Calling asteroids rocky is a misnomer. Recent space missions have shown them to be surprisingly loose agglomerations of pebbles that can barely hold themselves together gravitationally. And that may explain the phenomenon of asteroid pairs. Reporting online today in Nature, researchers say that asteroids can literally spin themselves apart, as in the simulations above, essentially giving birth. The split happens when sunlight heats the irregular surface of an asteroid unevenly, causing it to begin spinning, like a pinwheel in the wind. Eventually the asteroid rotates so fast that a big chunk breaks off. The researchers have observed 35 asteroid pairs that they think formed in this way, with the offspring less than 60% of the mass of the parent. The resulting pairs don't form binary systems, however: Lacking sufficient gravity from the parent, the newborn asteroid goes its own way, but follows its parent's original orbit.
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