Abstract : We describe a technique to determine the temperature of a Resident Space Object (RSO) from multiple infrared (IR) bands. The characteristic temperature of an object is the temperature of the Planck function that has the closest least squares fit to the observed irradiance in at least three infrared bands. The characteristic temperature and the effective solid angle are free parameters in a formulation that requires simultaneous minimization, across all bands, of chi-square expressions using modeled irradiances and the measured irradiances and their errors. Solutions are determined from a multi-dimensional Levenberg-Marquardt fitting algorithm. The advantage of this approach is that it provides a single, best-fit solution to the RSO modeled as a gray body radiator. In contrast, a 2-band (color) temperature approach using three or more bands produces different solutions for different band combinations with no objective way of determining which solution is best. We apply this technique to IR measurements of RSOs obtained by the Midcourse Space Experiment (MSX) satellite. The AFRL MSX database of serendipitous RSO observations contains multi-band IR measurements for hundreds of objects, including payloads, rocket bodies, and debris. Using this technique, we have obtained object characteristic temperatures and Infrared Cross Sections (IRCS) under a large variety of phase angle and solar illumination conditions, including eclipse. We examine specific cases in detail. We also compare and contrast results for population groups based on orbit type (LEO - low earth orbit, MEO - medium earth orbit, and GEO - geostationary earth orbit) and other parameters of interest. In addition, we look at a number of cases where color and characteristic temperatures and solid angles have been determined for the same object measurements and show that the characteristic parameters are more consistent with the Planck function model when expressed as their equivalent isophotal emissions.
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