Asteroid discoveries are essential for planetary-defence efforts aiming to prevent impacts with Earth1, including the more frequent2 megaton explosions from decametre impactors3-6. 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 models10-14. 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 techniques16-18. 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'.

Astrobiology Research Unit University of Liège Liège Belgium

Department of Astronomy and Planetary Science Northern Arizona University Flagstaff AZ USA

Department of Earth Atmospheric and Planetary Sciences Massachusetts Institute of Technology Cambridge MA USA

Department of Medical Physics and Acoustics Carl von Ossietzky University of Oldenburg Oldenburg Germany

ESA PDO NEO Coordination Centre Frascati Italy

Faculty of Mathematics and Physics Astronomical Institute of Charles University Prague Czech Republic

Florida Space Institute University of Central Florida Orlando FL USA

Leiden Observatory Leiden University Leiden The Netherlands

Lowell Observatory Flagstaff AZ USA

Max Planck Institut für Astronomie Heidelberg Germany

Max Planck Institut für extraterrestrische Physik Garching Germany

Space Science and Astrobiology Division NASA Ames Research Center Moffett Field CA USA

Space sciences Technologies and Astrophysics Research Institute University of Liège Liège Belgium

Tycho Tracker Parrott's Studio LLC Oklahoma City OK USA

Université Paris Saclay Université Paris Cité CEA CNRS AIM Gif sur Yvette France

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