You can think of our Milky Way galaxy and the neighboring Andromeda galaxy as two giant aircraft carriers accompanied by a flotilla of smaller warships. Those ships in this imaginary battle fleet are dwarf galaxies, a fraction the size and mass of the giant spiral galaxies. Our Milky Way has about 70 known dwarf galaxies, and Andromeda appears to have three times as many. The dwarf galaxies provide clues as to how the Milky Way and Andromeda evolved over billions of years. The satellites tell a markedly different story for each system. Our Milky Way has led a relatively placid life, while it's been a game of bumper cars around Andromeda—including a major collision several billion years ago. In an ambitious observing program, the Hubble Space Telescope was used to inventory all of the known dwarf galaxies surrounding Andromeda.