Explorer 21 (IMP 2) 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. October 4, 1964 at 03:45 UTC · Launch Successful Thor Delta C — United States Air Force, to Elliptical Orbit Space Launch Complex 17A, Cape Canaveral SFS, FL, USA The world's 363rd orbital launch attempt, and 70th of 1964. Data: Launch Library 2, The Space Devs. Image by NASA (NASA Image and Media Guidelines) via The Space Devs.