The problem, in part, is that the atmosphere itself gets compressed well before the asteroid hits the tank. And that would cause the atmosphere to be heated up even higher than the surface temperature of the sun.
No more presumptuous than to think that humans are causing climate change.
I am very convinced by @Colibri’s analysis BUT also come away with another thought - if not hitting the threshold to cause ejecta into the atmosphere, then more damage is had by having a greater number of separate impacts than fewer, along the same lines of analysis about the damage of the Hiroshima bomb:
Point remaining that I don’t think it is as simple as a linear correlation between kinetic energy and damage. Ejecta or not and what gets hit where both matter, spread out evenly like a cluster bomb would cause damage that say three hits each of which are below the threshold to trigger ejecta into atmosphere would not. I don’t think my question has a simple straightforward answer.
Wait a minute. Now you’re moving the goalposts. We’ve gone from 1,000 1-km asteroids to millions of tiny fragments.
The ejecta didn’t heat the air of the planet overall so that vegetation everywhere burst into flames. It only did that in the areas where it fell. If all your fragments fell on just one side of the planet, it wouldn’t necessarily heat the air on the other side enough so vegetation burst into flames.
A shattered asteroid doesn’t behave like ejecta. The fragments will indeed heat the air, but the effect of each one will be localized. The ejecta thrown out by larger impacts (which are fragments of the crust) will spread destruction over a wider area depending on how far they are thrown.
We are focusing on two different things here. You are focusing on the energy, I am focusing on how widespread the destruction is. And the destruction depends on what area the energy is spread over, not its total amount. The same energy spread over a larger area of the planet will be more destructive
It also should be stressed that the fires could not have engulfed the entire planet. Many ancient lineages of plants survive on all continents, including some very large trees; redwoods and sequoias go back to the Jurassic. There must have been some pockets that were not affected by severe fires, so the entire atmosphere wasn’t superheated.
Sorry, I don’t think I am moving the posts. The question was if it is better to shatter the asteroid (but it still hits us) or to leave it in one piece. If we shatter it, and I just went to the extreme shattering, were it doesn’t even hit the ground, the energy is still there and it dissipates in the air, warming it up by about 40°C (which is a lot) on one side of the planet. The consequences are devastating. If we don’t break it up, the consequences are like 66 Mio. years ago: devastating too.
Perhaps there is a sweet spot where we break the asteroid into fragments of say 83 yards each (I just made this figure up) and we get 15 km divided by 83 yards or about 6 Mio. pieces (really quick and dirty calculation here), the effect would not be as horrible. I doubt it.
If we shatter it enough the pieces behave like the ejecta and warm the atmosphere, I must insist on that point. Actually, they are even “hotter” than the ejecta, as those only falls with the escape velocity (11 km/s), while the fragments came with between 12 and 20 km/s accordind to the estimates, and as the formula reads E = m v² the term v is squared, that makes a significant difference for the worse. There would be no earthquake and no tsunami, but catastrophic winds on top of a lot of heat. Yes, I concentrate on the energy, because the damage is done by the energy. Either directly (in my shattered scenario) or by way of re-entry (ejecta) in your scenario.
So what I am actaully saying is that we must deflect the asteroid or we are doomed. Because the energy will cause catastrophic damage. Leaving it whole or breaking it up will change some details, but not the devastation.
And I do not believe we will be able to deflect such a mass as the original asteroid in the forseeable future. It is just too big.
Now for the good part: we know of no such asteroid. Let us hope it stays that way.
So it is probably better not to shatter the biest. Or have I miscalculated? By a huge factor, I hope, because my numbers do not look good.
Concerning your examples: Redwood and sequoias might have survived as seeds, I don’t know. Did any big trees survive? They would not in my scenario, as supersonic winds would fell them. But again: I may be wrong in my calculations, which nobody seems to have checked.
People emphasize how catastrophic the event was. And while it killed off 75% of all species, 25% of all species survived. As for plants, the effects varied regionally, with North America being hardest hit, but even there more than 40% of the flora survived.
All major clades of mammals survived. As for amphibians:
It wasn’t like the entire surface of the Earth was subjected to a blowtorch and an acid bath. There had to have been some areas that were sheltered from the worst effects where significant numbers of animals and plants survived.
I’m not saying that both scenarios wouldn’t be catastrophic. I’m just saying that the effects of shattering the asteroid would be somewhat less widespread, and thus slightly better. We would be screwed either way. Shattering the asteroid wouldn’t save us.
