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Explorer 18 (IMP 1) was a solar-cell and chemical-battery powered spacecraft instrumented for interplanetary and distant magnetospheric studies of energetic particles, cosmic rays, magnetic fields, and plasmas. Initial spacecraft parameters included a local time of apogee of 1020 h, a spin rate of 22 rpm, and a spin direction of 115 deg right ascension and -25 deg declination. Each normal telemetry sequence of 81.9 s duration consisted of 795 data bits. After every third normal sequence there was an 81.9-s interval of rubidium vapor magnetometer analog data transmission. The spacecraft performed normally until May 30, 1964, then intermittently until May 10, 1965, when it was abandoned. The principal periods of data coverage were November 27, 1963 to May 30, 1964; September 17, 1964 to January 7, 1965; and February 21, 1965 to March 25, 1965; however, only the first of these periods was very useful.

Alternate Names

  • 00693
  • Explorer 18
  • IMP 1
  • S 74

Facts in Brief

Launch Date: 1963-11-27
Launch Vehicle: Thor-Delta
Launch Site: Cape Canaveral, United States
Mass: 138 kg

Funding Agency

  • NASA-Office of Space Science Applications (United States)


  • Space Physics

Additional Information

Questions and comments about this spacecraft can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Ms. Della StewartGeneral ContactNASA Goddard Space Flight Center
Dr. Alois W. SchardtGeneral ContactNASA Headquarters
Mr. C. J. CrevelingGeneral ContactNASA Goddard Space Flight Center
Mr. Frank W. GaetanoGeneral ContactNASA Headquarters
Dr. Frank B. McDonaldProject ScientistNASA Goddard Space Flight Center
Mr. Everett J. PyleGeneral ContactNASA Goddard Space Flight Center
Mr. Paul ButlerProject ManagerNASA Goddard Space Flight Center

Explorer 18 engineering model

Explorer 18 engineering model at the Smithsonian Air and Space Udvar-Hazy Center.

Explorer 18

Explorer 18 / IMP-A

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