You couldn’t stockpile that much. You would have to have a system to cultivate plants - even algae - but that would take space, water, and energy.
Fortunately, we’ve gotten a head start on that part.
I think that has already been adressed: it depends on many variables. But if the asteroid is big enough splitting it up would not really make much difference. The kinetic energy is the same: 1/2 m v²
Another thing: I guess the oxygen levels would drop significantly: first the fires, then much reduced photosynthesis, then a lot of weathering and rusting of stuff.
I don’t see the underground shelter working: to get the plants to grow underground would require humoungous amounts of energy, day after day. An internal combustion engine powering a generator? It would take a supertanker of fuel, and how do you store that securely? The impact should cause earthquakes that could burst any container. What generator does not break down in 100 years? The number of lamps needed is absurd. And then the sociological problems. Three or four generations that never saw the sun? Pandemic confinement is nothing in comparison, and look how people are reacting. Cabin fever would look like a vacation.
I guess humanity would have to try and some would do, but I cannot see them succeeding.
I don’t see how not making a difference follows from the kinetic energy being the same, or what basis there is to claim that it doesn’t matter if the energy hits only atmosphere, water or rock, or all in one spot or spread out … that such would make little difference.
Lots of counter examples.
A single bullet hitting a body and lodging in it is not the same damage in the thigh as in the brain despite the same kinetic energy.
Landing my body mass on a bed of ten thousand closely spaced nails with velocity V is not the same damage landing my same mass on a single dagger at the same velocity, even though both have the same kinetic energy.
A thousand bullets with total mass M hitting a tank across its complete hull does not cause the same effect as a single rocket of same total mass hitting at the same velocity with the same total kinetic energy.
But the damage the asteroid causes is not because it hits the Earth’s heart like a dagger, so that spreading the hit would dampen it. It is the energy that does the planet in, and that is quite constant from a certain size upwards. Of course one 1,000 kilo asteroid is much heftier than 1,000 one kilo asteroids, although both would not be an existential threat, but I doubt there is much difference in the effects of a 15 mile asteroid and of 15 one mile asteroids in a fast sequence.
Best thing would be we never have to find out, of course.
The total energy may be the same, but each individual impact will have much less. The individual impacts won’t loft debris as high into the atmosphere, and more will come down near the impact site. One of the most destructive factors was ejecta re-entering the atmosphere, being heated, and setting fires far from the impact site when they landed far away. With smaller impacts, fires would have been less widespread, possibly not extending to most of the planet.
It’s already been mentioned that the impactor was especially destructive because it hit sulfur-bearing deposits. If most fragments did not hit such deposits, then less sulfur dioxide and acid rain would have been produced.
I have no idea what point you are trying to make is.
Oumuamua was not an asteroid and it was not 15km.
An interstellar object, even if one is technically an asteroid by chemical composition, would still be following the same sort of trajectory as comets, which as I already pointed out:
I will say that interstellar objects could be a bit more dangerous, as if they don’t have a tail, they may be even harder to detect. They’ll also probably be coming in a little faster, which gives even less lead time, and does more damage if they hit.
It is thought that asteroids 1 km or larger hit the Earth on average every 600,000 years. I don’t think any global extinction event has been tied to such impacts, although they may have caused global climate change.
It’s not the local energy produced that does most of the damage. It’s how high and how far the ejecta are thrown. And they won’t be thrown as far by smaller individual impacts.
On what do you base that confident assertion?
@Colibri makes a very reasoned argument against it.
Metaphorically it may be that it is hitting a gas tank that is the existential threat, not the total kinetic energy.
Of course best to never have to find out.
Ah, playing pool with asteroids, what could go wrong?
Sadly, you can be sure, without a doubt, that if such a meteor were headed for Earth there would be no shortage of conspiracy theorists who would claim that it was a hoax and all part of some bigger nefarious government plan to control the citizenry.
@Colibri argues that the ejecta are the real damage, but the ejecta are only that what the energy of the asteroid can eject. If the same energy is spread over hundreds or thousands of smaller asteroids simultaneously, they would still have the same energy, would they not? And if they are spread over one hemisphere they will heat up the air there and, enough mass being assumed, cause the same fires the ejecta would cause.
Shall we try to calculate with some figures? Assume a cubic asteroid (easier to calculate than the spherical cow 
) with the figures given for Chicxulub like cited in Wikipedia: “The authors simulate one scenario using an impactor that is 17 km in diameter, with a density of 2,650 g/m3 and therefore a mass of about 6.82×1015 kg, striking Earth at 12 km/s with an angle of 60⁰ from horizontal. In another scenario that also approximately matches the evidence they analyzed, they simulate an impactor that is 21 km in diameter, with a mass of 1.28×10^16 kg, a speed of 20 km/s, and an impact angle of 45⁰. These values are not definitive; they merely illustrate one set of experts’ estimates based on current evidence. Their density parameter approximates that of a carbonaceous chondrite asteroid, which is often considered the likely type of the impactor.”
Therefore, according to the formula E = 1/2 m v² we would have
E = 1/2 x 1.28×10^16 kg x (12,000 m/s)² = 9.216 x 10^23 kg m²/s² or Joule.
How much is that? “The energy released by the Hiroshima bomb explosion (about 15 kt TNT equivalent, or 6x10^13 J) is often used by geologists as a unit when describing the energy of earthquakes, volcanic eruptions, and asteroid impacts.” (cite). According to my calculation the division equals
15,360,000,000 Hiroshima bombs (this took a while, please check it, I may have miscalculated: too many zeroes). That should be enough to heat up one hemisphere significantly and land us in deep trouble: As the surface of the Earth is about 510.000.000 km², that is the equivalent of about 3 Hiroshima bombs per square kilometer, rather six actually, as only one side is hit. The other side will feel the effects quite soon too, the winds will be enormous, so will the fires. The only thing missing in the break up scenario would be the earthquakes.
