Allison K. Glantzberg

The asteroid 2024 YR4 was discovered in late December 2024, and over the next several weeks as its orbit was refined its estimated probability of an Earth impact in 2032 rose above 3%. Per international protocols, the International Asteroid Warning Network (IAWN) issued a memo alerting the Space Mission Planning Advisory Group (SMPAG) and the United Nations Office of Outer Space affairs of a possibility of an impending impactor [1], the first time these protocols were triggered since they were established in 2014. Based on spectral information and absolute magnitude, the diameter of 2024 YR4 was estimated to be 40-90 m by IAWN. Interestingly, this range bracketed the 50-m diameter above which SMPAG begins mission options planning [2], increasing the importance of an accurate size measurement. Happily, one facility was able to make such a measurement of 2024 YR4 as it receded from the Earth and Sun: JWST.Via Director’s Discretionary Time, we obtained roughly 4 hours of observing time on JWST, which amounts to roughly 10 hours of observatory time when overheads such as slewing, settling, instrument changes, etc. are included. The main goal of the program was to determine the diameter of 2024 YR4 via modeling of mid-infrared spectrophotometry, and so the majority of the observing time utilized the Mid-Infrared Instrument (MIRI), though Near-Infrared Camera (NIRCam) observations were also included both to support the thermal modeling results and to provide astrometry for continued orbital improvements. Because 2024 YR4’s spin period is about 19.5 minutes [3], the MIRI and NIRCam exposures were set to 20 minutes’ length to allow an average measurement to be obtained over an entire rotation period and avoid lightcurve effects.JWST is limited to observations at solar elongations from 85-135°, and 2024 YR4 entered the JWST observing window on 8 March 2025. NIRCam images were successfully taken with the F150W2 and F322W2 filters on that day, though guide star acquisition issues prevented MIRI data from being collected. MIRI observations with the F1280W, F1000W, and F1500W filters were successfully made, along with additional NIRCam images with the F150W2 and F322W2 filters, on 26 March. At this writing, additional measurements are anticipated in May but have not yet occurred.The team very quickly performed thermal modeling on the data, finding the diameter of 2024 YR4 to be 60 ± 7 m, with a 92% chance of being above 50 meters in size. A memo was prepared for IAWN with this result for their use, and delivered to them approximately 30 hours after the data became available to the team. A more technical report was submitted to the Research Notes of the AAS around 24 hours later [4], as a means of rapidly informing the community and the public. In the time since these two communications, the team has continued to work on more sophisticated thermal modeling analysis.In addition to the thermal modeling, astrometric positions for 2024 YR4 were extracted from the NIRCam data obtained on both 8 March and 26 March and passed to the Minor Planet Center. Given the small field of view for NIRCam compared to typical search or follow-up telescopes, it was found that future astrometric campaigns using JWST would benefit from only considering observing times where the field of view containing asteroid of interest included several Gaia stars, rather than depending on kismet.While the impact probability of 2024 YR4 was already decreasing by the time the JWST observations began, obtaining, reducing, and analyzing the data were still worthwhile. The experience in observation design, communication within and outside the planetary defense community, and data analysis gained in early 2025 will be used to help prepare for the next potential impact situation and inform our response. JWST played a key role in obtaining critical information about 2024 YR4, and our lessons learned will help optimize its use, if needed, in the future.We will present an overview of the JWST observations of 2024 YR4, how it might have performed in the hypothetical scenarios discussed at various conferences and tabletop exercises, and how it might best be used in the coming era of Rubin and NEO Surveyor.References: [1] International Asteroid Warning Network, “Potential for Impact of Near-Earth Asteroid 2024 YR4 on 22 December 2032”, [2] SMPAG-RP-003, “Recommended criteria & thresholds for action for a potential NEO impact threat" [3] International Asteroid Warning Network, “2024 YR4 Home”, [4] Rivkin, A. S., et al. "JWST Observations of Potentially Hazardous Asteroid 2024 YR4." Research Notes of the AAS9.4 (2025): 70.

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)

The destructive power of an impacting asteroid is primarily estimated by knowledge of its size. Asteroid 2024 YR4 reached a peak 2032 impact probability with Earth of 3%, motivating a desire to determine its size. Owing to its infrared capabilities, JWST is uniquely suited for such assessment, especially for decameter-scale objects. We used JWST to observe 2024 YR4 and find a diameter of 60 ± 7 m. This size range corresponds to an albedo for 2024 YR4 of 8%–18%, consistent with observation that it is an S-type asteroid (International Asteroid Warning Network, https://iawn.net/obscamp/2024YR4/index.shtml), if at the low-albedo end of that asteroid taxon. Future observations through 2025 May will help further refine the asteroid’s thermophysical model.

