellite) 1958 Alpha) was the first successful United States satellite, launched on January 31, 1958 from [Cape Canaveral](https://www.edgepedia.org/cape-canaveral), Florida, and the spacecraft whose cosmic ray instrument led to the discovery of the Van Allen radiation belts around Earth.[1](https://science.nasa.gov/mission/explorer-1/about/) • [2](https://airandspace.si.edu/collection-objects/satellite-explorer-i/nasm_A19610039000) Its success, less than four months after the Soviet Union's Sputnik 1, marked the beginning of the U.S. [Space Age](https://www.edgepedia.org/space-age).[3](https://www.jpl.nasa.gov/missions/explorer-1/)
| Key fact | Detail |
|---|---|
| Launch | January 31, 1958, 10:48 p.m. EST, Cape Canaveral, Florida, on a Jupiter-C (Juno I) rocket[1](https://science.nasa.gov/mission/explorer-1/about/) |
| Spacecraft | 80.75 inches (2.05 m) long, 30.66 pounds (13.90 kg), of which 18.35 pounds (8.32 kg) was instrumentation[1](https://science.nasa.gov/mission/explorer-1/about/) |
| Orbit | Perigee 224 miles, apogee 1,575 miles, period 114.9 minutes; spin-stabilized at 750 rpm[4](https://www.nasa.gov/history/sputnik/expinfo.html) |
| Principal science | A Geiger-counter cosmic ray detector designed by James Van Allen of the State University of Iowa[3](https://www.jpl.nasa.gov/missions/explorer-1/) |
| Discovery | Data from Explorer 1 and Explorer 3 revealed charged particles trapped by Earth's magnetic field, announced May 1, 1958 and later named the Van Allen belts[5](https://ntrs.nasa.gov/citations/19620000986) |
| Operations | Transmitted until batteries ran out on May 23, 1958; completed 58,376 orbits before burning up on reentry on March 31, 1970[1](https://science.nasa.gov/mission/explorer-1/about/) |
| Builders | Army Ballistic Missile Agency (rocket, directed by Wernher von Braun) and the Jet Propulsion Laboratory (satellite, directed by William Pickering)[1](https://science.nasa.gov/mission/explorer-1/about/) |
## Background: Sputnik and the American response
The Soviet Union launched [Sputnik 1](https://www.edgepedia.org/sputnik-1), the world's first artificial satellite, on October 4, 1957, and followed with the 1,120-pound [Sputnik 2](https://www.edgepedia.org/sputnik-2) carrying the dog Laika on November 3, 1957.[6](https://www.nasa.gov/history/sputnik/sputorig.html) The United States' first response, the Navy's Vanguard, failed publicly on December 6, 1957 when the rocket rose a few feet, fell back, and burst into flames.[7](https://www.nasa.gov/missions/explorer/explorer-1-fast-facts/)
The Army's path to orbit already existed on paper. In 1955 the Army Ballistic Missile Agency (ABMA) and the [Jet Propulsion Laboratory](https://www.edgepedia.org/jet-propulsion-laboratory) (JPL) had proposed launching a small satellite on Sergeant solid-fuel rockets atop a Redstone booster, a plan called Orbiter, but the Department of Defense selected the [Naval Research Laboratory](https://www.edgepedia.org/naval-research-laboratory)'s Vanguard instead.[8](https://www.nasa.gov/wp-content/uploads/2023/04/sp-4012v2.pdf) After Sputnik 1, ABMA was directed to proceed with a satellite launch using the Jupiter-C, a modified Redstone that had already been flight-tested in nose-cone re-entry tests; ABMA and JPL completed the job of modifying the rocket and building Explorer 1 in 84 days.[4](https://www.nasa.gov/history/sputnik/expinfo.html) JPL furnished the high-speed solid-propellant upper stages, and its Pasadena laboratory built the satellite under director William Pickering.[9](https://www.jpl.nasa.gov/videos/explorer-1-americas-first-satellite/) • [1](https://science.nasa.gov/mission/explorer-1/about/)
## The launch
After two days of delays caused by high winds, the Juno I booster lifted off at 10:48 p.m. EST on January 31, 1958.[1](https://science.nasa.gov/mission/explorer-1/about/) One NASA historical account gives the liftoff time as 10:55 p.m.[6](https://www.nasa.gov/history/sputnik/sputorig.html) [Confirmation](https://www.edgepedia.org/confirmation) of orbit took time: JPL's Pickering later described waiting through what he called the longest eight minutes of his life before the satellite's signal was picked up in California, and a jubilant press conference followed about two hours after launch, with Pickering, Van Allen, and von Braun displaying a model of the satellite.[10](https://science.nasa.gov/mission/explorer-1/)
Once in orbit, the instrument section at the front of the satellite and the empty scaled-down Sergeant fourth-stage casing orbited as a single unit, spinning around its long axis at 750 revolutions per minute.[4](https://www.nasa.gov/history/sputnik/expinfo.html)
## Science payload and the discovery of the radiation belts
The primary instrument was a cosmic ray detector, a [Geiger counter](https://www.edgepedia.org/geiger-counter) with a miniature tape recorder, designed by James Van Allen, head of the [University of Iowa](https://www.edgepedia.org/university-of-iowa) physics department, to measure the radiation environment in Earth orbit.[10](https://science.nasa.gov/mission/explorer-1/) • [3](https://www.jpl.nasa.gov/missions/explorer-1/) The satellite also carried micrometeorite detection equipment: a microphone and erosion gauges to register hits from tiny meteorites.[7](https://www.nasa.gov/missions/explorer/explorer-1-fast-facts/)
Once in space, the cosmic ray equipment indicated a much lower count than anticipated. Van Allen theorized that the instrument was being saturated by very strong radiation from a belt of charged particles trapped in space by [Earth's magnetic field](https://www.edgepedia.org/earths-magnetic-field).[4](https://www.nasa.gov/history/sputnik/expinfo.html) According to a modern account, the Geiger counters aboard Explorer 1—built with only cosmic ray intensities in mind—became saturated during dead time, which revealed how intense the fluxes of trapped particles actually were.[11](https://doi.org/10.1029%2F2018JA025940) Data recorded by a network of seventeen receiving stations, combined with the first orbits of data from Explorer 3 (launched March 26, 1958), resulted in the announcement on May 1, 1958 of the region of high-intensity radiation surrounding Earth.[5](https://ntrs.nasa.gov/citations/19620000986) Later spacecraft carrying Van Allen's instruments, Pioneer 3 and Explorer 4, provided data that revealed a second, outer radiation belt encircling the inner one; the belts are named the Van Allen Belts in honor of their discoverer.[1](https://science.nasa.gov/mission/explorer-1/about/) The inner belt is composed predominantly of protons and the outer belt mostly of electrons.[12](https://www.nasa.gov/solar-system/studying-the-van-allen-belts-60-years-after-americas-first-spacecraft/) This has been called the first major scientific discovery of the Space Age.[11](https://doi.org/10.1029%2F2018JA025940)
## Operations and end of mission
Explorer 1 carried a high-power and a low-power transmitter, both battery powered. The high-power transmitter operated properly until February 12, 1958 and the low-power system until about 7:00 UT on March 16, 1958, with battery exhaustion believed to be the cause of both failures; the satellite's signal ended on May 23, 1958 when its batteries ran out.[13](https://ntrs.nasa.gov/api/citations/19750016139/downloads/19750016139.pdf) • [1](https://science.nasa.gov/mission/explorer-1/about/) The spacecraft remained in orbit until March 31, 1970, completing 58,376 orbits before burning up on reentry over the [Pacific Ocean](https://www.edgepedia.org/pacific-ocean).[1](https://science.nasa.gov/mission/explorer-1/about/)
The mission also produced an early lesson in spacecraft dynamics. Designed as a spin-stabilized satellite spinning about its minimum inertia axis, Explorer 1 was assumed to be inherently stable as a rigid body. That assumption was false: energy dissipation in its flexible wire whip telemetry antennas had a destabilizing effect, and the vehicle eventually wound up in a flat spin about its maximum moment of inertia axis.[14](https://ntrs.nasa.gov/api/citations/20230005922/downloads/20230005922.pdf)
## Significance
Explorer 1 answered Sputnik, but its public impact was limited by its size. In contrast with the 1,121-pound Sputnik II, the thirty-one-pound Explorer 1 seemed small, and Soviet leader [Nikita Khrushchev](https://www.edgepedia.org/nikita-khrushchev) mocked the even smaller [Vanguard 1](https://www.edgepedia.org/vanguard-1) as a "grapefruit satellite."[15](https://cgsr.llnl.gov/sites/cgsr/files/pubs/2024-08/STATEGIC_LATENCY_Book-WEB.pdf) Its scientific return, however, was substantial: the radiation belt discovery opened a field of research that continues today, because particles accelerated to high energies in the belts are a hazard to both astronauts and spacecraft.[12](https://www.nasa.gov/solar-system/studying-the-van-allen-belts-60-years-after-americas-first-spacecraft/) NASA launched the twin [Van Allen Probes](https://www.edgepedia.org/van-allen-probes) in 2012 to study particle behavior in the region with radiation-hardened instruments that go far beyond Explorer 1's Geiger counter.[12](https://www.nasa.gov/solar-system/studying-the-van-allen-belts-60-years-after-americas-first-spacecraft/)
The satellite itself survives in museum form. The Army Ballistic Missile Agency transferred an Explorer 1 spacecraft to the Smithsonian Institution in September 1958, and a replica hangs in the Smithsonian's National Air and Space Museum.[2](https://airandspace.si.edu/collection-objects/satellite-explorer-i/nasm_A19610039000) • [16](https://www.smithsonianmag.com/arts-culture/explorer-i-satellite-12431712/)
## References
1. [About Explorer 1, NASA Science](https://science.nasa.gov/mission/explorer-1/about/)
2. [Satellite, Explorer I, National Air and Space Museum collection](https://airandspace.si.edu/collection-objects/satellite-explorer-i/nasm_A19610039000)
3. [Explorer 1, NASA Jet Propulsion Laboratory](https://www.jpl.nasa.gov/missions/explorer-1/)
4. [Explorer Information, NASA History](https://www.nasa.gov/history/sputnik/expinfo.html)
5. [Ludwig, G. H. (1961). Radiation Observations with Satellite 1958 Alpha (Explorer I), NASA NTRS](https://ntrs.nasa.gov/citations/19620000986)
6. [Sputnik and the Origins of the Space Age, NASA History](https://www.nasa.gov/history/sputnik/sputorig.html)
7. [Explorer 1 Fast Facts, NASA](https://www.nasa.gov/missions/explorer/explorer-1-fast-facts/)
8. [Space Science and Applications, NASA SP-4012](https://www.nasa.gov/wp-content/uploads/2023/04/sp-4012v2.pdf)
9. [Explorer 1: America's First Satellite, NASA JPL](https://www.jpl.nasa.gov/videos/explorer-1-americas-first-satellite/)
10. [Explorer 1, NASA Science mission page](https://science.nasa.gov/mission/explorer-1/)
11. [Li, X. et al. (2019). Earth's Van Allen Radiation Belts: From Discovery to the Van Allen Probes Era, Journal of Geophysical Research: Space Physics](https://doi.org/10.1029%2F2018JA025940)
12. [Studying the Van Allen Belts 60 Years After America's First Spacecraft, NASA](https://www.nasa.gov/solar-system/studying-the-van-allen-belts-60-years-after-americas-first-spacecraft/)
13. [EXPLORER 1/EXPLORER 4 data report, NASA NTRS](https://ntrs.nasa.gov/api/citations/19750016139/downloads/19750016139.pdf)
14. [NASA Lessons Learned, Appendix A-1: Explorer-1 (1958), NASA NTRS](https://ntrs.nasa.gov/api/citations/20230005922/downloads/20230005922.pdf)
15. [Strategic Latency: World War I to the Present, LLNL Center for Global Strategic Resources](https://cgsr.llnl.gov/sites/cgsr/files/pubs/2024-08/STATEGIC_LATENCY_Book-WEB.pdf)
16. [Explorer I Satellite, Smithsonian Magazine](https://www.smithsonianmag.com/arts-culture/explorer-i-satellite-12431712/)
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*Topic: Encyclopedia › Technology and the built world › Transport and spaceflight › Spaceflight › Spaceflight history and chronology › Spacecraft launches by year › Spacecraft launched 1957–1962*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license