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Comment by philipkglass | Hacker News Reader

There was plenty of technical headroom left for making more fearsome weapons in the early 1960s. Fortunately, the pace of innovation plummeted after atmospheric testing ended, and none of the later nuclear weapons states have surpassed the high water mark set by Cold War era superpower rivalry. Chuck Hansen's book The Swords of Armageddon is a fascinating account of the vast hidden empire of the post war Atomic Energy Commission, and how it developed atomic weapons so far beyond the early atomic bombs used on Japan. And then again optimized thermonuclear weapons so far past the early hydrogen bombs.

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They quit making bigger booms because there was no need for them. Two factors are at work:

1) Square-cube law. You do more damage with a collection of smaller warheads than with one big one with the same total yield.

2) The Earth isn't flat. Once a boom gets big enough the area of damage doesn't go up much as the shockwave ends up separating from the ground and the atmosphere above the bomb just ends up blasted off. Note that this does *not* limit the usefulness of big bombs detonated high up for thermal and EMP effects. Very big booms in low orbit could be very nasty with total surprise.

>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.

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.

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/S009457652...

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?

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.

>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.

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.

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.

Note that following pulverisation, the dust cloud would be under the effect of the gravitational field of the cloud itself, and tend to re-coalesce.

The initial pulverisation would presumably result in a distribution of velocities amongst particles. Some of those would achieve escape velocity and leave the cloud. Some would not.

Time to re-coalesce would depend on the total cloud mass and maximum extant itself. In total, it would be relatively quickly.

The resulting re-agglomerated impactor would then resemble a rubble-pile or the World's Largest Dust Bunny. It would be significantly disrupted by both tidal forces and interactions with the atmosphere, though of course, how greatly it is disrupted by atmospheric friction would again be a function of mass.

I don't think it will coalese, surface gravity of ceres is 0.2 metwrs oer second, and ceres is 500km across. Gravity of 10 km rock will be measure in cm or mm. Any process that will pulveriae an asteroid is going to give those dust particles a lot of energy

I've somewhat deliberately left the pulverising mechanism unspecified.

But the point remains: under the influence of the cloud's own gravity a cloud of merely separated particles will re-coalesce. You can look up in the night sky much of the time to see an example of this yourself, the Earth's moon.

And again, depending on the mechanism, a fair fraction of particles, quite possibly / probably the majority, will approach the original centroid of the mass.

In practice, pulverising planets and/or planetoids is likely a challenging proposition. Early experimentation on animal models tends to support this hypothesis:

https://youtube.com/watch?v=V6CLumsir34

Send a second bomb into the cloud shortly after, ignite and disperse it from within. Poof!

Effectively, yes.

Though the point remains that what you've got to do is avoid impacting the asteroid entirely, whether it's a single solid core or a dispersed dust-cloud. Or more precisely, the total delivered energy has to be considered and kept to a survivable maximum.

That said, I'm now wondering what it would look like to suddenly have a sub-lethal dust-cloud impact Earth. I imagine there'd be an initial few stray shooting stars, then a rapidly-growing glow as the dust struck the atmosphere. Hrm...

The atmosphere is going to deal with a bunch of little rocks a hell of a lot better than one big one.

>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_propuls...

https://en.wikipedia.org/wiki/Nuclear_weapons_of_the_United_...

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.

> And then again optimized thermonuclear weapons

Here in Blighty, during the early cold war we built fall-out shelters and equipped a civilian force with "Green Goddess" fire engines to enable them to extinguish fires from nuclear flash.

Yet this all stopped after development and adoption of the Hydrogen Bomb..... because unlike nuclear weapons (kT yield), trying to defend against thermonuclear weapons (MT yield) is pointless. This was a very real and honest defence policy.

https://en.wikipedia.org/wiki/Green_Goddess

[In the UK, some younger people may recognise the Green Goddess Fire engine because they were seen during fireman strikes].

...Huh. On an EXTREMELY tangential note, I wish I'd known about this when I was drawing a political cartoon comparing California to a burning building which [redacted] was turning a "Green New Deal" ("Green Dream or whatever") fire engine away from. Tracing and recoloring a random US fire truck didn't quite have the same kick this would have.

Of course civil defense against the H bomb isn't pointless. In fact, the bigger the bomb, the larger the perimeter zone where survival becomes a matter of knowing how to react appropriately.

"Duck and cover" won't work if you're too close to the fireball, but most people won't be.

It's worth noting, in case anyone thinks that we somehow slowed down testing with the atmospheric test ban...that we went on to conduct 1,352 nuclear weapon detonations. That's roughly 3/4 of all nuclear testing. It really is mind-boggling that we set off so many.

But really, how much more do you want? The US upped the ante several times, developing all kinds of more advanced super weapons, as well as modes of delivery, the first long range bombers, the Hydrogen bomb, ICBM's, MIRV, the Neutron bomb ... I mean, as you say it was FAR beyond the early atomic weapons. And they're continuing with development, in secret of course ...

Americans keep developing their missile shield so China and Russia need to keep developing ways to outsmart it. It will never end.

A missile shield that actually works will actually precipitate a nuclear conflict, because it means an end to MAD. If you know that your adversary will become invulnerable to your counter-strike in three months, the optimal game-theory solution is to strike him today.

Optimistically, military planners are aware of this, and are just doing pork barrel spending.

Pessimistically, military planners don't care about this, and are actively working on 'winning' a nuclear war.

This doesn't really follow IMO and is a result of deep oversimplifications.

Even if you make those simplifications though: If your choices are pre-emptive strike where MAD applies now (near-total destruction) vs. live to 3 months from now, where you have a post-MAD world where one adversary dominates, the post-MAD dominant adversary no longer has the incentive to pre-emptively strike since they aren't worried about a pre-emptive strike from their adversaries since they'd have an effective missile defense system.

> If you know that your adversary will become invulnerable to your counter-strike in three months, the optimal game-theory solution is to strike him today.

Striking today means MAD happens and your country gets wasted. That only makes sense if the shield-building adversary is some sort of psychopath who wants to see you dead.

The 3 big nuclear powers don't seem to be that stupid. Maybe the calculus would be different with Israel vs Iran or India vs Pakistan, where the conflict has religious aspects.

> That only makes sense if the shield-building adversary is some sort of psychopath who wants to see you dead.

Or if they may become a psychopath who wants to see you dead - in the name of self-defense, of course. Or if they think that you are a psychopath who wants to see them dead. Or if they think you may become a psychopath who wants to see them dead.

It's difficult to assure your counterparty that you will never behave in a psychotic manner in the future - or to assure yourself that they will not.

If you're not planning on starting a nuclear war, and you assume that your adversary isn't going to, either, why do you need a missile shield, anyways? MAD is a necessary and sufficient deterrent. If you're building a shield, you are very conspicuously betting that at least one of these two assumptions is no longer true...

> Or if they may become a psychopath who wants to see you dead - in the name of self-defense, of course. Or if they think that you are a psychopath who wants to see them dead. Or if they think you may become a psychopath who wants to see them dead.

These assumptions better be strongly supported by evidence, if one's going to decide to get their country annihilated by attempting a first strike.

Striking first won't protect the country from a hypothetical future attack, it will just make it happen now. I don't really see a sane reason to do it if the threat isn't considered immediate.

Which is precisely why during the Cold War there was a treaty limiting anti-missile defence (ABM Treaty). