Near-Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss. The activity manifests itself in the form of ejection events emitting up to hundreds of millimeter- to centimeter-scale particles. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer spacecraft monitored particle activity for a 10-month period that included Bennu's perihelion and aphelion. Novel and classical methods were utilized to detect the particles and characterize their orbital and physical properties. Roughly 30 of the observed particle mass escaped to heliocentric orbit. A majority of particles fell back onto the surface of Bennu after ejection, with the longest-lived particle surviving for 6 days on a temporary orbit. Particle ejection events appear to preferentially take place in the afternoon and evening and from low latitudes, although they can occur at any time or latitude. The reaccumulation of material is biased toward low latitudes resulting in the possible in-fill of craters and growth of Bennu's equatorial bulge. Of the potential mechanisms behind this activity that were investigated in focused studies, meteoroid impacts, thermal fracturing, and ricochet-but not water ice sublimation-were found to be consistent with observations. While phyllosilicate dehydration was not investigated with a focused study, it remains a possible mechanism. These mechanisms are not unique to Bennu, suggesting that many near-Earth asteroids may exhibit activity that has gone undetected thus far. Spacecraft missions with wide-field imagers are encouraged to further characterize this phenomenon. Plain Language Summary Near-Earth asteroid Bennu is the target of the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer asteroid sample return mission. Navigational imagery acquired by the spacecraft showed that pebble-sized particles of rock were being ejected repeatedly from the surface of Bennu. Modified and new techniques were deployed to study these particles and the ejection events that launched them. Some particles escape into space, others temporarily orbit Bennu, and most fall back onto Bennu's surface after being launched. Ejections most often occur in the local afternoon and evening. The most well-characterized potential mechanisms causing the ejection events are cracking of surface rocks due to thermal stress and impacts by meteoroids. Ricochet, when a particle reimpacts the surface and bounces off again, can also occur. Bennu thus belongs to a class of solar system objects known as "active asteroids", meaning those that shed mass from their surface, a characteristic once thought to be reserved for comets. The discovery of Bennu's activity, which is modest enough that it could only have been detected by a nearby spacecraft (not by telescopes), suggests that there may be more active asteroids than previously believed.
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