The wreckage barely hurries. Boulders up to 22 feet across, dislodged when NASA's DART spacecraft struck the asteroid Dimorphos, drifted away at roughly half a mile per hour, according to Hubble observations. Slow does not mean invisible. Against the darkness, the telescope identified 37 rocks among the faintest objects it has ever photographed within our solar system.
Deflection leaves loose ends. DART tested kinetic impact, a method that transfers a spacecraft's momentum to an asteroid, and shortened the time Dimorphos takes to orbit its larger companion, Didymos, by about 33 minutes. The rocks complicate that picture. Their estimated sizes, ranging from roughly three to 22 feet, suggest that the collision did more than produce the conspicuous dust plume seen after impact. These were probably passengers. Researchers suspect the boulders were already resting on the asteroid's surface, rather than being chunks freshly broken from its interior, although the precise launch mechanism remains uncertain.
Faintness carries scientific weight. Because these rocks reflect so little sunlight, detecting them gives researchers evidence about ejecta, the material expelled by an impact, that brighter images of the dust cloud cannot fully supply. Gravity sets the terms. Around a small body with weak gravitational attraction, material can escape at speeds that would seem leisurely on Earth, leaving a dispersed population whose movement helps constrain what happened at impact. Success has a shadow. Beside the measurable change in an asteroid's orbit hangs a quieter image: stones large enough to dwarf a person, departing almost without apparent haste.
