STS-56 (Discovery / ATLAS-2)

Background

STS-56 was the second flight of the Atmospheric Laboratory for Applications and Science (ATLAS-2), a pallet-mounted suite of instruments carried in the Space Shuttle's payload bay to study solar radiation and the chemistry of Earth's atmosphere. The mission continued a series of periodic ATLAS flights intended to build a long-term record of the Sun's energy output and correlate it with changes in the stratospheric ozone layer—a scientific priority that had grown considerably more urgent in the years following the discovery of the Antarctic ozone hole. By flying the same complement of instruments on multiple missions spread across the eleven-year solar cycle, researchers hoped to distinguish natural variability in solar output from the effects of human-produced chemicals such as chlorofluorocarbons.

Space Shuttle Discovery was chosen to carry the mission, flying from Launch Complex 39-B at Kennedy Space Center. The five-person crew reflected the increasingly diverse makeup of the astronaut corps in the early 1990s. Commander Kenneth Cameron and Pilot Stephen Oswald provided experienced leadership; mission specialists Michael Foale, Kenneth Cockrell, and Ellen Ochoa rounded out the team. Ochoa, making her first spaceflight, became the first Hispanic woman to fly in space—a milestone that lent the mission an additional layer of historical significance beyond its scientific goals.

Crew and Preparations

The crew trained extensively with the ATLAS-2 payload team and with engineers responsible for SPARTAN-201, a small free-flying satellite stowed in the payload bay. SPARTAN-201 was designed to observe the solar corona and the nascent solar wind in ultraviolet and white-light wavelengths that are blocked by Earth's atmosphere and therefore inaccessible from the ground. Because SPARTAN operated independently of the Shuttle for a period of roughly two days, its deployment and retrieval demanded careful orbital mechanics planning and practiced use of the robotic Canadarm. Ellen Ochoa, who held a doctorate in electrical engineering and had conducted research related to optical processing, was assigned as the primary Canadarm operator for those critical maneuvers—a role that drew on both her technical background and her training as a mission specialist.

The ATLAS-2 pallet itself carried instruments from the United States, Europe, and Japan measuring quantities ranging from solar ultraviolet irradiance and solar spectral flux to trace gases including ozone, water vapor, methane, nitrogen dioxide, and carbon dioxide in the middle atmosphere. The coordinated dataset was designed to feed into international assessments of stratospheric health.

The Flight

Discovery lifted off on 8 April 1993. Within the first nine minutes of flight the Shuttle reached its planned orbital altitude and the crew began configuring the ATLAS-2 instruments for continuous atmospheric observations. The pallet instruments operated largely in an automated mode, requiring crew intervention mainly for calibration sequences and anomaly response, which freed the crew to manage the parallel SPARTAN operations.

Roughly twenty-five hours into the mission, Ochoa used the Canadarm to lift SPARTAN-201 from its support structure and release it into a slightly different orbit, allowing it to drift free of Discovery so its sensitive detectors would be uncontaminated by the Shuttle's own thruster firings and outgassing. For approximately forty-four hours the small satellite pointed its instruments at the Sun, recording data on coronal structures and measuring properties of the solar wind close to its source region. The retrieval required Oswald to maneuver Discovery to a precise rendezvous with the tumbling satellite, after which Ochoa again operated the arm to capture SPARTAN and berth it safely in the payload bay. The deployment and retrieval were executed without significant difficulty, validating SPARTAN-201's design for future reflights.

Throughout the mission the ATLAS-2 instruments accumulated observations across a wide range of solar activity conditions. Scientists on the ground monitored the incoming data in near-real time, allowing them to adjust observation strategies when solar events of particular interest occurred. The nine-day mission provided a substantially larger dataset than the first ATLAS flight, STS-45 in 1992, helping investigators begin to build the multi-year baseline they needed.

After more than nine days in orbit, Discovery's crew performed the deorbit burn at approximately 221 hours and 30 minutes into the mission. The orbiter descended through the atmosphere and landed at Kennedy Space Center roughly thirty-nine minutes later, completing the mission at the launch site and simplifying post-flight access to the science payload.

Legacy

STS-56 reinforced the value of the ATLAS program as a disciplined, repeatable approach to Earth and solar science. By flying the same instruments at different points in the solar cycle, the series generated data that researchers could use to separate solar-driven changes in atmospheric chemistry from anthropogenic trends—a distinction with direct relevance to international policy debates over ozone-depleting substances and the effectiveness of controls established under the Montreal Protocol.

The SPARTAN-201 payload went on to fly additional missions aboard the Shuttle, continuing its investigation of the solar corona and contributing to the scientific groundwork that later informed dedicated solar observatories. The operational techniques refined during STS-56's deployment and retrieval sequences—particularly the choreography between orbiter proximity operations and robotic arm work—were carried forward into subsequent free-flyer missions.

For Ellen Ochoa personally, the mission launched one of the most distinguished careers in American spaceflight. She would go on to fly three additional Shuttle missions, log nearly one thousand hours in space, and eventually serve as Director of NASA's Johnson Space Center, the first Hispanic person to hold that position. Her debut on STS-56 thus marked not only a symbolic milestone but the beginning of a sustained record of achievement.

STS-56 stands as a representative example of the Space Shuttle's role as a platform for systematic, long-duration scientific observation—combining a large attached payload, a free-flying auxiliary spacecraft, and a skilled crew into a mission whose value accumulated over years as its data were compared with those of successive ATLAS flights.