5 ms·

>They quit making bigger booms because there was no need for them Obviously there's no need for them on Earth or anywhere nearby. But it might become useful t

>They quit making bigger booms because there was no need for them Obviously there's no need for them on Earth or anywhere nearby. But it might become useful to scale nuclear explosions to an arbitrary extent if/when something large on a collision course with Earth is discovered.

Arainach 5y ago
Less than you'd think. At the scale of object which poses a serious collision risk to Earth, nuclear weapons could (at best) break up the object, but would have no realistic chance of significantly reducing the mass/momentum of the object. Unless you can break it up into dust (which at that scale you can't), most of it is still going to land on Earth and do just as much damage.
- hwbehrens 5y ago
  Actually, one recent work on n-body gravitational methods found that "nuclear explosives remain an indispensable element of the planetary defense portfolio." [0] In particular, although these dispersion methods (as you pointed out) don't typically reduce the impact energy to zero, they offer a roughly 2 order-of-magnitude decrease in impact energy while requiring only month-scale warning. In contrast, diversion techniques which can reduce the impact energy to 0 (e.g. by shifting the impactor to 'miss' our planet) can require decade-scale to accomplish their goal. [0]: https://www.sciencedirect.com/science/article/pii/S0094576521003921 https://www.sciencedirect.com/science/article/pii/S009457652...
  - berberous 5y ago
    Why a decade? Is the idea that we can only nudge it slightly, so we have to go 10 years towards it so that the nudge compounds to a large enough distance to miss?
- dredmorbius 5y ago
  A point to keep in mind: even if it arrives as a literal dust cloud, an asteroid impact at ~20--40 km/s has all the kinetic energy of the original solid impactor, and the only way for that energy to be expended is in either thermal energy (frictional force against the atmosphere) or kinetic transfer (from impact). It was from a reply to a question I'd asked recently here on HN that an eyewitness to the Chelyabinsk impactor reported on the heat of that event. Even as the impactor largely broke apart and vaporised 20--40km above the Earth, it shed a huge amount of heat over a large area. Not enough to be lethal, but enough to be felt. A larger mass would deliver correspondingly more energy. And energy is conserved.
  - everybodyknows 5y ago
    >the only way for that energy to be expended is in either thermal energy Correct, much of the atmosphere would become superheated, perhaps to plasma, and radiate the energy away non-directionally. So only half would radiate toward the surface -- not that that's a whole lot of help.
    - dredmorbius 5y ago
      Note that for an asteroid reaching terminal velocity, that's already largely the radiative pattern for thermal energy and the bulk of the energy transfer. That is, for Chelyabinsk, roughly half the heat radiated directly to space. For a sufficiently large single impactor ... there's also an ejecta cloud, and for sufficient sizes (e.g., Chixulub+), that also results in atmospheric warming by a similar mechanism, though from secondary rather than primary fireballs. Several descriptions of the K-T mass extinction go into this mechanism in detail. Even at the antipodes, most exposed terrestrial life would have been subjected to at the very least uncomfortable, and probably lethal, heat. I've had the experience of feeling radiated heat from a bushfire (and a relatively small one at that), and even at 100s of meters distance, it could be distinctly felt, with the sense that this wasn't just a nearby small heat source (a radiator or firepit), but a large conflagration. The prospect of a wall of flame sweeping down a hillside, or of a sky on fire, is slightly more real to me since that.
  - ClumsyPilot 5y ago
