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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.

The spacecraft was basically a truncated cone attached to an octagonal platform, 48.2 cm high and 66.5 cm across the flats of the octagon. The platform, 14 cm thick and fabricated of nylon honeycomb and fiberglass with an aluminum cover, housed most of the electronic equipment and experiments. Four solar paddles and an 81 cm column supporting a magnetometer package extended from the platform. Total spacecraft mass was 37.6 kg. The solar paddles held 5600 solar cells that powered 13 silver-cadmium batteries. A two-watt transmitter operated at a frequency of 136.02 MHz. Orientation knowledge was provided by a photocell optical sensing system.

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

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

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

Explorer 12 backup

Explorer 12 backup spacecraft at the Smithsonian Air and Space Udvar-Hazy Center.

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