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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.