Abstract DGP2026-13

Accurate radiometric size determination for sub-kilometer asteroids: 2024 YR4 and the planetary defense case

T. Müller (1), E. MacLennan (2), A. Burdanov (3), B. Holler (4), A. Rivkin (5)

(1) Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstraße 1, D-85748 Garching, Germany, (2) Department of Physics, P.O. Box 64, 00560FI-00560 University of Helsinki, Finland, (3) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA, (4) Space Telescope Science Institute, Steven Muller Building, 3700 San Martin Drive, 21218 Baltimore, MD, USA, (5) Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD, 20723, USA

The potentially hazardous Apollo-type near-Earth asteroid 2024 YR4 has very close encounters with the Earth-Moon system approximately every four years. Based on extensive observing efforts in early 2025, it is currently listed with a 4% impact probability for the Moon on December 22, 2032. The optical light astrometric and photometric observations are compatible with an object size ranging from roughly 30 m (high-albedo case) to about 125 m (very low, cometary albedo). According to the UN-endorsed planetary defense framework, objects with sizes around 50 m or larger can cause serious regional damage in the event of an impact. For such cases, the Space Mission Planning Advisory Group (SMPAG) would initiate coordinated activities and consider mitigation strategies. In addition to asteroid's orbit, the size information is therefore crucial for decision makers around the world.

We present our project to determine the size of YR4 via JWST-MIRI mid-infrared observations. The MIRI measurements at 10, 12.8, and 15 µm were obtained on March 26, 2025, when the asteroid was at 1.81 au from the Sun and 1.08 au from JWST, and seen under a phase angle of –28.5°. The µJy fluxes were interpreted with a range of simple to highly sophisticated thermal models. YR4 turned out to be much colder than expected. The combined optical and infrared data are pointing to an object that rotates with a period of about 19 minutes, seen nearly equator-on, where a high thermal inertia transports heat to the nightside, making the entire equatorial zone nearly isothermal. Its roughly 60-meter size places it above the critical threshold for serious planetary defense actions. The current low Earth-impact probability has meanwhile eliminated it from the top of the high-risk object list, however, a 4% impact probability for the Moon remains. Our upcoming JWST measurements in March 2026 might change the impact probabilities again.

The coordinated activities for orbit and body characterization provide many important lessons for future discoveries of potentially hazardous asteroids. Our study focuses on strategies to obtain reliable size information for individual sub-kilometer size objects. The study is also relevant in the context of ESA's planned NEOMIR space mission to discover and study near-Earth objects via infrared imaging.