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

TL;DR

This paper reports a rapid observational campaign of asteroid 2024 YR4 during a high-impact risk alert, revealing its physical properties, rotation, and taxonomy to aid planetary defense efforts.

Contribution

It provides the first detailed physical characterization of asteroid 2024 YR4 during a Torino Scale 3 impact alert, emphasizing rapid international response importance.

Findings

Rotation period of approximately 19.46 minutes, classifying it as a fast rotator.

Spectral analysis suggests an Sq or K taxonomic type.

Shallow phase curve slope indicating surface properties.

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.