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| An Explorer 28 launch cover from "SpaceCraft Swanson", postmarked at Cape Canaveral with a hand cancel on May 29, 1965, signed by Paul Butler, the Project Manager for Explorer 28. |
Explorer 28 (IMP-C) was a NASA Interplanetary Monitoring Platform launched on May 29, 1965. It was particularly relevant to NASA's developing lunar program because understanding the radiation environment and solar activity in cislunar space was important for future human missions. NASA's stated objectives for the IMP-C mission specifically included studying the radiation environment of cislunar space and developing solar-flare prediction capabilities for Apollo. Explorer 28 operated successfully for almost two years, transmitting scientific data until May 1967, with the spacecraft eventually reentering Earth's atmosphere in July 1968.
