Space Shuttle Discovery's STS-39 mission began with a launch from Kennedy Space Center, Florida, on April 28, 1991. This mission was particularly significant as it marked the first unclassified space operation for the United States Department of Defense. Unlike previous missions which often dealt with classified payloads and sensitive military tech, STS-39 focused on military research experiments that could be shared with the public. The flight lasted approximately 7 days, during which the crew executed a series of experiments that included the deployment of satellites and various scientific investigations, particularly in the fields of atmospheric and space physics.

Commander Frederick D. Gregory led a capable crew consisting of Pilot Charles D. Gemar, Mission Specialists John M. Lounge, John K. Lounge, and Payload Specialists. The mission successfully carried out important tests, including the deployment of the Midcourse Space Experiment (MSX), designed to collect data and assess advanced sensor technology for missile detection and tracking systems. This publicly acknowledged mission also fostered a more collaborative approach to military space operations, reducing the veil of secrecy that typically surrounded them. Overall, the mission illustrated the shuttle's multifaceted capabilities in supporting diverse objectives beyond scientific exploration.

As the world was entering a new era of international relations following the Cold War, the unclassified nature of STS-39 allowed the U.S. to engage more openly in the realm of space exploration as well as military advancements. The successful completion of STS-39 contributed to the ongoing evolution of military and defense-related space activities, indicating a shift towards transparency and cooperation in space missions. Furthermore, the mission helped pave the way for future shuttle operations that would increasingly blur the lines between civilian and military objectives in space.