JWST sighting of decametre main-belt asteroids and view on meteorite sources
February 2025 • 2025Natur.638...74B
Abstract • Asteroid discoveries are essential for planetary-defence efforts aiming to prevent impacts with Earth1, including the more frequent2 megaton explosions from decametre impactors3, 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) population8,9. 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 models10, 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 techniques16, 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 meteorites13,14 'in situ'.
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