Coordinated Planetary Defence in action: astrometric follow-up of asteroid 2024 YR4

Asteroid 2024 YR4 was discovered on 27 December 2024 by the Chilean station of the ATLAS survey, as a fast-moving magnitude 16.5 object. In less than 24 hours, prediscovery data and immediate follow-up from the Catalina Sky Survey, as well as additional follow-up from other stations, allowed for its designation by the MPC, followed by an initial assessment of its impact threat, which proved non-negligible for a possible impact in December 2032.Regular follow-up continued over the following days, while the object receded from Earth, becoming fainter and slower. During these initial phases, the brightness of the object made it an easy target for small apertures, but at the same time its fast angular motion across the sky introduced additional challenges due to trailing and/or timing precision.By early January the object had been observed by many follow-up facilities, not only for astrometry, but also for physical characterization purposes. These physical characterization datasets, typically consisting of multiple high SNR images from moderately large telescopes, were also made available by the observers for astrometric purposes, and allowed for the extraction of high-precision astrometry, with accurately determined astrometric uncertainties. When incorporated into the orbit determination process, the data significantly improved the orbital accuracy, leading to the noticeable increase in impact probability that brought 2024 YR4 to the attention of the entire world.Once the object had become sufficiently prominent, even larger apertures came into play. By the end of January, the asteroid had crossed into magnitude 22 territory, where most amateur follow-up stations cannot provide adequate follow-up. From then on, increasingly larger professional facilities became the dominant astrometric contributors: multiple 2-meter-class telescopes provided excellent data until early February 2025, followed by larger facilities.Once 10-meter-class telescopes became necessary, the important coordination role of IAWN ensured that active observers with access to these facilities could exchange information about which instruments they were planning to use, and on the timeline of the planned observations. This allowed the community to obtain astrometry from multiple facilities, ensuring resilience to possible station-specific biases, while at the same time optimizing the use of valuable telescope resources. Fortunately, late February observations by some of these large telescopes, including ESO’s VLT, resulted in an orbital improvement sufficient to completely exclude the possibility of an impact with Earth in 2032.A successful proposal to JWST, submitted earlier on by a broad collaboration of astronomers, nevertheless gave the community the opportunity to test the capabilities of the telescope for planetary defense purposes. Observations with both the near-infrared NIRCam and the mid-infrared MIRI instruments were collected in March 2025, and provided a further extension of the astrometric coverage. One additional set of images, pushing the limits of how faint JWST can observe, is still scheduled for May 2025 at the time of this writing: if successful, it will prove the value of JWST as a follow-up asset for very faint asteroids, uniquely capable of detecting threatening asteroids that are otherwise too faint for ground-based telescopes.As of 2025, a non-negligible chance of impact with the Moon in 2032 remains: although no longer a planetary defense threat, 2024 YR4 is still a high priority scientific target, and will likely remain under scrutiny at least until the next observational opportunity in 2028.The case of 2024 YR4 represented a unique opportunity for the community of NEO observers to test their technical and collaborative capabilities in a high-profile situation. The results were overall extremely successful, from an astrometric and dynamical perspective: the object was effectively followed up, ensuring near-constant coverage with limited waste of observational resources.In this talk, we will present the results of this international effort, with a particular focus on the lessons learned while dealing with the specific challenges posed by this target, and how the community can be ready for a future high-profile situation like 2024 YR4.