[Ankaios]

There have been many studies about what could be done to deal with asteroids that are on track to impact earth and the simple answer is 'nothing'.

The National Academies report that you link to down-thread did not state that nothing can be done. They discuss multiple options, and they discuss which options would be most effective given different amounts of warning lead time. Figure 5.5 (https://nap.nationalacademies.org/read/12842/chapter/7#85) is a great high-level view of the option space.

A real message to take away from the "Mitigation" section of that report is that it's important to identify dangerous objects as early as possible.

[jacquesm]

No, the lead author is on the record as saying exactly that. The report is a fantastic read and chapter 5 (the mitigation section) leaves little doubt about the futility of any attempts to mitigate such a threat. Your chances of making things worse are just as large as making things better and given the masses involved your 'window of opportunity' is very likely going to be preclude doing something about an impact that you are 100% sure is going to happen. Of course if you want to feel good about this that's fine with me but this is just blunt physics, an impactor that weighs 8.4 billion metric tons (500 meter asteroid) isn't going to respond to much of anything that we do to it in the time between detection and the time that it will intersect Earth's orbit. And smaller ones are likely to go undetected right up to the moment that they hit.

The Siberian one was a very nice illustration of how completely blindsided we were and now that Arecibo is gone we have lost one very powerful tool in our arsenal that could have helped with this.

But don't let it ruin your day, the chances of this happening are very low, one in a million or larger.

Here is some more recent stuff on this subject:

https://earthsky.org/space/dart-mission-deflected-asteroid-u...

(Doesn't make a huge difference but gives experimental validation of their previous theories.)

Some choice quotes from the report:

" As addressed in Chapter 5, the time required to mitigate optimally (other than only via civil defense) is in the range of years to decades, but this long period may require acting before we know with certainty that an NEO will impact"

"The amount of destruction from an event scales with the energy being brought by the impacting object. Because the range of possible destruction is so huge, no single approach is adequate for dealing with all events. For events of sufficiently low energy, the methods of civil defense in the broadest sense are the most cost effective approach for saving human lives and minimizing property damage.[+] For larger events, changing the path of the hazardous object is the appropriate solution, although the method for changing the path varies depending on the amount of advance notice available and the mass of the hazardous object. For the largest events, from beyond global catastrophe to events that cause mass extinctions, there is no current technology capable of sufficiently changing the orbital path to avoid disaster."

[+] So, in the case of say the Siberian meteor if we had seen it coming (which we did not) you could have called on all the people in a 100 km (1/20th of a second of travel!) radius or so to go to the nearest shelter. This likely would have caused more injuries and casualties than the event itself did, but if the impact had been a bit more steep and closer to a city (or even in a city) then it may well have saved (some) lives. Note that that was only 18m across, was going close to 70 K km/hour, weighed 9000 tons and that it exploded nearly 30 Km up in the air.

"Finding: No single approach to mitigation is appropriate and adequate to fully prevent the effects of the full range of potential impactors, although civil defense is an appropriate component of mitigation in all cases. With adequate warning, a suite of four types of mitigation is adequate to mitigate the threat from nearly all NEOs except the most energetic ones."

Note the careful qualifications, 'adequate warning' does a lot of heavy lifting there.

Pages 70 and onwards are pretty realistic and I think that the table really tells it all, none of the methods outlined are going to be practical given realistic times of warning and the kind of effect that you would have to create to make a meaningful difference in the outcome. Unless you happened to be able to pinpoint the trajectory with extreme precision and you had plenty of time and the impactor would be small enough. But that's playing the lottery. 'Civil defense' is code for 'shelter and evacuation', but assuming we're talking about an impact the size of the one that we are talking about here (400 meters, a couple of hours notice) utterly futile, especially if you don't know exactly where it is going to come down, you might end up moving people in the wrong direction, besides the mass panic. I don't want to be overly pessimistic but I'm with Jewitt in the sense that I do not believe we are geared up to deal with a challenge at that level.

Here it is in his own words in case you don't believe me:

https://youtu.be/4Wrc4fHSCpw?t=871