This spacecraft carried instrumentation for the study of energetic particle fluxes and spectra and the resulting dose rates. A major objective of the experiment package was to obtain data with which to check approximations made in theoretical dose calculations. The spacecraft had a slowly varying tumble period of tens of seconds. Spacecraft performance initially was normal. However, the onboard clock and the tape recorder failed on December 1, 1965, and on January 13, 1966, respectively. Limited real-time operations were carried out until total spacecraft failure in April 1967.