COLD CLASH Thanks to results from New Horizons, planetary scientists are more convinced than ever that Pluto's precursor collided with a somewhat smaller object early in solar system history. This mashup of ice-encrusted worlds left Pluto spinning on its side and five moons - sizable Charon and four small siblings - orbiting nearby. ILLUSTRATION BY CASEY REED Sk yandTelescope.com December 2016 37