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Explorer 28 (IMP 3) 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 2020 h, a spin rate of 23.7 rpm, and a spin direction of 64.9-deg right ascension and -10.9-deg declination. Each normal telemetry sequence was 81.9 s in duration and consisted of 795 data bits. After every third normal telemetry sequence there was an 81.9-s interval of rubidium vapor magnetometer analog data transmission. Performance was essentially normal until late April 1967, then intermittent until May 12, 1967, after which no further data were acquired.

Alternate Names

  • Explorer 28
  • S 74B
  • IMP 3
  • 01388

Facts in Brief

Launch Date: 1965-05-29
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Mass: 128 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
Mr. C. J. CrevelingGeneral ContactNASA Goddard Space Flight Center
Mr. Everett J. PyleGeneral ContactNASA Goddard Space Flight Center
Mr. Paul ButlerProject ManagerNASA Goddard Space Flight Center
Dr. Frank B. McDonaldProject ScientistNASA Goddard Space Flight Center
Ms. Della StewartGeneral ContactNASA Goddard Space Flight Center
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