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Explorer 1



Explorer 1 was the first successfully launched U. S. spacecraft. Launched late on 31 January 1958 (10:48 p.m. EST, or 03:48 UTC on 01 February) on an adapted Jupiter-C rocket, Explorer 1 carried instrumentation for the study of cosmic rays, micrometeorites, and for monitoring of the satellite's temperature.

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.

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.

Explorer 1 was the first spacecraft to successfully detect the durably trapped radiation in the Earth's magnetosphere, dubbed the Van Allen Radiation Belt (after the principal investigator of the cosmic ray experiment on Explorer 1, James A. Van Allen). Later missions (in both the Explorer and Pioneer series) were to expand on the knowledge and extent of these zones of radiation and were the foundation of modern magnetospheric studies.

Alternate Names

  • 00004
  • 1958 Alpha 1
  • Explorer1

Facts in Brief

Launch Date: 1958-02-01
Launch Vehicle: Jupiter C (Juno I)
Launch Site: Cape Canaveral, United States
Mass: 13.97 kg

Funding Agency

  • Department of Defense-Department of the Army (United States)


  • Space Physics
  • Earth Science

Additional Information

Questions and comments about this spacecraft can be directed to: Dr. Edwin V. Bell, II



NameRoleOriginal AffiliationE-mail
Mr. J. FroelichProject ManagerNASA Jet Propulsion Laboratory
Dr. Albert R. HibbsProject ScientistNASA Jet Propulsion Laboratory

Selected References

  • Some preliminary reports of experiments in satellites 1958 Alpha and 1958 Gamma, IGY Satell. Rep. Ser., No. 3, May 1958.
  • Victor, W. K., U.S. Explorer satellites, Inpro. 071875.
  • Van Allen, J. A., 1994 Kuiper prize lecture: Electrons, protons, and planets, Icarus, 122, No. 2, 209-232, doi:10.1006/icar.1996.0121, 1996.
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