When rubble-pile asteroid 2008 TC₃ impacted Earth on 7 October 2008, the recovered rock fragments indicated that such asteroids can contain exogenic material^1,2. However, spacecraft missions to date have only observed exogenous contamination on large, monolithic asteroids that are impervious to collisional disruption^3,4. Here, we report the presence of metre-scale exogenic boulders on the surface of near-Earth asteroid (101955) Bennu—the 0.5-km-diameter, rubble-pile target of the OSIRIS-REx mission⁵ that has been spectroscopically linked to the CM carbonaceous chondrite meteorites⁶. Hyperspectral data indicate that the exogenic boulders have the same distinctive pyroxene composition as the howardite–eucrite–diogenite (HED) meteorites that come from (4) Vesta, a 525-km-diameter asteroid that has undergone differentiation and extensive igneous processing^7,8,9. Delivery scenarios include the infall of Vesta fragments directly onto Bennu or indirectly onto Bennu’s parent body, where the latter’s disruption created Bennu from a mixture of endogenous and exogenic debris. Our findings demonstrate that rubble-pile asteroids can preserve evidence of inter-asteroid mixing that took place at macroscopic scales well after planetesimal formation ended. Accordingly, the presence of HED-like material on the surface of Bennu provides previously unrecognized constraints on the collisional and dynamical evolution of the inner main belt.

当碎石小行星2008 TC ₃于2008年10月7日撞击地球时，回收的岩石碎片表明此类小行星可能含有外源物质^1,2。但是，迄今为止，航天器飞行只观察到了大型，整体式小行星的外源性污染，这些小行星无法通过碰撞破坏^3,4。在这里，我们报告了近地小行星（101955）Bennu表面存在米级外生巨石（OSIRIS-REx任务⁵的直径为0.5公里的碎石堆目标，已通过光谱法将其与CM碳质球粒陨石⁶。高光谱数据表明，外力作用巨石具有相同的独特的组合物，辉石作为古铜钙无粒陨石-钙长辉长无粒陨石-古铜无球陨石（HED），该来自（4）的Vesta，已经历分化和广泛火成岩处理的直径525公里的小行星陨石^{7,8- ，9}。交付方案包括将Vesta碎片直接倒入Bennu或间接倒入Bennu的母体，后者的破坏是由内源性和外源性碎片的混合物产生的Bennu。我们的发现表明，碎石小行星可以保留小行星形成结束后宏观尺度上发生小行星间混合的证据。因此，在Bennu的表面上存在类似HED的材料，这对内部主皮带的碰撞和动力学演变提供了以前无法识别的限制。