Rapid-response characterization of near-Earth asteroid 2024 YR4
On 27 December 2024 the newly-discovered near-Earth asteroid 2024 YR4 was identified as a virtual impactor whose impact probability later pushed it to Torino Scale 3—the first object ever to trigger an International Asteroid Warning Network notification. We performed a multi-facility observing campaign combining time-series photometry (LDT, VLT, CSS), broadband visible–NIR colours (LDT, TNG, VLT/HAWK-I) and low-resolution spectroscopy (GTC/OSIRIS).
A Fourier-series analysis of four independent light-curves yields a synodic rotation period of 19.4633 ± 0.0002 min. The composite reflectance spectrum is best matched by Sq/K-type taxonomy. Phase-curve photometry in the R band gives H_R = 23.8 and an unusually shallow slope (G = 0.50). Combining the recent results from the JWST (D=60 +- 7m) and our determination of the H magnitude, we found that YR4 should display a moderately low albedo that would be more consistent with a K-type classification rather than an S-type.
These results, obtained within weeks of discovery, illustrate the effectiveness of coordinated rapid-response strategies for planetary defence. The precise spin state, taxonomic class and photometric properties derived here would have been critical inputs for impact-probability refinement and mitigation-option studies had the threatening trajectory persisted. 2024 YR4 therefore serves as a touchstone case for future fast-paced characterization of hazardous asteroids.