Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets, is a 1997 book by University of Arizona Planetary Sciences professor emeritus John S. Lewis that describes possible routes for accessing extraterrestrial resources, either for use on Earth or for enabling space colonization. Each issue or proposal is evaluated for its effects on humanity, physics and economic feasibility based on planetary science. For instance, Chapter 5 ("Asteroids and Comets in our Backyard") exhaustively catalogs the types of near-Earth objects (asteroids and extinct comets whose orbits intersect Earth's), assessing both the harms likely from possible collisions with Earth (the subject of Prof. Lewis's previous book, Rain of Iron and Ice) on the one hand, and their potential for profitable exploitat