NASA’s OSIRIS-REx spacecraft has found that Bennu is an active asteroid and is covered in large rocks that will make grabbing a sample more difficult
Source: https://www.newscientist.com/article/2197008-asteroid-bennu-is-spewing-out-dust-and-rocks-to-create-its-own-moons/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home