JWST sighting of decameter main-belt asteroids and view on meteorite sources.

Asteroid discoveries are essential for planetary-defense efforts aiming to prevent impacts with Earth ¹ , including the more frequent ² megaton explosions from decameter impactors ^3-6 . While large asteroids (≥100 km) have remained in the main belt since their formation ⁷ , small asteroids are commonly transported to the near-Earth object (NEO) population ^8,9 . However, due to the lack of direct observational constraints, their size-frequency distribution -which informs our understanding of the NEOs and the delivery of meteorite samples to Earth-varies significantly among models ^10-14 . Here, we report 138 detections of the smallest asteroids (⪆ 10 m) ever observed in the main belt, which were enabled by JWST's infrared capabilities covering the asteroids' emission peaks ¹⁵ and synthetic tracking techniques ^16-18 . Despite small orbital arcs, we constrain the objects' distances and phase angles using known asteroids as proxies, allowing us to derive sizes via radiometric techniques. Their size-frequency distribution exhibits a break at ~ 100 m (debiased cumulative slopes of q = - 2.66 ± 0.60 and  - 0.97 ± 0.14 for diameters smaller and larger than ~ 100 m, respectively), suggestive of a population driven by collisional cascade. These asteroids were sampled from multiple asteroid families -most likely Nysa, Polana and Massalia- according to the geometry of pointings considered here. Through additional long-stare infrared observations, JWST is poised to serendipitously detect thousands of decameter-scale asteroids across the sky, probing individual asteroid families ¹⁹ and the source regions of meteorites ^13,14 "in-situ".
(© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)