The Milky Way has enjoyed a relatively quiet history in recent eons, but newcomers continue to stream in. Stargazers in the Southern Hemisphere can spot with the naked eye a pair of dwarf galaxies called the Large and Small Magellanic Clouds. Astronomers long believed the pair to be our steadfast orbiting companions, like moons of the Milky Way. Then a series of Hubble Space Telescope observations between 2006 and 2013 found that they were more like incoming meteorites. Nitya Kallivayalil, an astronomer at the University of Virginia, clocked the clouds as coming in hot at about 330 kilometers ...