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. May 29, 1965 at 12:00 UTC · Launch Successful Thor Delta C — United States Air Force, to Elliptical Orbit Space Launch Complex 17B, Cape Canaveral SFS, FL, USA The world's 441st orbital launch attempt, and 48th of 1965. Data: Launch Library 2, The Space Devs. Image by NASA (NASA Image and Media Guidelines) via The Space Devs.