Explorer 28

Explorer 28, also called IMP-C, IMP-3 and Interplanetary Monitoring Platform-3, was a NASA satellite launched on 29 May 1965 to study space physics, and was the third spacecraft launched in the Interplanetary Monitoring Platform program. It was powered by chemical batteries and solar panels. There were 7 experiments on board, all devoted to particle studies. Performance was normal until late April 1967, when intermittent problems began. It stayed in contact until 12 May 1967, when contact was lost. The orbit decayed until it re-entered the atmosphere on 4 July 1968. The spacecraft design was similar to its predecessors Explorer 18 (IMP-A), launched in November 1963, and Explorer 21 (IMP-B), launched in October 1964, though this satellite was a few kilograms lighter. The successor Explorer 33 (IMP-D) began the use of a new design. Explorer 28 (IMP-3) 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. Initial spacecraft parameters included a local time of apogee of 20:20 hours, a spin rate of 23.7 rpm, and a spin direction of 64.9° right ascension and -10.9° declination. Each normal telemetry sequence was 81.9-seconds in duration and consisted of 795 data bits. After every third normal telemetry sequence there was an 81.9-seconds interval of rubidium vapor magnetometer analog data transmission. 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 2.6 to 190 MeV, 13.3 to 26 MeV, 26 to 94 MeV, and 94 to 190 MeV) and charge spectra (Z