In April, Planetary Resources, a newly formed private space company, announced that it would begin mining asteroids for water in 2020. Asteroids, the firm said, could also be a valuable source of platinum-group elements (PGEs), six metals used in industrial chemical reactions and devices such as catalytic converters. Earth contains only four high-grade PGE deposits, and the demand for the metals is increasing. But is there enough information to know where to dig? The basics are clear—how asteroids form, where they are located, and, roughly, what they're composed of—but details are scarce. Upcoming missions, including those by Planetary Resources' own prospecting spacecraft, may fill in some of the holes. But if space mining is to work, prospectors will need much more accurate information about the solar system and its minerals.