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Wouldn't breaking up a planetary defense worthy asteroid into medium to even large boulder size asteroids be considered a win? Whats the limit on asteroid size

Wouldn't breaking up a planetary defense worthy asteroid into medium to even large boulder size asteroids be considered a win? Whats the limit on asteroid size that can make it through the atmosphere? And even then, intuitively, it would seem like the destructiveness is going to scale with the size of the individual asteroid, so a rain of many smaller asteroids making it to ground would be far lower than one large asteroid..

icegreentea2 3y ago
It depends on what the threshold for 'planetary defense worthy' is I guess. If you shattered an asteroid, but all of the asteriod pieces still entered the earth's atmosphere, then you're going to end up releasing pretty much an identical amount of energy. At "city buster" size of impact, this would probably be beneficial. But beyond that? Probably not hugely beneficial. Probably effectively the difference between a ground detonation and a partial airburst.. so maybe less ejecta (reduced global dimming after). But... probably not a meaningful change.
- burnte 3y ago
  Yes the energy is identical, but the volume across with the energy is dispersed is the benefit. Even if you "only" break up an asteroid so that the rubble falls all over the world across a few days or weeks, that's nothing compared to the asteroid in one piece hitting. It's not the fall, it's the sudden stop at the end. Make that fall over days/weeks and a million times the area, you're fine.
  - mcbutterbunz 3y ago
    Still depends on the amount of energy being released. Burning up a large enough asteroid would cook the planet, even at days/weeks/months. I guess it really depends on the rate at which heat is lost to space.
  - LorenPechtel 3y ago
    It's all going to fall at basically the same instant. At small rocks destroying them is a good thing. At big rocks destroying them is worse than not shooting: Blast radius goes at the cube root of energy, blast area goes at the square of blast radius. Thus if you were to break an incoming rock into 8 equal parts the blast radius of each impact would be halved, the blast area quartered--but there would be 8 hits, for a final result of doubling the blast area. The smaller the pieces the worse the effect until you get down to the point where they burn rather than hit. (And that brings it's own problems with flash heating.) Thus big rocks must only be deflected, not destroyed.
    - EthanHeilman 3y ago
      All of this depends on the actual parameters we are talking about > last radius goes at the cube root of energy, blast area goes at the square of blast radius. Only up to the point at which the blast wave touches atmosphere then strange things happen because the blast is now being tunneled into a vacuum. This likely reduces the blast of very big asteroids by channeling most of the energy back into space. However blast area isn't the only concern. An ice age can be triggered if enough matter is blasted into the upper atmosphere. You might increase the blast area by breaking the asteroid into smaller pieces but completely eliminate the risk of depositing light blocking particles in the upper atmosphere. It is by no means guaranteed that if you break a big asteroid into eight pieces, they will all hit. The energy to split an asteroid may confer sufficient momentum that small if not all the pieces miss the Earth.
    - burnte 3y ago
      > It's all going to fall at basically the same instant. No, not for any sane definition of "at the same instant". See Shoemaker-Levy 9 as a great analog. Completely natural forces broke it up and it fell across a huge swath of Jupiter. An engineered breakup would of course be much more fine tuned to create both chunks and vectors of our choosing. If something were to head at us, and we took it from one big chunk to multiple smaller chunks but with such a terribly small explosion that we turn it into a closely-spaced fleet of rocks, then yes, but hopefully if we go through the effort of trying to destroy/deflect, we'd use a reasonable amount of