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| An Explorer 33 launch cover from "Orbit Covers", postmarked at Cape Canaveral with a hand cancel on July 1, 1966. |
Explorer 33 (IMP-D) was a NASA scientific spacecraft launched in 1966, originally intended to become the first U.S. spacecraft to enter lunar orbit. Part of NASA’s Interplanetary Monitoring Platform (IMP) program, it was designed to study the space environment between Earth and the Moon, including the solar wind, cosmic rays, magnetic fields, and radiation—information important for understanding the conditions astronauts would encounter during future lunar missions. Although it failed to achieve lunar orbit, Explorer 33 successfully gathered valuable data on the Earth–Moon environment, contributing to NASA’s preparation for the Apollo missions.
