When the formation of the Moon and the phase of giant impacts had mostly run its course, a final round-up of the remaining planetesimals took place. Gravitational forces exerted by the eight planets scoured interplanetary space: some planetesimals were flipped into the Sun, others thumped into terrestrial or giant planets, and the remainder were expelled to interstellar space by gravitational slingshot effects. Unsurprisingly, gravity’s purge of the solar system was not an entirely clean sweep. Comets, asteroids, meteors (meteorites once they make the journey through Earth’s atmosphere) and mere specks of dust lingered. In our age, this debris has become an indispensable archival source of data for revealing how, over billions of years, the elements have been sieved and sorted for distribution throughout the solar system. Detailed investigations of the composition of asteroids and meteorites became a hot area of planetary science research at the beginning of the present century. In 2015, the NASA spacecraft Dawn began a three-year survey mission to Ceres, the largest asteroid.