"We just don't know," says Yeomans. "No one saw the object prior to impact."

Indeed, there was no warning. The object emerged from darkness, unknown and uncatalogued, and—wham!—before anyone could photograph the body intact, it had become a cloud of debris.

The cloud's chemical composition holds clues to the nature of the impactor. Orton says ground-based observers are now analyzing light reflected from the cloud to figure out what it is made of. "If the spectra contain signs of water, that would suggest an icy comet. Otherwise, it's probably a rocky or metallic asteroid."