Science: Experts can't say exactly when the next Earth-bound asteroid will heave into view, says Richard Stone in Science magazine but they are confident that humanity has the tools to defend itself. There are several deflection scenarios; in most, the straightforward objective would be to change an asteroid's speed so that it arrives too early or too late to hit Earth. In an accompanying piece Stone looks at the network of new telescopes will spot thousands of near-Earth asteroids and comets. If one is headed our way, will world leaders be ready to respond?