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

TL;DR

This paper documents the extensive astrometric follow-up of near-Earth asteroid 2024 YR4 during a high-profile Torino level 3 alert, highlighting how a mature, coordinated global community refined the orbit and reduced Earth-impact risk. It details discovery, rapid global follow-up, prediscovery and precovery efforts, and a JWST extension that attempted to further constrain the trajectory, illustrating the crucial role of IAWN in coordinating resources across ground- and space-based facilities. The study reports 504 astrometric observations from 63 stations over six months and emphasizes the value of public image archives for potential precovery, ultimately downgrading the Earth-impact threat while maintaining Moon-impact considerations. The work serves as a practical test-case for planetary defense readiness ahead of future Rubin Observatory surveys and dedicated space missions like NEOMIR and NEO Surveyor.

Abstract

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.

Figures (6)

• Figure 1: Discovery images of 2024 YR4 from the ATLAS station at Rio Hurtado, Chile. The two images, exposed on 2025 December 27 at 05:43 and 05:46 UTC, are stacked here without motion, displaying the asteroid as two distinct sources. The sources are slightly trailed in the motion direction, as expected for a $30\hbox{\rmfamily s}$ exposure of an object moving at $23"/\hbox{\rmfamily min}$. The pixels visible in the image are the native pixels of the camera, each $1.86"$ wide, while the field of view covered by this cropped image is approximately $8' \times 8'$.
• Figure 2: Brightness evolution (V magnitude) of 2024 YR4 from the time of discovery to when it became too faint even for space assets like JWST. Full moon nights, and the time of the IAWN notification, are marked on the plot. The histogram in the background indicates how many observations were reported on each UTC day. Horizontal lines indicate the approximate aperture of facilities that can meaningfully contribute astrometric measurements until that point.
• Figure 3: Stack of 55 images obtained with ESO's VLT on 2025 March 23, for a total of $50\hbox{\rmfamily min}$ of exposure time. The object is the faint circled source a the center of the image. A few additional images, exposed in the same sequence, were removed from this stack because the object was transiting in the vicinity of a bright field star. This is the last ground-based optical detection of 2024 YR4 obtained during its 2024-2025 discovery apparition.
• Figure 4: Number of stars in the NIRCam long wavelength field of view as a function of the observing time, for the observing window used in the 2024 YR4 campaign. The three epochs when the observations were acquired are marked by the vertical red lines.
• Figure 5: Detection of 2024 YR4 from the first integration of the first JWST NIRCam visit, acquired on 2025 March 8 at 21:57 UTC. The effective exposure time is approximately $107 \hbox{\rmfamily s}$. The target is circled in the frame, while the other round sources are cosmic ray hits on the array, which are not removed at processing level since the "jump" processing step was skipped. The brightest source in the frame is an example of a trailed field star.