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

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.

• @ARogueAnt. Thanks. Those are the kinds of factors that I am trying to identify. Commented Nov 29, 2021 at 20:35
• Not sure why you are seeking this answer, but, assuming it is some sort of story based scenario. You may find it more productive if you worked the problem backwards. There are so many variables to consider even outside of pure chance. Find the mass/reflectivity of the impactor, which you have. From there you can work out virtually any trajectory and time frame it could be discovered. It could come directly from the ort cloud or come in though earths orbit around the sun and impact from that direction. It could come from the other side of the sun and blindside us without any warning at all. Commented Nov 29, 2021 at 20:47
• @Gillgamesh The idea is more to figure out a warning time that is within reason and then to back into an impactor that would fit that scenario. Commented Nov 29, 2021 at 22:49
• RIght, but my point is, there are so many variables that any answer is applicable. under the right circumstances your object could be detected as far out as the asteroid belt or as a blinding second sun headed right for you. Depending mainly on 5 things. the objects size, its reflectivity. its relative position of it the earth and sun. And who happens to be looking in its direction at what time. That's ALOT of balls in the air. Interesting to know if there are any ways to detect an impactor that a layperson would be unaware of though? That could narrow your search to a fit answer. Commented Nov 30, 2021 at 12:20

## 1 Answer

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.

• This is helpful. Also, I've linked to what I think you are referencing in your answer, let me know if you are thinking of something else: en.wikipedia.org/wiki/Chelyabinsk_meteor Commented Nov 29, 2021 at 20:34
• Yes, that's the Chelyabinsk bolide I referenced. As I recall, the meteoroid was no more than ten meters across -- and completely undetected before it started to burn in daylight. Commented Nov 29, 2021 at 20:36
• Detection of objects this small is essentially luck. The bigger the object, the better the odds of detection with longer warning, but if its orbit brings it toward Earth from (visually) near the Sun, we probably won't know until "boom". Even if it comes in the other way (so it continually lit), a collision course means it moves very little or not at all against the star field, reducing odds of detection by telescopes on Earth or in LEO. Commented Nov 29, 2021 at 20:40
• There is a planned mission called NEO to park a telescope at L1 to have a better chance of seeing objects coming in out of the sun. Commented Nov 29, 2021 at 20:56
• @ohwilleke Question goes up to 1500 meters, not 1500 kilometers. Objects smaller than Eros and Bennu. Commented Nov 30, 2021 at 12:02