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Explorer 12



Explorer 12 was a spin-stabilized, solar-cell-powered spacecraft instrumented to measure cosmic-ray particles, trapped particles, solar wind protons, and magnetospheric and interplanetary magnetic fields. It was the first of the S 3 series of spacecraft, which also included Explorers 14, 15, and 26. A 16-channel PFM/PM time-division multiplexed telemeter was used. The time required to sample the 16 channels (one frame period) was 0.324 s. Half of the channels were used to convey eight-level digital information, and the other channels were used for analog information. During ground processing of the telemetered data, the analog information was digitized with an accuracy of 1/100th of full scale. One analog channel was subcommutated in a 16-frame-long pattern and was used to telemeter spacecraft temperatures, power system voltages, currents, etc. A digital solar aspect sensor measured the spin period and phase, digitized to 0.041 s, and the angle between the spin axis and sun direction to about 3-deg intervals. The spacecraft functioned well until December 6, 1961, when it ceased transmitting data apparently as a result of failures in the power system. Good data were recorded for approximately 90% of the active lifetime of the spacecraft. The initial spin rate was 28.0 rpm, and the spin axis direction was right ascension 48 deg, declination -28 deg. The direction was nearly constant with time, and the spin rate slowly increased with time to 34.3 rpm. Apogee direction varied from about 1200 h to 0600 h local time.

Alternate Names

  • S 3
  • 00170
  • 1961 Upsilon 1
  • Explorer12
  • EPE-A

Facts in Brief

Launch Date: 1961-08-16
Launch Vehicle: Thor-Delta
Launch Site: Cape Canaveral, United States
Mass: 37.6 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: Dr. John F. Cooper



NameRoleOriginal AffiliationE-mail
Mr. Paul ButlerProject ManagerNASA Goddard Space Flight Center
Dr. Frank B. McDonaldProject ScientistNASA Goddard Space Flight Center
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