Item

Abstract

Most asteroids are irregularly shaped celestial bodies tens of kilometers in size or smaller that orbit the Sun inside the orbit of Jupiter. They are mostly rocky and have a wide range of compositions. This distinguishes them from comets, which originate further out in the solar system and contain a large fraction of ice that turns into gas when heated by the Sun and leads to the formation of the characteristic dust tail. On page 1217 of this issue, Lauretta et al. (1) found centimeter-sized debris ejected from the surface of the likely ice-free asteroid 101955 Bennu in images taken by NASA's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft. The cause of this particle ejection remains somewhat puzzling, but the observations show that asteroids are far from being inert bodies (see the figure).