An Early Warning System for Asteroid Impact
  Earth is bombarded by meteors, occasionally by one large enough to cause a
significant explosion and possible loss of life. Although the odds of a deadly
asteroid strike in the next century are low, the most likely impact is by a
relatively small asteroid, and we suggest that the best mitigation strategy in
the near term is simply to move people out of the way. We describe an "early
warning" system that could provide a week's notice of most sizable asteroids or
comets on track to hit the Earth. 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. In addition to detecting and warning of approaching asteroids, ATLAS
will continuously monitor the changing universe around us: most of the variable
stars in our galaxy, many micro-lensing events from stellar alignments,
luminous stars and novae in nearby galaxies, thousands of supernovae, nearly a
million quasars and active galactic nuclei, tens of millions of galaxies, and a
billion stars. With two views per day ATLAS will make the variable universe as
familiar to us as the sunrise and sunset.
