The Vera C. Rubin Observatory's LSST will discover approximately one to two meter-sized asteroids per year on collision courses with Earth. This doubles the current detection rate. The improvement is real — better optics, deeper survey, Southern Hemisphere coverage that fills the gap left by Northern facilities.
The warning time is 1.57 days.
Chow, Jurić, and colleagues simulated pre-impact observations for 343 meter-sized objects from NASA's fireball database. The typical discovery occurs less than two days before the asteroid hits the atmosphere. Detection and impact are nearly simultaneous events on any human planning timescale. You learn the rock exists and learn where it lands in the same news cycle.
This is not a failure of the telescope. It is a consequence of geometry. Meter-sized objects are faint. They become detectable only when close, and close means fast-arriving. The survey's depth determines how far away it can see a given size; the object's velocity determines how quickly “far away” becomes “here.” For meter-sized impactors, these two numbers conspire to produce a warning window measured in hours to days, not weeks or months.
The objects are too small to cause ground damage — they burn up in the atmosphere as bright fireballs. The scientific value is in the detection itself: characterizing the population, measuring the size distribution, and validating the survey's completeness. But the pattern scales. Larger objects are detectable farther away, buying more time. The 1.57-day warning for meter-sized rocks maps onto weeks for ten-meter objects and months for hundred-meter ones.
The telescope that finds the danger is the same telescope that proves how little time finding it buys.