Coordinated Planetary Defence in action: orbit determination and impact monitoring of asteroid 2024 YR4

Asteroid 2024 YR4 was discovered on 27 December 2024 by the ATLAS survey, and within days, preliminary orbit determinations revealed a non-negligible probability of Earth impact in December 2032. Less than a month after discovery, the impact probability (IP) had increased significantly, eventually reaching 1% which caused the Torino Scale (TS) to raise to 3, the highest value recorded post-Apophis. This critical threshold triggered immediate coordination among international centres, including ESA’s NEO Coordination Centre (NEOCC), NASA’s CNEOS, and NEODyS, and prompted the International Asteroid Warning Network (IAWN) to issue an official alert to the Space Mission Planning Advisory Group (SMPAG). The rapid evolution of the scenario, together with the unusually high TS, made 2024 YR4 a unique and instructive test case for the global planetary defence infrastructure.

In this contribution, we present a detailed overview of the methodologies employed to monitor and quantify the threat posed by YR4, with a particular focus on dynamical modelling and impact monitoring techniques. Our operational workflow relied on continuous updates to the orbital solution, as new astrometric data became available, and on a systematic treatment of observational uncertainties. Particular care was taken to ensure the stability and consistency of the predictions in the presence of heterogeneous data inputs.

We show the temporal evolution of the IP and the associated uncertainty region, highlighting key decision points where the estimated risk crossed critical thresholds and needed renewed assessments. The first critical moment occurred on 27 January, when the IP reached 1.3%, causing the Torino Scale rating to rise to level 3. A subsequent increase led to a peak IP of approximately 3% on 18 February, but two days later the impact probability began to decrease, and on 8 March, the potential Earth impact scenario for 2032 was formally excluded. This sequence of events illustrates the dynamic nature of impact risk evaluation and the importance of closely tracking the evolution of uncertainty over time to support timely and well-informed decisions. Special attention is devoted to the characterization of the uncertainty region at various epochs, which played a central role in understanding the risk dynamics and guiding response strategies. The assessment also included future statistical projections based on the observability conditions, which were primarily used to understand the expected evolution of the risk over time and to anticipate how the IP might respond to future observations. Particular attention was devoted on the role on the expected contribution of James Webb Space Telescope observations, that were later performed in mid-March.

An additional aspect emerged during the monitoring phase: in addition to the impact probability with Earth, a non-negligible probability of impact with the Moon was also identified. This lunar risk is currently under assessment and continues to be monitored as new data become available, adding an additional dimension to the case, and illustrating the broader scope of modern impact monitoring efforts.

We also reflect on the coordination between centres during the high-alert phase, which included regular exchanges of orbital solutions, comparisons of uncertainty regions, and extensive discussions on the treatment and weighting of observational data. These exchanges were facilitated by timely sharing of information from a diverse set of observing stations and by a collaborative spirit aimed at ensuring coherent and robust impact monitoring. Particular effort was devoted to identifying and mitigating the influence of outliers or biased data points in the astrometric input, thereby ensuring convergence among independent analyses. Despite some differences in methodology and implementation, the independent results aligned closely, ultimately supporting a consistent assessment that led to the exclusion of any 2032 Earth impact scenario.

This case represents a valuable opportunity to evaluate and reflect on our operational procedures in a real-world near-crisis context. It underscores the importance of transparent, multi-centre collaboration, timely data sharing, and robust modelling capabilities for maintaining an effective planetary defence strategy. We conclude by outlining the main lessons learned and suggesting possible enhancements to current monitoring frameworks, including the integration of systematic lunar impact risk evaluation and improved coordination mechanisms for rapid and effective information dissemination within the global Planetary Defence Community.