Explorer 20 (IE A, Ionospheric Explorer A, TOPSI, Top Side Explorer or S 48) was designed to measure electron distribution, ion density and temperature, and to estimate cosmic noise levels between 2 and 7 MHz. August 25, 1964 at 13:43 UTC · Launch Successful Scout X-4 — National Aeronautics and Space Administration, to Low Earth Orbit Space Launch Complex 5, Vandenberg SFB, CA, USA The world's 353rd orbital launch attempt, and 60th of 1964. Data: Launch Library 2, The Space Devs. Image by U.S. Air Force (Department of Defense Image Use Policy) via The Space Devs.