A Thermophysical Model Study for 2024 YR4 based on JWST/MIRI Measurements

The Apollo-type near-Earth asteroid (NEA) 2024 YR4 was discovered on 25 December 2024, only a few hours after it had passed Earth at a distance of approximately 2.2 lunar distances. In December 2028, it will again approach the Earth-Moon system, this time at a distance of about 20 lunar distances. As of this writing, it has a 3.8% impact probability with the Moon on 22 December 2032.The orbit of 2024 YR4 is highly eccentric and has a low inclination (semi-major axis a = 2.52 au, eccentricity e = 0.66, inclination i = 3.4°), crossing the orbits of Earth (perihelion at 0.85 au), Mars, and even extending beyond the main asteroid belt (aphelion at 4.18 au). The object is estimated to be about 60 ± 7 meters in diameter (Rivkin et al. 2025a), with a rotation period of approximately 19.5 minutes, based on lightcurves showing ~0.4 mag variations (P. Pravec, SBO-NASA mailing list, 03-Jan-2025). An initial lightcurve inversion analysis by Bolin et al. (2025) suggests an oblate shape with an axial ratio of ~3:1 and a spin axis oriented towards (l, b) = (~42°, ~−25°). Its measured color indices are consistent with S-type or L-/K-type asteroids.To date, no radar observations or successful occultation measurements have been obtained, and the size estimate is based solely on a simple radiometric analysis using single-epoch, three-band JWST/MIRI observations.We conducted a combined JWST/NIRCAM and MIRI multi-epoch observing campaign (Rivkin et al. 2025b). Here, we re-analyzed all available MIRI imaging data from March and May 2025 using advanced post-processing and filtering techniques. Our flux extraction and uncertainty estimates were validated against faint calibration stars and then applied consistently across various data products: from single calibrated integration images and per-dither images to combined 4-dither, multi-integration mosaics per band. The observations on March 26, 2025 (r = 1.81 au, D =1.08 au, a = 28°) in filters F1000W, F1280W, and F1500W covered one full rotation of the asteroid, while the May F1000W observations (YR4 located at the inner asteroid belt region beyond 2 au, but still seen under a similar phase angle of about 28°) captured three full rotations.We applied simple thermal models, including the Near-Earth Asteroid Thermal Model (NEATM; Harris 1998) and the Fast-Rotation Model (FRM; Lebofsky et al. 1978), to derive estimates of size, albedo, and beaming parameter. In parallel, we used thermophysical modeling (TPM; Lagerros 1998; Delbo et al. 2015, and references therein) to test various spin and shape solutions (Bolin et al. 2025; MacLennan et al. 2025), constrain thermal properties, and refine the size–albedo solution. The final TPM results are discussed in the context of other decameter-scale asteroids (e.g., Burdanov et al. 2025).Our analysis also provides a basis for predicting future JWST observing opportunities for YR4 and offers improved constraints on non-gravitational forces acting on the asteroid, which are relevant for long-term orbit predictions and impact risk assessment.Acknowledgement:P.P. has been supported by the "Praemium Academiae" award by the Academy of Sciences of the Czech Republic, grant AP2401.References:Bolin, B.T., Hanuš, J., Denneau, L., Bonamico, R., Abron, L.-M., et al., The Discovery and Characterization of Earth-crossing Asteroid 2024 YR4, The Astrophysical Journal Letters, 984, L25, 12 pp (2025) Burdanov, A. Y., de Wit, J., Brož, Müller, T.G., Hoffmann, T., et al., JWST sighting of decametre main-belt asteroids and view on meteorite sources, Nature, 638, 74 (2025) Delbo, M., Mueller, M., Emery, J., Rozitis, B, Capria, M.T., Asteroid Thermophysical Modeling, in Asteroids IV, Patrick Michel, Francesca E. DeMeo, and William F. Bottke (eds.), University of Arizona Press, Tucson, 895 pp, ISBN: 978-0-816-53213, p107-128 (2015) Harris, A.W., A Thermal Model for Near-Earth Asteroids, Icarus 131, 291-301 (1998) Lagerros, J.S.V, Thermal physics of asteroids, PhD Thesis, Uppsala University, 377L (1998) Lebofsky, L.A., Veeder, G.J., Lebofsky, M.J., Matson, D.L., Visual and Radiometric Photometry of 1580 Betulia, Icarus, 35, 336-343 (1978) MacLennan, E., et al., Shape and Spin Properties of 2024 YR4 from Multi-filter Lightcurve Observations, EPSC/DPS 2025 (2025) Rivkin, A.S., Müller, T.G., MacLennan, E., Holler B., Burdanov, A., et al., JWST Observations of Potentially Hazardous Asteroid 2024 YR4, Research Notes of the AAS, 9, 70 (2025a) Rivkin, A.S., et al., JWST MIRI and NIRCAM Observations of Potentially Hazardous Asteroid 2024 YR4, EPSC/DPS-2025 (2025b)