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The P-21A was a suborbital probe designed to collect nighttime ionospheric data needed to improve Earth-space communications. It carried a continuous-wave propagation experiment to study electron density and associated parameters of the ionosphere; a swept-frequency probe to make measurements of electron density; and an ion trap to determine positive ion concentration. The spacecraft was an 8-sided cone which contained the experiments and transmitters. It was launched on a Scout X-2 (ST-9) and flew to 6291 km altitude and 7040 km downrange from Wallops Station, Virginia in a 97 minute flight. P-21A returned good data on nighttime ionosphere conditions. It described the newly discovered helium layer under nighttime conditions for the first time.

Alternate Names

  • Electron Density Profile Probe

Facts in Brief

Launch Date: 1962-03-29
Launch Vehicle: Scout
Launch Site: Wallops Island, United States
Mass: 43 kg


  • Space Physics

Additional Information

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



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