The Didymos Reconnaissance and Asteroid Camera for OpNav (DRACO) was the only instrument on board NASA’s Double Asteroid Redirection Test spacecraft. DRACO had a 2628.326 mm focal length Ritchey–Chrétien telescope with a 208.28 mm aperture and a 0.29° field of view (FOV). The camera used a front-side-illuminated complementary metal–oxide–semiconductor (CMOS) sensor, BAE CIS2521F, with 2048 × 2048 illuminated pixels. DRACO worked with an onboard image processor to support autonomous targeting and ensure impact with Dimorphos, the secondary asteroid in the 65803 Didymos system, on 2022 September 26. This paper covers the calibration and in-flight performance of DRACO, including descriptions of ground and in-flight activities, instrument characterization, the calibration pipeline, and delivered Planetary Data System (PDS) products. DRACO exhibited low read noise and dark current that enhanced the instrument’s sensitivity, although the sensor’s nonlinearity and popcorn noise in global shutter mode required careful characterization and compensation to optimize terminal navigation performance. DRACO had a relatively circular point-spread function of less than two binned pixels across the full FOV. The combined calibration efforts achieved an absolute radiometric calibration (in both rolling and global shutter modes) of better than 20% and a relative calibration across an image of <0.2%, despite no formal radiometric requirements. DRACO acquired 259,290 images in flight, which have been delivered to the PDS in both raw and calibrated forms. DRACO’s excellent performance indicates that similar off-the-shelf CMOS detectors would be well suited for future planetary missions in low-radiation environments.

Observations made by Earth-based radar telescopes and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provided compelling evidence for water ice in Mercury's polar craters. In our investigation, we constructed higher-resolution (125 m pixel⁻¹) digital elevation models (DEMs) for four of the largest northernmost craters, Kandinsky, Tolkien, Chesterton, and Tryggvadóttir. The DEMs were leveraged to model solar illumination and the thermal environment, products that were used to identify permanently shadowed regions and simulate surface temperatures. From these models, we predicted the regions of surface stability for ice and volatile organic compounds. These predictions were then compared against the Arecibo radar, Mercury Laser Altimeter (MLA), and Mercury Dual Imaging System (MDIS) data. Our radar analysis shows that areas of high radar backscatter are correlated with areas predicted to host surface ice. Additionally, we identify radar backscatter heterogeneities within the deposits that could be associated with variations in ice purity, mantling of the ice, or ice abundances. The MDIS analysis did not reveal conclusive evidence for ice or volatiles at the surface, while MLA results support the presence of water ice at the surface in these craters. However, evidence for boundaries between the surface ice and low-reflectance volatile organic compounds, as suggested could be present by our models, was inconclusive owing to the limited MESSENGER data in these regions. BepiColombo’s upcoming orbital mission at Mercury has the opportunity to obtain new measurements of these high-latitude craters and test our predictions for the distribution of surface volatiles in these environments.

Mercury’s south polar region is of particular interest since Arecibo radar measurements show many high-reflectance regions consistent with ice deposits. However, current elevation information in Mercury’s southern hemisphere is not sufficient to perform detailed modeling of the illumination and thermal conditions at these radar-bright locations and to constrain properties of the volatiles potentially residing there. In this work, we leverage previously existing elevation maps of Mercury’s surface from stereo-photogrammetry at 665 m pix⁻¹, Mercury Dual Imaging System Narrow Angle Camera images, and Shape-from-Shading tools from the Ames Stereo Pipeline, to provide the first high-resolution topographic maps of the south pole with a resolution of 250 m pix⁻¹ poleward of 75°S. We show that the increased resolution and level of detail provided by our new elevation model allow for a more realistic recovery of illumination conditions in Mercury’s south polar region, thus opening the way to future thermal analyses and for the characterization of potential ice and volatile deposits. We compare both the old and new topographic models to the Mercury Dual Imaging System Narrow Angle Camera images to show the higher level of fidelity with our products, and we assess the improved consistency of derived permanently shadowed regions with reflectance measurements by Arecibo’s antennas.