In a comment to my ATLAS post, Hop David asked whether the Planetary Resources Arkyd Telescopes (congrats on their successful fundraising efforts, by the way) could serve as an early-warning system, especially on the daylight side of the Earth where ATLAS wouldn’t be able to see asteroids because of the bright blue sky. My response was that they would probably help, but would still have the problem of a solar keep-out zone (also called a solar avoidance zone), where telescopes can’t point close to the sun. Hubble, for example, can’t look within 50 degrees of the sun (though it’s unclear from the Wikipedia text whether that’s a full 50 degree cone (100 degrees around) or a half (25 degree diameter)).
The best place to look for asteroids is INSIDE the orbit you’re worried about. The B612 Foundation’s Sentinel craft will do just that, traveling inside Earth’s orbit on a multiyear survey of Earth-crossing asteroids. By the way, they’re doing a fundraiser for it now (ugh! it ends on July 7th!)…a much more important fundraiser than a nearby art museum expansion. Sentinel, however, will not always be near Earth. Its orbit will take it away from our home planet, and while this is good for the asteroid survey it’s bad for serving as an early warning craft. Another craft placed at the Sun-Earth L1 point would serve as an excellent early warning craft. It might even be able to be the same design as Sentinel, and would require similar (perhaps less) launch ‘kick’ to get to its location. This spacecraft would always face Earth, looking for asteroids that are getting close. The amount of warning time it could provide depends on the specifics of the design, with a plain telescope providing a short warning, and a spinner survey-type craft having a wider field of view that would give a longer warning. Maybe a hybrid of both sensors would be the best.