STS-66 (Atlantis / ATLAS-3)

Background

By the early 1990s, mounting concern over stratospheric ozone depletion and the broader question of how solar energy drives terrestrial climate had pushed Earth-observing science to the forefront of NASA's shuttle manifest. The Atmospheric Laboratory for Applications and Science (ATLAS) programme was designed to answer those questions from low Earth orbit, flying a suite of instruments in the payload bay across successive missions timed to sample different seasons and different phases of the solar cycle. STS-66 carried the third installment of that programme, ATLAS-3, launching aboard Orbiter Atlantis on 3 November 1994. Coming roughly two years after the eruption of Mount Pinatubo had seeded the stratosphere with aerosols, the mission arrived at a scientifically decisive moment: researchers needed to characterise how the ozone column was recovering — or failing to recover — as volcanic debris slowly cleared.

The crew assembled for STS-66 brought complementary depth to a scientifically intensive flight. Commander Donald McMonagle was on his third shuttle mission; Pilot Curtis Brown was flying for the second time. Mission specialists Ellen Ochoa, Joseph Tanner, Jean-François Clervoy, and Scott Parazynski rounded out a six-person team whose responsibilities ranged from around-the-clock instrument operations to the deployment and retrieval of a free-flying satellite. Ochoa, already distinguished as the first Hispanic woman to reach orbit, served as a lead science operator. Clervoy, flying for the French space agency CNES under an ESA assignment, added European expertise to a payload that itself reflected international collaboration.

Payload and Scientific Objectives

ATLAS-3 was not a single instrument but a coordinated ensemble mounted on Spacelab pallets in Atlantis's payload bay. The suite included spectrometers, radiometers, and photometers tuned to measure solar irradiance, trace-gas concentrations, and the thermal emission of the middle atmosphere across ultraviolet, infrared, and millimetre-wavelength bands. The overarching goals were threefold: to quantify the solar energy input that ultimately regulates surface climate; to map the distribution of ozone, water vapour, methane, nitrogen oxides, and other chemically active species in the stratosphere and mesosphere; and to observe how those distributions changed with latitude, longitude, and the progression of Northern Hemisphere autumn.

Complementing the pallet instruments was CRISTA-SPAS — the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere attached to a Shuttle Pallet Satellite. Unlike the fixed ATLAS instruments, CRISTA-SPAS could be released to fly independently, giving it an unobstructed view of the limb in all directions without contamination from the orbiter's own outgassing. The satellite was deployed early in the mission and flew in close proximity to Atlantis for several days before being retrieved by the robotic arm and returned to the payload bay.

The Flight

Atlantis lifted off from Kennedy Space Center's Launch Complex 39B on schedule at the opening of its launch window on 3 November 1994. Within minutes of clearing the tower the crew began configuring the ATLAS instruments for operations, and science data collection was underway before the vehicle had completed its first orbit. The mission flew at an inclination designed to sweep the ground track across a wide band of latitudes, maximising the geographic coverage of the atmospheric measurements.

The deployment of CRISTA-SPAS, accomplished in the mission's first day, was one of the operational centrepieces of the flight. Once free of the payload bay, the satellite used its own attitude-control system to point its cryogenically cooled infrared telescopes at the atmospheric limb, recording the faint thermal emission of molecules at altitudes ranging from the upper troposphere to the lower thermosphere. Atlantis station-kept at a safe distance while the ATLAS pallet instruments continued their own observations in parallel. After the planned free-flight period, the robotic arm captured the satellite and berthed it back in the payload bay, completing the deploy-and-retrieve sequence successfully.

Throughout the mission the crew worked rotating shifts to keep the instruments operating continuously. The science team on the ground at Marshall Space Flight Center's Payload Operations Control Center coordinated with principal investigators from NASA, ESA, and research universities to assess data quality in near-real time and adjust instrument pointing or operational modes when opportunities arose. The disciplined cadence of the operations — calibrations against stellar sources, cross-comparisons between instruments measuring the same atmospheric constituent — was engineered to produce a data set that could be directly compared with ATLAS-1 and ATLAS-2 results, allowing trends to be identified across the series.

Landing and Legacy

After nearly eleven days in orbit, Atlantis executed its deorbit burn and descended through the atmosphere to a runway landing at Edwards Air Force Base in California, closing out a mission that had proceeded with few significant anomalies. The total science return was substantial: the instruments collectively accumulated hundreds of hours of atmospheric measurements spanning a latitudinal range from polar to tropical regions.

The data from STS-66 proved valuable to the atmospheric science community in several lasting ways. Measurements of stratospheric ozone taken in the aftermath of the Pinatubo aerosol loading provided a benchmark against which the effects of the Montreal Protocol's phasedown of chlorofluorocarbons could eventually be assessed. The simultaneous solar-irradiance observations contributed to the long-term climate record that scientists use to separate natural variability from anthropogenic forcing. CRISTA-SPAS, with its ability to produce three-dimensional maps of trace gases at high spatial resolution, demonstrated measurement techniques that influenced the design of later satellite instruments.

STS-66 also reinforced the scientific logic behind flying the same core instrument suite on multiple missions. By preserving continuity across ATLAS-1, ATLAS-2, and ATLAS-3, investigators could distinguish seasonal signals from longer-term trends in a way that no single flight could achieve. The mission thus stands as a demonstration of how systematic, repeat-measurement campaigns — rather than one-off observations — underpin the most durable advances in Earth and atmospheric science.