Explorer 21

Explorer 21, also called IMP-B, IMP-2 and Interplanetary Monitoring Platform-2, was a NASA satellite launched as part of Explorer program. Explorer 21 was launched on 4 October 1964, at 03:45:00 GMT from Cape Canaveral (CCAFS), Florida, with a Thor-Delta C launch vehicle. Explorer 21 was the second satellite of the Interplanetary Monitoring Platform, and used the same general design as its predecessor, Explorer 18 (IMP-A), launched the previous year, in November 1963. The following Explorer 28 (IMP-C), launched in May 1965, also used a similar design. 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. Each normal telemetry sequence of 81.9 seconds in duration consisted of 795 data bits. After every third normal sequence there was an 81.9 seconds 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° right ascension and 47.4° declination. A charged-particle, solid-state telescope was used to measure range and energy loss of galactic and solar cosmic rays. The experiment was designed to study particle energies (energy per nucleon intervals approximately proportional to Z squared/A, for protons 0.9 to 190 MeV, 6.5 to 19 MeV, 19 to 90 MeV, and 90 to 190 MeV) and charge spectra (Z