When I visited Arecibo [...], the intern giving me a tour told me that Arecibo was essentially the main way we had to detect asteroids that might hit earth. Ar

When I visited Arecibo [...], the intern giving me a tour told me that Arecibo was essentially the main way we had to detect asteroids that might hit earth. Arecibo wasn't used for detecting asteroids—it was used to characterize asteroids that were detected by other means. Arecibo observed asteroids via radar. The power necessary for a radar observation goes up very quickly with distance to the target (proportional to distance to the fourth power), so radar would generally be a poor choice for hunting for asteroids.