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 Earth¹, including the more frequent² megaton explosions from decametre impactors^{3, 4, 5–6}. Although large asteroids (≥100 kilometres) have remained in the main belt since their formation⁷, small asteroids are commonly transported to the near-Earth object (NEO) population^8,9. 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^{10, 11, 12, 13–14}. 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 asteroids¹⁵ and synthetic tracking techniques^{16, 17–18}. 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 families¹⁹ and the source regions of meteorites^13,14 ‘in situ’.