How Far In Advance Could People See A 100-1500 Meter Earth Bound Impactor Before It Hit

Question

How much advanced notice would we likely have in advance of an Earth bound impactor with a place of origin outside Earth's orbit (e.g. a meteor, asteroid, comet, space debris that doesn't rotate around the Earth) of 100 to 1500 meters in diameter with astronomy technology that currently exists or will exist in the near future (i.e. next decade or so telescopes and computing technology with non-vaporware instrumentation that is already in advanced planning stages)?

What is the range of time frames in which we would likely have notice (presumably there would be some variability)?

What factors would influence how much advanced notice we had of such an impactor with current technology (e.g. direction of approach, current speed, composition of object, etc.)?

Presumably, we would have a more accurate estimate of where it would hit as it got closer at some time after we initially noticed it was heading in the general direction of Earth.

How much notice would we have before we could pin its impact point down to a roughly 150 km radius impact area on Earth?

Assume that nothing but gravity and its current position and velocity influence its route (e.g. not the case of a working space ship), and that there is no "tracker" on the object, or compact radiation source, that makes it easier to pinpoint its location and trajectory.

Answer 1

Based on when we have detected objects in this class in the recent past (last couple years), we might detect a small object like this with centuries of warning and high uncertainty of impact until the last decade or so (because there are so many objects in the Solar System that can perturb orbits, and they're chaotic -- that is, sensitively dependent on initial conditions) -- or we might detect it only by the fireball and explosion when it either breaks up as the air gets thick or strikes the ground or sea still at several kilometers per second.

The problem is, asteroids are dark, and roughly half of those that pose a threat of impact will come from inside Earth's orbit -- more or less "out of the Sun" -- which is half of the sky that's difficult to search with current technology.

Every year, half a dozen or more objects in your size class are discovered after closest approach, at distances ranging from several Lunar orbit distances to a fraction -- and several smaller objects (like the one that exploded over Chelyabinsk a while back) are discovered only by the impact. A similar or slightly greater number are discovered with only weeks, days, or even hours of warning (a couple years ago, a small object was discovered barely before it started to burn in the upper atmosphere; an object that exploded over Africa and littered an area the size of a large city with meteorites).

As things stand, our average warning before impact cannot be counted on to be a positive number of hours, never mind days, weeks, months, or years.