I would think any kind of underground bunker would be completely overwhelmed by people trying to get in. Billions of people who would otherwise die will get very desperate. One of my favorite “Twilight Zone” episodes touched on this topic. An air raid siren goes off during a neighborhood dinner party. The host sends everyone home and his family goes into their bomb shelter. The neighbors break down the door to his bomb shelter to get in. That’s pretty much what I would imagine happening to any kind of bunker setup to survive the meteor strike.
So again: it was bad, but breaking it up into many tiny pieces seems worse. I think we have reached the end of our speculation, I for one don’t know what eIse to argue; I wish someone like @Chronos would step in, I believe he really knows about that stuff. The fact that he does not hints at the possibility that we both (and @DSeid) are way off the mark here and he does not consider us worthy of even refutal, that would be a pity.
PS: I like amphibians (and bats!) very much too. Can you recommend a monography on frogs? I need that for a comic I am developing, but also out of simple interest. I know that sounds weird, but at post #91 I hope it is not a hijack.
So we basically agree. I am glad to read that, I do not wish to enter a pointless feud.
It is past midnight in Europe, I am going to sleep. If you answer my quest for the monography (Spanish or German would be OK too, if that happens to be the best one) I will read it tomorrow. Good night.
Of course it’s not a feud. The discussion was interesting, but I think we were emphasizing different things.
I can’t say I know any “monograph” which implies a technical work. While I am not familiar with it personally, this book looks like it could fit your needs.
https://www.amazon.com/Frogs-Toads-World-Chris-Mattison/dp/0691149682/
Yeah. The Moon might be a big enough crumple zone. Assuming the impactor didn’t split the Moon a la Seveneves.
I’m imagining more like a tube out in space that’s about 10,000 miles long maybe double the diameter of the impactor and full of … something … . Maybe something very thin and gaseous at the entrance end slowly thickening to something more like rock at the run-out end.
Then we just have to align that tube perfectly so the impactor flies into one end and runs down the entire length, quickly turning the whole thing into an energetic plasma. Easy. Not. 
To the degree I checked you seem to be within a reasonable range … my issue is not with the calculations but the jumps made with “That should be enough to …” and that the calculations do not address the actual question - other than to say “this too would be bad” - and specifically do not support the claim that damage is pretty much linear to kinetic energy end.
Actual modelling I think is lot more difficult. Here’s a more recent attempt. That one looking specifically at the Chicxulub crater specifies how much the kinetic energy alone is not the whole story
You mean like this?
Or it’s also possible that I spent most of the day at my mom’s house making pierogi. Nah, it’s got to be that other thing.
@Colibri has a point that, if all of the asteroid fragments hit the Earth at once, no single impactor would have enough energy to throw hot ejecta all the way to the antipodes, and that half the planet would thus be spared the worst of the immediate effects. That’s probably true… but it’s also dependent on all the fragments hitting at once. If an asteroid were smithereened, the fragments wouldn’t just spread out laterally, but also longitudinally, resulting in different impact times, and allowing the planet to rotate some between impacts.
And what’s all this about preparing to survive in bunkers for a century? The estimates I’ve seen have been for more like five years, before the outside climate is again conducive to agriculture.
Plus, of course, some places already have much of the infrastructure needed. In Iceland, for instance, hydroponic agriculture under artificial light, powered by geothermal generators, is already the norm. I’m sure that they’d suffer from a loss of trade with the rest of the world, but even if another dino-killer struck right now, with no advance warning, they’d probably survive. And there are people with multiple years’ worth of food stockpiled in their homes: They’d probably make it, too, if they have the know-how to start up with farming once they’re able (which many of them do).
There is a potential win-win here - stack all the old mattresses and tyres into a big pile, boing the asteroid back into deep space AND vaporise all that old crap so we never have to deal with it again.
Pierogi sounds nice, and thanks for your dose of realistic optimism. I mean, five years sounds feasible, that is great.
@Colibri: thanks for the link!
Except that the earth is not staying in place during that time. Its orbital velocity is very fast compared to how fast it rotates - by the time the earth has rotated even a quarter turn it has moved 650,000 km away from where it was, about 51 earth diameters away. (Earth diameter about 12,756 km and orbital distance traveled by earth each day about 2.6 million km).
By the time the earth rotates much at all it is long gone from the same spot.
The equivalent is 1 15-mile asteroid and 3,400 1-mile asteroids.