Explorer 18, also called IMP-A, IMP-1, Interplanetary Monitoring Platform-1 and S-74, was a NASA satellite launched as part of the Explorer program. Explorer 18 was launched on 27 November 1963 from Cape Canaveral Air Force Station (CCAFS), Florida, with a Thor-Delta C launch vehicle. Explorer 18 was the first satellite of the Interplanetary Monitoring Platform (IMP). Explorer 21 (IMP-B) launched in October 1964 and Explorer 28 (IMP-C) launched in May 1965 also used the same general spacecraft design.
Mission
Explorer 18 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 10:20 hours, a spin rate of 22 rpm, and a spin direction of 115° right ascension and -25° declination. Each normal telemetry sequence of 81.9 seconds 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. The spacecraft performed normally until 30 May 1964, then intermittently until 10 May 1965, when it was abandoned. The principal periods of data coverage were 27 November 1963 to 30 May 1964; 17 September 1964 to 7 January 1965; and 21 February 1965 to 25 March 1965; however, only the first of these periods was very useful.
Experiments
Cosmic-Ray Range versus Energy Loss
A charged-particle, solid-state telescope was used to measure the 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) and charge spectra (Z<=6). The detector was oriented normal to the spacecraft spin axis. The detector accumulators for each energy interval were telemetered six times every 5.46 minutes. Each accumulation period was about 40 seconds (the initial spacecraft spin period was about 2 seconds). 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. A malfunction limited alpha studies to particles with E>30 MeV.
Cosmic Rays
This experiment consisted of two detector systems. The first was a dE/dx versus E telescope with thin and thick Caesium iodide (CsI) scintillators (one each) and an anticoincidence plastic scintillation counter. The telescope axis was normal to the spacecraft spin axis. Counts of particles penetrating the thin CsI scintillator and stopping in the thick CsI scintillator were accumulated during one 39.36-seconds interval every 5.46 minutes. 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. Each telescope consisted of two colinear GM tubes. The parallel and perpendicular telescopes measured, respectively: 1) the sum of counts due to protons above 70 MeV and electrons above 6.5 MeV and 2) the sum of counts due to protons above 65 MeV and electrons above 6 MeV. Counts registered in any one of the four GM tubes were also accumulated. These omnidirectional counts were due to protons above 50 MeV plus electrons above 4 MeV. The parallel, perpendicular, and omnidirectional count rates were obtained for one 40-seconds accumulation interval during successive normal 81.9-seconds telemetry sequences. Thus, any one count rate was measured for 40 seconds once each 5.46 minutes. Both detector systems worked well from launch until 26 May 1964.