Explorer 19 was the second in a series of 3.66-m inflatable spheres placed into orbit to determine atmospheric densities. December 19, 1963 at 18:49 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 291st orbital launch attempt, and 67th of 1963. Data: Launch Library 2, The Space Devs. Image by U.S. Air Force (Department of Defense Image Use Policy) via The Space Devs.