There may be a million asteroids with masses far greater than ocean liners in Earth-approaching orbits, nearly all of which telescopes have yet to see. Some of those Near-Earth Objects are potential killers. Those at least 40 meters across could wipe out a metropolis; a kilometer-wide asteroid could devastate part of a continent and shroud the planet in soot. NASA-sponsored searches, mostly from the ground, have found most of the kilometer-size, civilization-threatening asteroids, but only a fraction—perhaps 1%—of the smaller, but still menacing, objects. Convinced that NASA will not finish the job, two former astronauts have founded a private foundation to raise $450 million for a space mission to find asteroids that may threaten our planet.