Binary asteroid in Jupiter's orbit may be icy comet from solar system's infancy
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A bound pair of icy comets similar to the dirty snowballs circling outside the orbit of Neptune has been found lurking in the shadow of Jupiter. Astronomers at the University of California, Berkeley, working with colleagues in France and at the Keck Telescope in Hawaii, have calculated the density of a known binary asteroid system that shares Jupiter's orbit, and concluded that Patroclus and its companion probably are composed mostly of water ice covered by a patina of dirt.
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