Rapid-response characterization of near-Earth asteroid 2024 YR4 during a Torino Scale 3 alert

TL;DR

This study documents a rapid, real-world observational campaign for asteroid 2024 YR4, the first object to reach Torino Scale 3, to determine its rotation, phase behavior, and surface composition under an evolving Earth-impact threat. Using multi-instrument time-series photometry, color photometry, and spectroscopy, the authors derive a synodic rotation period of $P = 19.46341 \pm 0.00008$ minutes and a shallow phase slope with $G = 0.51 \pm 0.11$, along with $H_R = 23.82 \pm 0.09$ mag (and $H_V = 24.14 \pm 0.25$ mag after color corrections). Taxonomic analysis points to Sq or K type, with diameter estimates from JWST implying $D \approx 60 \pm 7$ m and $p_V \approx 0.11 \pm 0.05$, illustrating the ambiguities that can arise from limited phase-angle coverage. The campaign demonstrates the feasibility and value of rapid international coordination for planetary-defense readiness, providing critical physical parameters to inform risk assessment and potential mitigation, even as the Earth-risk ultimately subsided. It also highlights remaining needs, such as polarimetric albedo constraints and extended low-phase-angle observations, to reduce ambiguities in composition and size estimates for similar threats in the future.

Abstract

On 27 December 2024, near-Earth object (NEO) 2024 YR$_4$ was discovered by the ATLAS survey and identified as a virtual impactor. A few weeks later, it eventually reached level 3 on the Torino Scale and was the first and only asteroid to be ever classified at that level. Here we report an intensive observational campaign combining time-series photometry in the visible, broadband visible and near-infrared colors, and low-resolution visible reflectance spectroscopy to assess its physical properties. Fourier analysis of the lightcurves yields a synodic rotation period of $P = 19.46341 \pm 0.00008$ min, placing 2024 YR$_4$ among the fast rotators, even if such rotation is common for objects of similar $H$ magnitude. Its visible and near-infrared colors and spectra are most consistent with an Sq or K taxonomic classification, though some ambiguity remains. Finally, its phase curve exhibits a notably shallow slope ($G = 0.51 \pm 0.11$), from which we derive an absolute magnitude of $H_\mathrm{R} = 23.82\pm0.09$ mag. After color correction and taking into account other models for the phase function, we report an absolute magnitude of $H_\mathrm{V} = 24.14\pm0.25$ mag. These characterizations, rotation period, taxonomy, and surface properties, would have been crucial for risk assessment and mitigation planning had the initially high impact probability scenario been confirmed, underscoring the importance for planetary defense of a rapid, coordinated international response.

Figures (8)

• Figure 1: Top row: Lightcurves obtained at the Danish 1.54 m (DK154; W74) Telescope on 3 and 4 January 2025. Bottom panel: Phased lightcurve of the two DK154 datasets according to a rotation period of 19.464 minutes.
• Figure 2: Lightcurve obtained at the 4.3 m Lowell Discovery Telescope (LDT; G37). 21 individual acquisitions of 30 seconds each were obtained spanning a total time of 37 minutes.
• Figure 3: Lightcurve obtained at the 8.2 m Very Large Telescope (VLT; 309). 45 individual acquisitions of 15 s each were obtained spanning a total time of 40 minutes
• Figure 4: Lightcurve obtained at the 1 m Mount Lemmon telescope (I52). 32 individual acquisitions of 8 s each were obtained spanning a total time of 8 minutes
• Figure 5: Phased lightcurve of YR4 according to a period of 19.46341 minutes. The best Fourier series fit of order 5 is plotted as a continuous line.