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I'm looking for a science based answer based on current technology. Given that we do commit some small amount of resources and time to searching for stellar objects, with our current technology, how long before impact would we probably detect an asteroid large enough to (possibly) lead to a mass extinction event?

We do have some effort currently spent on tracking things which could impact earth, but I know we have far from complete coverage. A network of various astronomy departments, amateur astronomers, and agencies like NASA do share information, which does increase our chances of spotting things, but our coverage of even the known objects in our solar system is patchy. We have a hard time even keeping track of all the random junk our space programs have deposited in low earth orbit.

Given all this, but also given the fact that an asteroid capable of conceivably wiping out the human race (or at least a sizable majority of our population would have to be pretty large, pretty dense, and traveling at a pretty high relative velocity, how long before impact would modern society probably detect it?

The size of the rock is big enough to blast a pretty significant crater in the earth, probably reshaping a continent in the process and creating a new inland sea (unless it hits the Pacific or something). It is large enough to throw up enough debris to effect people all over the earth (possibly with nuclear winter effects, etc).

Edit: the Chicxulub crater was probably caused by an asteroid 10-15 km in diameter. This impact does not need to be as powerful as that one, but that might provide a rule of thumb for what constitutes an "extinction event" sized asteroid strike.

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    $\begingroup$ there are several issues that can slide this between days or years, it is actually really hard to detect something coming directly at us especially if they are not particularly reflective. We are also not watching much of the sky at a time, if it from outside the solar system we might get no warning at all. $\endgroup$ – John Feb 5 '18 at 7:45
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Minimum times

ATLAS is an automated asteroid early-warning system, designed to detect potential impactors. On their homepage, they claim to provide one day's warning for a 30-kiloton "town killer," a week for a 5-megaton "city killer," and three weeks for a 100-megaton "county killer".

Maximum times

A century or higher. NASA is keeping track of these too. Here are 75 asteroids that have impact probabilities between 1e-3 and 1e-11. At least one of them doesn't have a potential simulated impact until 2880. So we could get warning centuries to millenia in advance, potentially. Incidentally, the shortest time between detection and first potential impact on this list is 3 years (for a 0.5 km diameter asteroid), so there is some evidence that we will have good advance warning.

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You will be able to justify any outcome from no warning to decades. These examples are upper and lower bounds for the answer, anything in between are probabilities, not possibilities.

  • The Chelyabinsk meteor was not detected before impact. We might have missed something with ten times the diameter (and 1,000 times the impact) as well.
  • This list has probable close encounters in the next decades and centuries.
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Problem is not to have a mapping of the possible candidates for your Armageddon meteorite, but to have a reasonable certainty it will actually hit.

Various space agencies are currently tracking all "likely candidates" and thus problem is not spotting a rogue, but fact is space is big and a "close encounter" is very likely to result in a "near miss" (this has happened multiple times in recent years).

All likely candidates have a (relatively) small mass and so they are very easily deflected by larger bodies, including Earth, Moon and all other major bodies.

This means computing actual trajectory of such a body is very difficult and certainty of hit will come only after it has passed the last obstacle (the moon orbit).

In practice we can detect "dangerous" objects with months warning, but reasonable certainty of actual hit is only days before impact.

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    $\begingroup$ Don't forget origin of the object: if it's in the plane of the solar system then we might have a fair idea that it might hit decades in advance. If it shoots in on a weird orbit from north or south, isn't out-gassing and is fairly dark in colour we might not notice it until it's on top of us. $\endgroup$ – Murphy Feb 5 '18 at 16:25
  • $\begingroup$ We are tracking likely candidates--near Earth objects in the ecliptic of the solar system. Something in a highly elliptical orbit out of the plane of the ecliptic and approaching out of the sun would not give much warning at all. An interstellar visitor that grazed the sun and approached us a couple of degrees away from the solar disk (far enough out to not be spotted by occultation, close enough to be lost in the glare so nobody's looking there) might not be spotted until it hit the atmosphere. $\endgroup$ – Loren Pechtel Feb 6 '18 at 1:26

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