An asteroid the size of the Roman Coliseum crashes into the ocean somewhere on Earth, producing a transient cavity miles across. The spectacular collapse launches tsunamis in all directions, casting the asteroid's impact energy far and wide. Everyone living on these shores may be in for a bad day.
Based partly on the assumption that relatively minor asteroid impacts spawn dangerous tsunamis, the National Research Council has recommended that NASA and the National Science Foundation design and build a Large Synoptic Survey Telescope (LSST) to image the entire sky every 7 days and detect thousands of asteroids to the 24th magnitude, especially those down to 300-m diameter on near-Earth orbits. The LSST will have many astronomical uses apart from asteroid hunting; nevertheless, before $100 million to $200 million gets spent, one must examine the justifications. Are sub-kilometer asteroids really a significant threat? If not, then existing survey telescopes are adequate to catalog −98% of the 1100±100 near-Earth asteroids larger than 1 km diameter in the next few decades. Of these, half have been detected so far, and most of the rest have orbits that are harder to discover from Earth. Perhaps the LSST money should instead be dedicated to a space-borne observatory for bagging “holdouts” larger than a kilometer, those capable of causing global catastrophe.