JWST /MIRI constraints on the nucleus of interstellar comet 3I/ATLAS and the interstellar object size distribution

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ID: 339550
2026
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Abstract
Abstract We analyse James Webb Space Telescope Mid-Infrared Instrument (MIRI) F1800W imaging of comet 3I/ATLAS obtained on 2025 December 22 at heliocentric and JWST-centric distances of 2.38 and 1.80 au. We model the coma and fit an empirical point-spread function (PSF) to a candidate unresolved component using four extraction prescriptions. Assuming beaming parameter η = 1, the Near-Earth Asteroid Thermal Model gives D ≃ 2.2 km; fitting radii of 0.8–1.2 arcsec give 2.14–2.24 km. Varying η = 0.6–2.2 gives 1.78–3.20 km. Combining the thermal diameter with the optical nucleus magnitude gives geometric albedo pV = 0.053 ± 0.020 at η = 1. Using approximate discovery rates and diameters of 0.2, 0.8 and 2.2 km for interstellar objects (ISOs) 1I, 2I and 3I, we estimate mean populations of 13, 0.04 and 0.019 above these respective thresholds within 3 au. An approximate joint Poisson fit gives cumulative size–frequency exponent $q=2.87^{+0.93}_{-1.02}$ (68 per cent credible interval). An illustrative uniform prior over η = 0.6–2.2 gives $q=2.77^{+0.97}_{-1.05}$. These slopes are compatible with collisional-cascade and Jupiter-family comet distributions, but will need to be confirmed in future observations of ISOs.
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Authors Bryce Bolin, Marco Delbò
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Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag2142
URL
https://doi.org/10.1093/mnras/stag2142
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