The Discovery and Characterization of Earth-crossing Asteroid 2024 YR4

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We describe observations and physical characteristics of Earth-crossing asteroid 2024 YR4, discovered on 2024 December 27 by the Asteroid Terrestrial-impact Last Alert System. The asteroid has semimajor axis a = 2.52 au, eccentricity e = 0.66, inclination i = 3 . ° 41, and a ∼0.003 au Earth minimum orbit intersection distance. We obtained g-, r-, i-, and Z-imaging with the Gemini South/Gemini Multi-Object Spectrograph on 2025 February 7 and Y and J imaging with the Keck/Multi-Object Spectrometer for Infrared Exploration on 2025 February 12. We measured a g − i spectral slope of 13% ± 3%/100 nm and color indices g −​ r = 0.70 ± 0.10, r − i = 0.25 ± 0.06, i − Z = −0.27 ± 0.10, and Y − J = 0.41 ± 0.10. 2024 YR4 has a spectrum that best matches R-type and Sa-type asteroids and a diameter of ∼30–65 m using our measured absolute magnitude of 23.9 ± 0.3 mag and assuming an albedo of 0.15–0.4. The lightcurve of 2024 YR4 shows ∼0.4 mag variations with a rotation period of ∼1170 s. We use photometry of 2024 YR4 from Gemini and other sources taken between 2024 December to 2025 February to determine the asteroid’s spin vector and shape, finding that it has an oblate, ∼3:1 axial ratio and a pole direction of λ, β = ∼ 42∘, ∼−25∘. Finally, we compare the orbital elements of 2024 YR4 with the near-Earth object population model and find that its most likely sources are resonances between the inner and central Main Belt.