explosive. While your energy dispersal functions in a vacuum are correct, you're completely neglecting everything else about the situation. All the energy we're putting into the system to change the vectors to start with, you're acting like we barely tap it. Look at what happened with DART and that had zero explosive, that was _purely_ ballistic. You hit a rock several months out and send pieces flying in 18,000 directions, some hit us, some don't, they hit across weeks, they hit across the world, and you get the benefit of our atmosphere. Presuming optimal angle of attack for the interloper, everything under 100 meters in diameter is vapor. More acute angles take that number up, and at really steep angles the objects skip off the atmo. Larger rocks will airburst from the entry into atmo, and lots of stuff will make landfall, but again, ALL THAT ENERGY IS DISPERSED ACROSS SPACE AND TIME. I know you said it won't, but the math, and real-world examples, say it will. > Thus big rocks must only be deflected, not destroyed. The problem is most things aren't just big rocks, the things that endanger us also consist of rock piles and slush balls, which you cannot deflect. So we MUST engineer for destruction as well. And this discussion itself also ignores a hundred other variables such as orbital mechanics, how much time we get, composition, etc. To just say "It'll all hit at the same time and we can only deflect" is only correct, pardon the pun, in a vacuum. There's more to consider.
- akavel 3y ago
  But won't they burn earlier in the atmosphere due to exposing bigger surface to it? AFAIU small asteroids burn in the atmosphere and thus don't reach the surface or reach it in very small pieces (and presumably more distributed if a bunch of them at once), vs. big ones burning less in total and hitting more concentrated; do I have it wrong?
  - icegreentea2 3y ago
    Yes, but planet killers can be mind boggling large. The 'dinosaur killer' was estimated to be 72 teratonnes of TNT equivalent. Even if you spread that perfectly evenly over 10 days, that's a 83 MT nuke going off every second. Or taken another way, the dinosaur killer had an total estimated energy of 300ZJ - 3x10^24J. Total solar energy is 44 quadrillion W (4.4x10^16 J/s). Even spread over 10 days (864000 seconds), that results in an excess 3.5x10^18 J/s. Which is why I said it depends on what your threshold is. Certainly somewhere between city buster and dinosaur killer lies the tipping point.
    - LorenPechtel 3y ago
      But you're not going to meaningfully spread the energy over time. If the bits are going to hit that means they didn't move more than 4000km after the strike. Assuming they're distributed in a sphere that means none hits more than 4000km early, either (and likewise 4000km late)--and since we are talking about objects with a relative velocity of a few km/sec that means it's dispersed over at most a couple of hours. By the back of the envelope you're looking at about a megasun. Once again, a scenario where letting the rock just hit is not as bad--energy spent gouging a huge crater is energy not spend blasting civilization.
      - LargoLasskhyfv 3y ago
        Since you're discussing Teratonnes, I'll link a discussion from X which is about what a 10GT nuclear warhead ignited at 80km altitude would do just thermally. Crossed my timeline just recently and has a link to https:// https:// apps.dtic.mil/sti/tr/pdf/ADA 383988.pdf if you don't like https://twitter.com/ToughSf/status/1689305541422575623 https://twitter.com/ToughSf/status/1689305541422575623
- kjs3 3y ago
  Over a certain size object, it's "do you want to get shot with a rifle or a shotgun".
simonh 3y ago
Dangerous asteroid sizes span many orders of magnitude from the smallest real global danger to the largest. Breaking one up would work for asteroids over maybe tgevlowest order of magnitude size range, but that still leaves many more orders of magnitude in larger sizes for which it wouldn’t help you much.
lamontcg 3y ago
Meteors smaller than 25m burn up in the Earth's atmosphere: https://www.nasa.gov/mission_pages/asteroids/overview/fastfacts.html https://www.nasa.gov/mission_pages/asteroids/overview/fastfa...

