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Explorer 21 (IMP 2) 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. The spacecraft was supposed to reach an apogee of 203,600 km, in order to study the transition region between the Earth's magnetosphere and interplanetary space. But a malfunction of the third stage 17 seconds after ignition resulted in a lower than planned injection velocity, and the spacecraft could only reach an apogee of about 94,000 km, severely impacting the mission. It still returned data from within the magnetosphere.

The spacecraft was an octagonal prism 20 cm (8 in.) high and 71 cm across, with a 183 cm (6 ft.) boom extending from the top holding a magnetometer sphere, and two 213 cm (7 ft.) booms holding magnetometers extending from the body. Four solar panel padddles also extended from the spacecraft. The solar panels held 11,520 solar cells intended to produce 38 W charging silver-cadmium batteries. It was planned to be spin-stabilized at 20 rpm. Perterbations introduced by the third stage failure shifted the spacecraft spin axis resulting in lower than planned power output from the solar panels.

Each normal telemetry sequence of 81.9 s in 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. Initial spacecraft parameters included a local time of apogee at noon, a spin rate of 14.6 rpm, and a spin direction of 41.4-deg right ascension and 47.4-deg declination. The significant deviation of the spin rate and direction from the planned values and the achievement of an apogee of less than half the planned value adversely affected data usefulness. Otherwise, spacecraft systems performed well, with nearly complete data transmission for the first 4 months and for the sixth month after launch. Data transmission was intermittent for other times, and the final transmission occurred on October 13, 1965.

Alternate Names

  • 00889
  • Explorer 21
  • IMP 2
  • S 74A

Facts in Brief

Launch Date: 1964-10-04
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Mass: 135 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. Everett J. PyleGeneral ContactNASA Goddard Space Flight Center
Mr. Paul ButlerProject ManagerNASA Goddard Space Flight Center
Ms. Della StewartGeneral ContactNASA Goddard Space Flight Center
Mr. C. J. CrevelingGeneral ContactNASA Goddard Space Flight Center
Dr. Frank B. McDonaldProject ScientistNASA Goddard Space Flight Center

Selected References

  • Carr, F. A., Flight report Interplanetary Monitoring Platform IMP-2, NASA, TN D-3353, Wash., D.C., June 1966.
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