Asteroid discoveries are essential for planetary-defense efforts aiming to prevent impacts with Earth¹, including the more frequent² megaton explosions from decameter impactors³⁻⁶. While large asteroids (≥100 km) have remained in the main belt since their formation⁷, 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 models¹⁰⁻¹⁴. 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 peaks¹⁵ and synthetic tracking techniques¹⁶⁻¹⁸. 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 families¹⁹ and the source regions of meteorites^13,14 “in-situ”.

小行星的发现对于旨在防止撞击地球¹ 的行星防御工作至关重要，包括十米撞击器^3-6 更频繁的² 百万吨爆炸。虽然大型小行星（≥100 公里）自形成以来一直留在主带⁷，而小型小行星通常被运送到近地天体 （NEO） 群体^8,9。然而，由于缺乏直接的观测限制，它们的大小-频率分布（这为我们了解近地天体和向地球运送陨石样本提供了信息——在模型^10-14 之间差异很大。在这里，我们报告了在主带中观察到的 138 次最小小行星 （⪆ 10 m） 的探测，这些探测是由 JWST 的红外功能实现的，覆盖了小行星的发射峰¹⁵ 和合成跟踪技术^16-18。尽管轨道弧很小，但我们使用已知的小行星作为代理来限制天体的距离和相位角，从而使我们能够通过辐射测量技术得出大小。它们的大小-频率分布在 ~ 100 m 处出现中断（直径小于和大于 ~ 100 m 时，q = − 2.66 ± 0.60 和 − 0.97 ± 0.14 的去偏累积斜率），表明种群由碰撞级联驱动。这些小行星是从多个小行星家族中采样的——很可能是 Nysa、Polana 和 Massalia——根据这里考虑的指向的几何形状。通过额外的长时间凝视红外观测，JWST 准备在天空中意外探测到数千颗十米级的小行星，在“原地”探测单个小行星家族¹⁹ 和陨石^13,14 的源区。