Astrometric follow-up of near-Earth asteroid 2024 YR4 during a Torino scale level 3 alert

The discovery of 2024 YR4 presented the planetary defense community with the most significant impact threat in almost two decades, reaching level 3 on the Torino scale. The community, now mature and well-organized, responded with a global observational effort. Astrometric measurements, forming the basis for orbital refinement and impact prediction, were a central component of this response. In this paper, we present the astrometric data collected by the international community, from the time of discovery until the object became too faint for all existing observational assets, including JWST. We also discuss the coordination role played by the International Asteroid Warning Network, and the importance of publicly available image archives to enable precovery searches.

💡 Research Summary

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The paper documents the comprehensive astrometric campaign undertaken for near‑Earth asteroid 2024 YR4, which briefly reached Torino Scale level 3—the highest impact threat rating observed in almost two decades. Discovered on 27 December 2024 by the Southern ATLAS station in Chile at V≈16.5 mag, the object moved at an angular speed of 23 arcsec min⁻¹, producing slightly trailed images but still allowing the ATLAS pipeline to deliver positions with ≈0.2 arcsec uncertainty. Within three hours the discovery was posted to the Minor Planet Center’s NEO Confirmation Page, prompting rapid follow‑up by the Catalina Sky Survey (Mt. Lemmon), the Moriyama station in Japan, and the iTelescope network in Australia. These early observations employed specialized trail‑fitting software to achieve sub‑0.1 arcsec precision despite the fast motion.

The International Asteroid Warning Network (IAWN) issued its first official potential‑impact notification on 29 January 2025, when 2024 YR4 had faded beyond magnitude 22 and only larger apertures could still provide useful astrometry. Approximately twenty‑four 2‑meter class facilities—including the Magdalena Ridge Observatory, the Nordic Optical Telescope, and the Faulkes Telescope North—contributed tens of measurements per night, maintaining a daily cadence of high‑precision positions (≤0.1 arcsec). During early February the impact probability peaked at about 3 %, temporarily raising the Torino rating to level 2.

From mid‑February onward, the campaign shifted to the world’s largest optical telescopes. The European Southern Observatory’s Very Large Telescope (VLT), Gemini North and South, and the W. M. Keck Observatory supplied astrometry at magnitudes 24–26 with uncertainties better than 0.05 arcsec. These data dramatically reduced the orbital covariance, leading to a rapid decline in impact probability and the eventual dismissal of the 2032 Earth‑impact scenario by late March. The final ground‑based detection was a V=26.5 mag observation with VLT on 23 March 2025.

Recognizing that ground‑based facilities would soon be unable to track the object, the team prepared a JWST program. The proposal, submitted while the impact risk still existed, called for near‑infrared imaging with NIRCam and mid‑infrared observations with MIRI. The goal was to obtain astrometric precision better than 50 mas—far surpassing the ≈200 mas accuracy of the standard JWST pipeline—by re‑processing the images with Gaia DR3 reference stars. The authors discuss challenges such as the limited field of view of NIRCam, the scarcity of Gaia stars in the target region, and the need for careful time‑tagging of fast‑moving objects. Although the JWST observations were ultimately unnecessary because the risk was removed, the exercise provides a valuable template for future space‑based astrometry of very faint NEOs.

A critical component of the effort was the pre‑covery search. The ATLAS team identified an earlier image pair from 25 December 2024, in which one frame contained a faint, heavily trailed detection of 2024 YR4 with an uncertainty of ±1.6 arcsec along the motion direction. This earlier astrometric point extended the observational arc and supplied a strong lever arm for orbit determination, underscoring the importance of publicly available image archives for rapid threat assessment.

The paper emphasizes the coordinating role of IAWN, which facilitated real‑time sharing of observing plans, minimized duplication, and ensured redundancy across sites. By orchestrating the use of limited large‑aperture resources, IAWN maximized coverage during the critical period when the impact probability was highest.

In summary, the study showcases (1) the speed and global scale of modern planetary‑defence response, (2) the effectiveness of high‑precision trail‑fitting and re‑measurement techniques, (3) a tiered observing strategy that transitions from modest to large ground‑based telescopes and finally to space‑based platforms, and (4) the strategic value of pre‑covery data from open archives. The 2024 YR4 episode validates the maturity of today’s planetary‑defence infrastructure and provides a concrete blueprint for handling future threats that reach Torino Scale 2 or higher.