CIRRIS

CIRRIS. The Cryogenic Infrared Radiance Instrumentation for Shuttle was a sophisticated United States Air Force experiment designed to measure the Earth's atmospheric infrared background from the Space Shuttle. Developed primarily by the Air Force Research Laboratory, it played a pivotal role in advancing the understanding of upper atmosphere phenomena and calibrating space-based sensor systems. Its data proved crucial for both fundamental atmospheric physics and the development of missile defense and surveillance satellite technologies.

Overview

The experiment was conceived to address critical data gaps in the characterization of the infrared environment in low Earth orbit. Prior to its flights, detailed spectral measurements of atmospheric airglow, aurora, and other natural emissions in the thermal infrared range were limited. It represented a major collaborative effort between military research entities like the Air Force Geophysics Laboratory and academic partners, including the University of Michigan and Utah State University. The instrument's design emphasized high spectral resolution and sensitivity to detect faint signals against the cold backdrop of space.

Development and Launch

Development began in the late 1970s under the direction of the Air Force Systems Command. Key engineering challenges involved creating a cryogenic cooling system to maintain the infrared detectors at extremely low temperatures, minimizing instrument self-emission. The sensor was integrated into a Spacelab pallet for shuttle operations. Its first flight occurred on the Space Shuttle Discovery during the STS-39 mission in April 1991, a dedicated Department of Defense flight. A subsequent, modified version flew on the Space Shuttle Endeavour as part of the STS-53 mission in December 1992.

Scientific Objectives and Instrumentation

Primary objectives included mapping the spatial and spectral structure of the infrared atmospheric limb, measuring the radiance of airglow layers, and observing signatures of the aurora australis and aurora borealis. The core instrument was a high-resolution Fourier-transform spectrometer capable of measuring across a broad infrared band. It was complemented by a low-light-level television camera for context imagery and a cryogenic vacuum system engineered by Ball Aerospace. This suite allowed simultaneous spectral and spatial data collection on targets such as the Milky Way and specific celestial objects for calibration.

Missions and Operations

During STS-39, the instrument operated successfully from the shuttle payload bay, conducting planned observations of the Earth's limb and deep space. The crew, including astronauts Michael Coats and Blaine Hammond, managed the experiment from the shuttle cockpit. The STS-53 mission featured an upgraded instrument and focused on different orbital geometries and target sets. Operations required precise pointing of the Space Shuttle using its Reaction Control System to track specific atmospheric regions or stellar fields, with data downlinked in real-time to stations like the White Sands Missile Range.

Key Findings and Scientific Contributions

The experiment provided the first comprehensive spectral maps of the infrared atmospheric limb, revealing detailed structure in the hydroxyl radical airglow and noctilucent cloud emissions. Data significantly improved models of atmospheric radiative transfer, particularly for the mesosphere and thermosphere. Measurements of the aurora provided new insights into energetic particle precipitation. Furthermore, its observations of calibrated infrared sources like Jupiter and Sirius became a gold standard for validating data from other systems, including the Midcourse Space Experiment and Defense Support Program satellites.

Legacy and Impact

The program's legacy is profound in both military and civilian space science. Its datasets remain archived and utilized for modern climate and atmospheric model validation, influencing later missions like the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. Technologically, it demonstrated the feasibility of complex cryogenic experiments on the Space Shuttle, paving the way for instruments on the Hubble Space Telescope and other observatories. The program directly informed the design and operation of next-generation overhead persistent infrared systems critical to United States Space Force operations and global security.