I know the data assumed are controversial, but even a factor of ten (up or down) would not make it a nice day.
Oh, I see that I have mixed up the higher mass estimate with the lower velocity estimate. Well, I guess that makes it mid-range. One assumption is even higher, the other is lower. Sorry, I am not going to recalculate, the difference should not be that much. Or is it? Damned sunday evening concentration low, I deserve a beer and I am going to get one.
Even being optimistic and looking to a decade of self reliance before climate and environments stabilise, you need to pack for:
-
People, plus food for them
-
Seeds and plant stock that was absolutely not going to be eaten, and enough room to propagate it for multiple generations
-
Any animals we might want, and the food for them, that were absolutely not going to be eaten
in enough locations that at least one will survive the impact.
The eruption of Tambora volcano had perceptible climatic effects for several years, so you’d expect some level of instability for quite a while, and you would need to pick the right time to come out and risk some of your seed stock.
Presumably, with the bigger end of the food chain gone, it will be party time for all manner of insects, so add bug spray and fly swatters to the list. And enough insect netting to cover all your crops and vaccinations for any possible insect-borne diseases for your lifestock.
Plus your highlights of arts and culture.
You again are missing my point. The fires weren’t caused directly by the asteroid impact, but by the ejecta superheating the air when they fell. The extent of the fires depended in part on what distance the ejecta traveled before they fell. The Chicxulub impact was so powerful it sent ejecta all the way around the planet. There are two areas where ejecta are concentrated, in the area around the impact site, and at the antipodal point where India was at the time. The fires were so destructive because they spread from opposite sides of the planet. As the planet rotated, the fires spread westward from both sites.
You mention the destruction being spread over “one hemisphere,” but it took place over both hemispheres. Lower energy individual impacts might not have thrown ejecta high and far enough so they reached the other side of the planet. If the fires had been limited to just the Western Hemisphere, then less soot would have been produced and a shorter impact winter.
The total energy may be the same, but if each individual impact has less energy, it can’t impart as much to ejecta, and although they may travel far enough to destroy one hemisphere, unless they have enough to reach the far side of the planet then they won’t cause as widespread destruction as a single big one.
This might not be a solution to something as large as a 15km asteroid, but I had been thinking recently if maybe there was a way to stop asteroids from dealing considerable damage if it was possible to construct a vast tank at the projected impact site filled with some compressible fluid (or maybe loosely packed granular solids that almost behave as a compressible fluid) that would cause the asteroid to lose its kinetic energy more slowly as it compresses the fluid, which have there be more time for the energy to dissipate, and hopefully not cause much in the way of problems when the asteroid pushes this fluid into the air in the process of landing.
I have absolutely no idea if there possibly could be such a fluid that would have properties that worked and could be manufactured on a scale sufficient for whatever asteroid might be a problem, but is this something that anyone has looked at? It certainly would be a tough sell politically if the projected area to be hit was heavily developed, and we’d look like awful fools if our prediction of where it was going to land was off, but something about this idea makes me think that it might work. I await to hear from experts why it wouldn’t.
I think it would be essentially impossible to calculate the impact site with such precision.
And any asteroid would hit with such energy that it is going to vaporize what it hits, compressible or not.
This is sort of relevant, re: an asteroid striking in water, deep or not. The quick link gives a very good visual about how much water is on earth. Developed by USGS and Woods Hole Oceanographic Institution.
Not debating PappaSan at all. But this link really sticks to ya. How much water is in Earth
I’m imagining a giant jello mold.
But not really. Anything that has enough energy to cause devastation is going to have too much to be absorbed by any sort of man made crumple zone. It would be a very narrow range of impact scenarios where it could possibly have any sort of positive effect.
I am sorry, but I think you are missing my point too. If we shattered the asteroid to pieces before the impact those pieces would act as the ejecta you mention did when they fell back to earth. The energy of those little stones, millions upon millions of them, would be 9.216 x 10^23 Joule (see above, with some margin of error, granted) or the equivalent of 15,360,000,000 Hiroshima bombs (same margin of error), and that energy would superheat the atmosphere in one side of the Earth. That would cause enormous winds because of the temperature difference between both hemispheres and those winds would stoke the flames.
The question was if it would be better to shatter the asteroid or to leave it in one piece. A shattered asteroid behaves like the ejecta you mention. Even if all the pieces were so small that they all would turn to vapor in the atmosphere and never reach the ground, the energy would heat the air. The energy must go somewhere, and that somewhere is the air. The effect would be the same as the dinosaur asteroid, minus the earthquake, plus enormous winds due to the temperature gradient.
Assuming the mass of the atmosphere is 5.3 × 10^18 kg (cite) that results in about 20,000 Joule for each kg of air, enough to warm the air about 20°C if I understood this site correctly, specifically the part that reads:
Specific heat capacity (Cp) air at 0°C and 1 bara: 1.006 kJ/kgK. If only one side is warmed up I will assume it heats up by 40°C (half the air, double the temperature increase) and the temperature gradient to the other side would blow the wind to supersonic speeds.
If you claim that leaving the asteroid in one piece would be even worse, well, that would be extremely bad indeed. But to me there is no worse than dead, and it seems to me that we would be.
Do you think there is some fundalmental error in my calculations? More than a factor of say two due to unknown precise mass and velocity of the asteroid?
Given that humans only entered space 60 years ago, I think it’s a bit presumptuous to think you know what will happen in 50 years.