The discovery and characterization of Earth-crossing asteroid 2024 YR$_4$

We describe observations and physical characteristics of Earth-crossing asteroid 2024 YR$_4$, discovered on 2024 December 27 by the Asteroid Terrestrial-impact Last Alert System. The asteroid has semi-major axis, $a$ = 2.52 au, eccentricity, $e$ = 0.66, inclination $i$ = 3.41$^{\circ}$, and a $\sim$0.003 au Earth minimum orbit intersection distance. We obtained g, r, i, and Z imaging with the Gemini South/Gemini Multi-Object Spectrograph on 2025 February 7 and Y and J imaging with the Keck/Multi-Object Spectrometer For Infra-Red Exploration on 2025 February 12. We measured a g-i spectral slope of 13$\pm$3 $\%$/100 nm, and color indices g-r = 0.70 $\pm$ 0.10, r-i = 0.25$\pm$0.06, i-Z = -0.27 $\pm$ 0.10, and Y-J = 0.41 $\pm$ 0.10. 2024 YR$_4$ has a spectrum that best matches R-type and Sa-type asteroids and a diameter of $\sim$30-65 m using our measured absolute magnitude of 23.9 $\pm$ 0.3 mag, and assuming an albedo of 0.15-0.4. The lightcurve of 2024 YR$_4$ shows $\sim$0.4 mag variations with a rotation period of $\sim$1170 s. We use photometry of 2024 YR$_4$ 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, $\sim$3:1 a:c axial ratio and a pole direction of $λ$, $β$ = $\sim$42$^{\circ}$, $\sim$-25$^{\circ}$. Finally, we compare the orbital elements of 2024 YR$_4$ with the NEO population model and find that its most likely sources are resonances between the inner and central Main Belt.