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

Asteroid discoveries are essential for planetary-defence efforts aiming to prevent impacts with Earth, including the more frequent megaton explosions from decametre impactors. Although large asteroids (≥100 kilometres) have remained in the main belt since their formation, small asteroids are commonly transported to the near-Earth object (NEO) population. However, owing to the lack of direct observational constraints, their size-frequency distribution (SFD)-which informs our understanding of the NEOs and the delivery of meteorite samples to Earth-varies substantially among models. Here we report 138 detections of some of the smallest asteroids (≳10 metres) ever observed in the main belt, which were enabled by JWST's infrared capabilities covering the emission peaks of the asteroids and synthetic tracking techniques. Despite small orbital arcs, we constrain the distances and phase angles of the objects using known asteroids as proxies, allowing us to derive sizes through radiometric techniques. Their SFD shows a break at about 100 metres (debiased cumulative slopes of q = -2.66 ± 0.60 and -0.97 ± 0.14 for diameters smaller and larger than roughly 100 metres, respectively), suggestive of a population driven by collisional cascade. These asteroids were sampled from several asteroid families-most probably Nysa, Polana and Massalia-according to the geometry of pointings considered here. Through further long-stare infrared observations, JWST is poised to serendipitously detect thousands of decametre-scale asteroids across the sky, examining individual asteroid families and the source regions of meteorites 'in situ'.