icegreentea2 3y ago

It depends on what the threshold for 'planetary defense worthy' is I guess. If you shattered an asteroid, but all of the asteriod pieces still entered the earth's atmosphere, then you're going to end up releasing pretty much an identical amount of energy. At "city buster" size of impact, this would probably be beneficial. But beyond that? Probably not hugely beneficial. Probably effectively the difference between a ground detonation and a partial airburst.. so maybe less ejecta (reduced global dimming after). But... probably not a meaningful change.

burnte 3y ago
Yes the energy is identical, but the volume across with the energy is dispersed is the benefit. Even if you "only" break up an asteroid so that the rubble falls all over the world across a few days or weeks, that's nothing compared to the asteroid in one piece hitting. It's not the fall, it's the sudden stop at the end. Make that fall over days/weeks and a million times the area, you're fine.
- mcbutterbunz 3y ago
  Still depends on the amount of energy being released. Burning up a large enough asteroid would cook the planet, even at days/weeks/months. I guess it really depends on the rate at which heat is lost to space.
- LorenPechtel 3y ago
  It's all going to fall at basically the same instant. At small rocks destroying them is a good thing. At big rocks destroying them is worse than not shooting: Blast radius goes at the cube root of energy, blast area goes at the square of blast radius. Thus if you were to break an incoming rock into 8 equal parts the blast radius of each impact would be halved, the blast area quartered--but there would be 8 hits, for a final result of doubling the blast area. The smaller the pieces the worse the effect until you get down to the point where they burn rather than hit. (And that brings it's own problems with flash heating.) Thus big rocks must only be deflected, not destroyed.
  - EthanHeilman 3y ago
    All of this depends on the actual parameters we are talking about > last radius goes at the cube root of energy, blast area goes at the square of blast radius. Only up to the point at which the blast wave touches atmosphere then strange things happen because the blast is now being tunneled into a vacuum. This likely reduces the blast of very big asteroids by channeling most of the energy back into space. However blast area isn't the only concern. An ice age can be triggered if enough matter is blasted into the upper atmosphere. You might increase the blast area by breaking the asteroid into smaller pieces but completely eliminate the risk of depositing light blocking particles in the upper atmosphere. It is by no means guaranteed that if you break a big asteroid into eight pieces, they will all hit. The energy to split an asteroid may confer sufficient momentum that small if not all the pieces miss the Earth.
  - burnte 3y ago
    > It's all going to fall at basically the same instant. No, not for any sane definition of "at the same instant". See Shoemaker-Levy 9 as a great analog. Completely natural forces broke it up and it fell across a huge swath of Jupiter. An engineered breakup would of course be much more fine tuned to create both chunks and vectors of our choosing. If something were to head at us, and we took it from one big chunk to multiple smaller chunks but with such a terribly small explosion that we turn it into a closely-spaced fleet of rocks, then yes, but hopefully if we go through the effort of trying to destroy/deflect, we'd use a reasonable amount of explosive. While your energy dispersal functions in a vacuum are correct, you're completely neglecting everything else about the situation. All the energy we're putting into the system to change the vectors to start with, you're acting like we barely tap it. Look at what happened with DART and that had zero explosive, that was _purely_ ballistic. You hit a rock several months out and send pieces flying in 18,000 directions, some hit us, some don't, they hit across weeks, they hit across the world, and you get the benefit of our atmosphere. Presuming optimal angle of attack for the interloper, everything under 100 meters in diameter is vapor. More acute angles take that number up, and at really steep angles the objects skip off the atmo. Larger rocks will airburst from the entry into atmo, and lots of stuff will make landfall, but again, ALL THAT ENERGY IS DISPERSED ACROSS SPACE AND TIME. I know you said it won't, but the math, and real-world examples, say it will. > Thus big rocks must only be deflected, not destroyed. The problem is most things aren't just big rocks, the things that endanger us also consist of rock piles and slush balls, which you cannot deflect. So we MUST engineer for destruction as well. And this discussion itself also ignores a hundred other variables such as orbital mechanics, how much time we get, composition, etc. To just say "It'll all hit at the same time and we can only deflect" is only correct, pardon the pun, in a vacuum. There's more to consider.
akavel 3y ago
But won't they burn earlier in the atmosphere due to exposing bigger surface to it? AFAIU small asteroids burn in the atmosphere and thus don't reach the surface or reach it in very small pieces (and presumably more distributed if a bunch of them at once), vs. big ones burning less in total and hitting more concentrated; do I have it wrong?
- icegreentea2 3y ago
  Yes, but planet killers can be mind boggling large. The 'dinosaur killer' was estimated to be 72 teratonnes of TNT equivalent. Even if you spread that perfectly evenly over 10 days, that's a 83 MT nuke going off every second. Or taken another way, the dinosaur killer had an total estimated energy of 300ZJ - 3x10^24J. Total solar energy is 44 quadrillion W (4.4x10^16 J/s). Even spread over 10 days (864000 seconds), that results in an excess 3.5x10^18 J/s. Which is why I said it depends on what your threshold is. Certainly somewhere between city buster and dinosaur killer lies the tipping point.
  - LorenPechtel 3y ago
    But you're not going to meaningfully spread the energy over time. If the bits are going to hit that means they didn't move more than 4000km after the strike. Assuming they're distributed in a sphere that means none hits more than 4000km early, either (and likewise 4000km late)--and since we are talking about objects with a relative velocity of a few km/sec that means it's dispersed over at most a couple of hours. By the back of the envelope you're looking at about a megasun. Once again, a scenario where letting the rock just hit is not as bad--energy spent gouging a huge crater is energy not spend blasting civilization.
    - LargoLasskhyfv 3y ago
      Since you're discussing Teratonnes, I'll link a discussion from X which is about what a 10GT nuclear warhead ignited at 80km altitude would do just thermally. Crossed my timeline just recently and has a link to https:// https:// apps.dtic.mil/sti/tr/pdf/ADA 383988.pdf if you don't like https://twitter.com/ToughSf/status/1689305541422575623 https://twitter.com/ToughSf/status/1689305541422575623
kjs3 3y ago
Over a certain size object, it's "do you want to get shot with a rifle or a shotgun".

simonh 3y ago

Dangerous asteroid sizes span many orders of magnitude from the smallest real global danger to the largest. Breaking one up would work for asteroids over maybe tgevlowest order of magnitude size range, but that still leaves many more orders of magnitude in larger sizes for which it wouldn’t help you much.

lamontcg 3y ago

Meteors smaller than 25m burn up in the Earth's atmosphere: https://www.nasa.gov/mission_pages/asteroids/overview/fastfacts.html https://www.nasa.gov/mission_pages/asteroids/overview/fastfa...