In a paper posted tonight on the Los Alamos astro-ph website, an astronomer is proposing an early warning system for asteroid impact. Key quote from the abstract:
This system, dubbed “Asteroid Terrestrial-impact Last Alert System” (ATLAS), comprises two observatories separated by about 100km that simultaneously scan the visible sky twice a night, and can be implemented immediately for relatively low cost. The sensitivity of ATLAS permits detection of 140m asteroids (100 Mton impact energy) three weeks before impact, and 50m asteroids a week before arrival. An ATLAS alarm, augmented by other observations, should result in a determination of impact location and time that is accurate to a few kilometers and a few seconds.