    If an object started off as a 10-km rock, and was pulverised to a dust cloud, that dust cloud will disperse across thousands of kilometres, some of it likely missing earth entirely. Area of impact for original rock: 78.5 km2 Area of impact for a 1000-km dust cloud: 785,398 km2 Asteroids that struck 65 million years ago had 3×10 ^23 joules In case of dusk cloud, that's 382,000 Megajoules per square meter impacted A column of air with a cross-sectional area of 1 square centimetre (cm2 has a mass of about 1.03 kilogram A square meter of air column weighs ~10 tons Heat capacity of air is 1.0035 joules per gram per degree centigrade So it's 10 MJ to heat up 1 m^2 of atmosphere by 1 degree So if all the energy was absorbed, converted to heat, and evenly distributed vertically (unrealistically), it the atmosphere would heat to 38,000 degrees. If this is in the right ballpark, then radiation will dominate this equation, which I can't estimate. Upper atmosphere will be heated to a glow, and will radiate away most of this energy. Whatever happens, not a happy scenario. For a dust cloud 5,000 km in diameter, situation is quite different: 3,821 megajoules per square meter, that's 382 degrees rise per square meter. For comparison, the Sun provides 115 Mj/m2 per day. I would anticipate that in case of 5000 KM dust cloud there wouldn't be a major loss of life - the atmosphere doesn't conduct heat very well, so most of it would be stuck in upper atmosphere and radiated away. I think a realistic 'space dust cloud' would disperse very quickly to greater than earth radius, but it's not going to be uniform dust.
- throwaway0a5e 5y ago
  The atmosphere is going to deal with a bunch of little rocks a hell of a lot better than one big one.
- perl4ever 5y ago
  >Less than you'd think. I am in favor of numbers in a discussion like this, even if wrong, (and I am not a physicist) they're more interesting than without. - The asteroids considered a threat to the Earth start at about 460 feet in diameter. This is about an order of magnitude less (in diameter, not mass) than the impact that got the dinosaurs but would still probably be a lot worse than anything in recorded history, if it hit land or sea. It's said to be about a 10,000 year level. - Density is uncertain, but I found a figure of 2.7 g / cm^3 as a mid range. - This equates to 3.9 million metric tons or more being probably what we would want to do something about. - Freeman Dyson wrote a paper in 1968 or so, about nuclear pulse rocket design, that described a hypothetical spacecraft that would mass forty million metric tons. - It would, however, accelerate very slowly; at 0.00003 g (supposedly). - But that's really not so slow. - Ignoring orbital mechanics, because I just can't right now, if something is headed for Earth, it should be enough to move it by one radius in some direction...right? - Earth is something like 7926 miles in diameter, and 6.4 megameters is ~half that. - d = 1/2at^2, 1/2a = 0.000147 m/s^2, so divide the radius by 0.000147, take the square root, and I get ~209,000 seconds. - That is...2.4 days! - However, it presumes a one megaton bomb going off every 100 seconds, for a total of around 2,100. - That sounds like a lot. On the other hand, it's less than 3% of the nuclear bombs the US has produced since 1945. - However they would have to be custom designed, is my impression, as "nuclear shaped charges" and not just recycled warheads. - The asteroid might have to be reinforced on the size that was being pushed. The spacecraft design included a 1mm thick sheet of copper to absorb thermal energy. - However, if the asteroid was four and not forty million tonnes, then less time and fewer explosions would be needed, I would assume. Would it be ten times less? I'm leaving that as an exercise for the reader. https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls... https://en.wikipedia.org/wiki/Nuclear_weapons_of_the_United_States https://en.wikipedia.org/wiki/Nuclear_weapons_of_the_United_...
  - perl4ever 5y ago
    The tl/dr might be: Moving an asteroid is a lot more plausible than you'd think, but it would require "normal" sized bombs, not super gigantic ones.

