[6] Explorer 21 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.
After every third normal sequence there was an 81.9 seconds interval of rubidium vapor magnetometer analog data transmission.
[7] A charged-particle, solid-state telescope was used to measure range and energy loss of galactic and solar cosmic rays.
The output from two 128-channel, pulse-height analyzers was obtained for one incident particle every 41 seconds and read out along with the detector accumulations.
The relative contribution to the count rate of various species (electrons between 3 and 12 MeV, ions with charge = 1 or 2, atomic mass = 1, 2, 3 or 4, and energy between 18.7 and 81.6 MeV/nucleon) and energy spectral information were determined by 512-channel, pulse-height analysis performed simultaneously on the output of both CsI scintillators six times every 5.46 minutes.
The second detector system consisted of two Geiger–Müller tube (GM) telescopes oriented parallel to and perpendicular to the spacecraft spin axis.
The parallel, perpendicular, and omnidirectional count rates were obtained for one 40-seconds accumulation interval during successive normal 81.9-seconds telemetry sequences.
Two collector plates were used to yield information about the angular variation out of the satellite spin plane.
[10] Each of two uniaxial fluxgate magnetometers, having dynamic ranges of plus or minus 40 nT, sampled the magnetic field 30 times within each of six 4.8-seconds intervals every 5.46 minutes.
[11] This experiment, designed to measure fluxes of geomagnetically trapped particles, consisted of a 7.6-cm diameter, Neher-type ionization chamber and two Anton 223 Geiger–Müller (GM) tubes.
The instrument experienced secondary electron contamination but returned essentially continuous data until 5 April 1965.
The planned monitoring of the interplanetary medium was not accomplished because the apogee that the satellite achieved was lower than expected.
The satellite equatorial plane was divided into three contiguous sectors (61°, 95° and 204°) by use of an optical aspect sensor.