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Description
The thermal emission from an asteroid is a consequence of its surface temperatures, and the object’s size can be directly estimated from observations in the infrared. Accurately estimating the surface temperature distribution, which depends on several factors, can improve the precision of these size measurements. Key factors include the asteroid’s shape, spin, and thermophysical properties such as thermal inertia and surface roughness. Thermophysical models (TPMs) calculate surface temperatures based on shape and spin parameters, incorporating subsurface heat conduction and small-scale topographic effects (i.e., roughness) that cause shadowing and self-heating effects that influence the surface temperatures. When the asteroid’s brightness is measured or estimated, its albedo can also be derived alongside its size from infrared observations. Additionally, depending on the data quality and observing conditions, the thermal inertia and surface roughness can be constrained to some degree.
We present new thermal infrared observations of the potentially hazardous near-Earth asteroid (1566) Icarus, obtained using the MIRSI instrument at NASA's IRTF during a close approach to Earth in June 2024. The thermal emission of Icarus was too weak to be detected in single, background-subtracted MIRSI frames, but optically bright enough at visible wavelengths to be tracked with the MIRSI Optical Camera (MOC). By employing blind stacking of frames acquired over several hours on three separate nights, we were able to detect and measure its thermal emission at 10-microns. Using a TPM along with pre-existing shape and spin parameters, we estimate the asteroid’s size and surface thermophysical properties. We also obtained simultaneous absolute optical photometry, enabling us to estimate Icarus's albedo and update its shape model.