Greater revelations await the New Horizons probe when it flies past Pluto in July. At about 1,440 miles wide, Pluto is the largest known member of the Kuiper Belt, an enormous zone of frozen bodies that extends far beyond Neptune. Over the years, we've learned a few basic details: Pluto has five known moons, and one is more than half its size. It rotates sideways and experiences extreme weather: In summer, the surface ice evaporates into an atmosphere, which then freezes back onto the ground each winter. Wild temperature swings may also trigger icy eruptions.