Originally, astronomers thought that the Chelyabinsk meteor came from a 1.24-mile-wide (2 kilometers) near-Earth asteroid called 1999 NC43. But a closer look at the asteroid's orbit and likely mineral composition, gained from spectroscopy, suggests few similarities between it and the Russian meteor. However, "the composition of [the] Chelyabinsk meteorite that was recovered after the event is similar to a common type of meteorite called LL chondrites," he added. "The near-Earth asteroid has a composition that is distinctly different from this." More generally, Reddy and his colleagues' work showed that it is difficult to make predictions about what particular asteroid could...