of low-energy trapped and auroral protons and electrons. It employed a 5-mg-thick powder phosphor scintillator with a 1000-A aluminum coating. Additional absorbers were inserted in the detector aperture by a 16-position stepped wheel. The aperture was pointed at 45° to the spin axis. Due to the thinness and type of phosphor, the detector in the pulse mode would respond only to low-energy ions, and, therefore, essentially measured the flux of protons that penetrated the absorbers and stopped in the phosphor. Both the pulse counting rate and the phototube current were telemetered once each frame period. Sixteen readings were telemetered in each wheel position, and thus one complete set of data was obtained every 256 frames (one wheel revolution = 80 s). Protons in seven energy ranges were measured. The high-energy limit was about 10 MeV for all ranges, and the low-energy cutoffs were 97, 125, 168, 295, 495, 970, and 1700 keV. The energy fluxes of electrons in three ranges were measured separately using scatter geometry, absorbers, and the phototube current. The low-energy cutoffs were 17, 33, and 75 keV, and the high-energy cutoff was about 100 keV for all three ranges. Solid-State Electron Detector Trapped electrons and protons in the earth's Van Allen belts were measured using a combination of six omnidirectional and directional solid-state particle detectors (silicon p-n junctions). Electrons were analyzed in the energy ranges E>1 MeV, E>3.5 MeV, and E>2.5 MeV with the three omnidirectional detectors (E1, E2, E3), and in the ranges E>0.3 MeV, E>0.45 MeV, and E>1.7 MeV with the three directional detectors (E5, E6, E7). Protons were analyzed in the energy ranges E>10 MeV, E>27 MeV, and E>21 MeV with the omnidirectional detectors, and in the ranges E>1.5 MeV, E>5.0 MeV, and E>16 MeV with the directional detectors. Species discrimination was not always possible. Omnidirectional data were accumulated and telemetered every 1.43 s. Directional data were accumulated for 0.145 s and telemetered every 0.29 s. The spacecraft spin period increased from 0.03 min to 0.5 min during the spacecraft life. Proton data were primarily useful in identifying proton contamination of electron counting rates. The instrument behaved well throughout the spacecraft life. Launch Explorer 26 was launched from Cape Canaveral (CCAFS) at Cape Canaveral Space Launch Complex 17 (LC-17A) on a Thor-Delta C launch vehicle on 21 December 1964 at 09:00:03 GMT. Mission The spacecraft systems functioned well, except for some undervoltage turnoffs, until