Arainach 5y ago

Less than you'd think. At the scale of object which poses a serious collision risk to Earth, nuclear weapons could (at best) break up the object, but would have no realistic chance of significantly reducing the mass/momentum of the object. Unless you can break it up into dust (which at that scale you can't), most of it is still going to land on Earth and do just as much damage.

hwbehrens 5y ago
Actually, one recent work on n-body gravitational methods found that "nuclear explosives remain an indispensable element of the planetary defense portfolio." [0] In particular, although these dispersion methods (as you pointed out) don't typically reduce the impact energy to zero, they offer a roughly 2 order-of-magnitude decrease in impact energy while requiring only month-scale warning. In contrast, diversion techniques which can reduce the impact energy to 0 (e.g. by shifting the impactor to 'miss' our planet) can require decade-scale to accomplish their goal. [0]: https://www.sciencedirect.com/science/article/pii/S0094576521003921 https://www.sciencedirect.com/science/article/pii/S009457652...
- berberous 5y ago
  Why a decade? Is the idea that we can only nudge it slightly, so we have to go 10 years towards it so that the nudge compounds to a large enough distance to miss?
dredmorbius 5y ago
A point to keep in mind: even if it arrives as a literal dust cloud, an asteroid impact at ~20--40 km/s has all the kinetic energy of the original solid impactor, and the only way for that energy to be expended is in either thermal energy (frictional force against the atmosphere) or kinetic transfer (from impact). It was from a reply to a question I'd asked recently here on HN that an eyewitness to the Chelyabinsk impactor reported on the heat of that event. Even as the impactor largely broke apart and vaporised 20--40km above the Earth, it shed a huge amount of heat over a large area. Not enough to be lethal, but enough to be felt. A larger mass would deliver correspondingly more energy. And energy is conserved.
- everybodyknows 5y ago
  >the only way for that energy to be expended is in either thermal energy Correct, much of the atmosphere would become superheated, perhaps to plasma, and radiate the energy away non-directionally. So only half would radiate toward the surface -- not that that's a whole lot of help.
  - dredmorbius 5y ago
    Note that for an asteroid reaching terminal velocity, that's already largely the radiative pattern for thermal energy and the bulk of the energy transfer. That is, for Chelyabinsk, roughly half the heat radiated directly to space. For a sufficiently large single impactor ... there's also an ejecta cloud, and for sufficient sizes (e.g., Chixulub+), that also results in atmospheric warming by a similar mechanism, though from secondary rather than primary fireballs. Several descriptions of the K-T mass extinction go into this mechanism in detail. Even at the antipodes, most exposed terrestrial life would have been subjected to at the very least uncomfortable, and probably lethal, heat. I've had the experience of feeling radiated heat from a bushfire (and a relatively small one at that), and even at 100s of meters distance, it could be distinctly felt, with the sense that this wasn't just a nearby small heat source (a radiator or firepit), but a large conflagration. The prospect of a wall of flame sweeping down a hillside, or of a sky on fire, is slightly more real to me since that.
- ClumsyPilot 5y ago
  If an object started off as a 10-km rock, and was pulverised to a dust cloud, that dust cloud will disperse across thousands of kilometres, some of it likely missing earth entirely. Area of impact for original rock: 78.5 km2 Area of impact for a 1000-km dust cloud: 785,398 km2 Asteroids that struck 65 million years ago had 3×10 ^23 joules In case of dusk cloud, that's 382,000 Megajoules per square meter impacted A column of air with a cross-sectional area of 1 square centimetre (cm2 has a mass of about 1.03 kilogram A square meter of air column weighs ~10 tons Heat capacity of air is 1.0035 joules per gram per degree centigrade So it's 10 MJ to heat up 1 m^2 of atmosphere by 1 degree So if all the energy was absorbed, converted to heat, and evenly distributed vertically (unrealistically), it the atmosphere would heat to 38,000 degrees. If this is in the right ballpark, then radiation will dominate this equation, which I can't estimate. Upper atmosphere will be heated to a glow, and will radiate away most of this energy. Whatever happens, not a happy scenario. For a dust cloud 5,000 km in diameter, situation is quite different: 3,821 megajoules per square meter, that's 382 degrees rise per square meter. For comparison, the Sun provides 115 Mj/m2 per day. I would anticipate that in case of 5000 KM dust cloud there wouldn't be a major loss of life - the atmosphere doesn't conduct heat very well, so most of it would be stuck in upper atmosphere and radiated away. I think a realistic 'space dust cloud' would disperse very quickly to greater than earth radius, but it's not going to be uniform dust.
throwaway0a5e 5y ago
The atmosphere is going to deal with a bunch of little rocks a hell of a lot better than one big one.
perl4ever 5y ago
>Less than you'd think. I am in favor of numbers in a discussion like this, even if wrong, (and I am not a physicist) they're more interesting than without. - The asteroids considered a threat to the Earth start at about 460 feet in diameter. This is about an order of magnitude less (in diameter, not mass) than the impact that got the dinosaurs but would still probably be a lot worse than anything in recorded history, if it hit land or sea. It's said to be about a 10,000 year level. - Density is uncertain, but I found a figure of 2.7 g / cm^3 as a mid range. - This equates to 3.9 million metric tons or more being probably what we would want to do something about. - Freeman Dyson wrote a paper in 1968 or so, about nuclear pulse rocket design, that described a hypothetical spacecraft that would mass forty million metric tons. - It would, however, accelerate very slowly; at 0.00003 g (supposedly). - But that's really not so slow. - Ignoring orbital mechanics, because I just can't right now, if something is headed for Earth, it should be enough to move it by one radius in some direction...right? - Earth is something like 7926 miles in diameter, and 6.4 megameters is ~half that. - d = 1/2at^2, 1/2a = 0.000147 m/s^2, so divide the radius by 0.000147, take the square root, and I get ~209,000 seconds. - That is...2.4 days! - However, it presumes a one megaton bomb going off every 100 seconds, for a total of around 2,100. - That sounds like a lot. On the other hand, it's less than 3% of the nuclear bombs the US has produced since 1945. - However they would have to be custom designed, is my impression, as "nuclear shaped charges" and not just recycled warheads. - The asteroid might have to be reinforced on the size that was being pushed. The spacecraft design included a 1mm thick sheet of copper to absorb thermal energy. - However, if the asteroid was four and not forty million tonnes, then less time and fewer explosions would be needed, I would assume. Would it be ten times less? I'm leaving that as an exercise for the reader. https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls... https://en.wikipedia.org/wiki/Nuclear_weapons_of_the_United_States https://en.wikipedia.org/wiki/Nuclear_weapons_of_the_United_...
- perl4ever 5y ago
  The tl/dr might be: Moving an asteroid is a lot more plausible than you'd think, but it would require "normal" sized bombs, not super gigantic ones.