STS-39 (Discovery)

Background

By the early 1990s, the Space Shuttle program had developed a quiet dual identity. Alongside its celebrated scientific and commercial missions, a significant share of shuttle flights were devoted to payloads originating from the Department of Defense. Most of those missions were conducted under strict secrecy, with crew communications blacked out and payload details withheld from the public. STS-39, which flew aboard orbiter *Discovery* in the spring of 1991, represented a notable departure from that pattern. Although the mission carried DoD-sponsored hardware and pursued objectives of direct interest to the military, it was designated unclassified — a relatively rare distinction that allowed scientists, engineers, and the general public a far clearer view of what the shuttle was actually doing on behalf of national defense research.

The mission assembled a seven-member crew under commander Michael Coats, a veteran astronaut making his third spaceflight. Pilot Blaine Hammond was joined by mission specialists Guion Bluford, Gregory Harbaugh, Richard Hieb, Donald McMonagle, and Charles Veach. Bluford, who had made history in 1983 as the first African American to fly in space, brought considerable experience to the team. The crew was trained to operate a complex suite of instruments around the clock, reflecting the continuous observational demands of the science program.

Objectives and Payload

The scientific heart of STS-39 was a program designated AFP-675, which stood for Air Force Program 675. Its primary purpose was to gather detailed data on the behavior of the upper atmosphere and to characterize the optical and infrared signatures produced by rocket plumes and spacecraft surfaces in the orbital environment. These measurements had both fundamental scientific value and clear strategic relevance: understanding how exhaust plumes from rockets and maneuvering thrusters appear against the background of Earth's atmosphere and the cold of space was essential to improving the discrimination capabilities of early-warning and surveillance systems.

To accomplish these goals, the mission made use of two distinct instrument platforms. The AFP-675 package itself hosted a collection of sensors designed to observe natural and artificial phenomena in the upper atmosphere from within the shuttle's payload bay. Complementing this was the SPAS-II — the Shuttle Pallet Satellite, a deployable free-flying platform developed through German-American cooperation. The SPAS-II was designed to be released from *Discovery* and to observe the orbiter from a distance, allowing sensors to record the infrared, ultraviolet, and visible signatures of the shuttle itself as it performed thruster firings and other maneuvers. This outside-in perspective was essential: no instrument riding inside the orbiter could measure what the orbiter looked like from space.

Additional experiments rounded out the payload, investigating phenomena such as aurora, airglow, and the faint chemical reactions taking place in the near-vacuum surrounding the spacecraft — a region sometimes called the spacecraft's "contamination environment."

The Flight

*Discovery* lifted off from Kennedy Space Center on April 28, 1991. Within the first minutes of flight, after the main engines cut off and the orbiter settled into its working orbit, the crew began activating the AFP-675 instruments. The science timeline was demanding. Because the phenomena under study — atmospheric emissions, auroral activity, and plume signatures — occurred at all hours and could not be scheduled to suit a single shift of operators, the crew divided into rotating teams that kept the instruments operating continuously throughout the mission.

The deployment of the SPAS-II was one of the mission's signature events. Released from the payload bay using the shuttle's robotic arm, the pallet satellite flew in formation with *Discovery* while its sensors recorded the orbiter's signatures. The crew then performed a series of scripted thruster firings and maneuvers, generating the plume signatures the DoD researchers most wanted to study. After its free-flight observation period, the SPAS-II was retrieved and stowed back in the payload bay. The combination of observations from aboard the shuttle and from the released platform gave researchers an unusually complete dataset.

The mission accumulated a substantial time on orbit. The deorbit burn was executed approximately 198 hours and 40 minutes after launch, and *Discovery* touched down at Kennedy Space Center approximately 199 hours and 22 minutes into the flight, completing a mission of just over eight days.

Legacy

STS-39 occupies a distinctive place in the history of the shuttle program's relationship with the defense community. The decision to conduct the mission as an unclassified flight, while unusual for a DoD payload, reflected a recognition that the atmospheric and plume-signature science it pursued had broad applications that extended beyond any single military program. Data gathered during the mission contributed to research programs examining how phenomena in the upper atmosphere might interfere with, or be exploited by, space-based sensor systems — questions that remained relevant long after the Cold War context that originally motivated them had shifted.

For the crew, the mission demonstrated the shuttle's value as a flexible research platform capable of sustained, around-the-clock operations. The use of the deployable SPAS-II to observe the orbiter from outside was a methodologically significant choice, demonstrating that the shuttle itself could serve as a target as well as a carrier — a perspective that yielded data unobtainable by any other means available at the time.

Guion Bluford's participation continued his record of contribution to major shuttle milestones. For several other crew members, including Harbaugh and Hieb, STS-39 represented early flights in careers that would go on to include spacewalks and further critical missions during the shuttle era.

STS-39 stands as evidence that even within the classified corners of the shuttle program, significant and lasting scientific work was being conducted — work that, in this instance, the public was permitted to see.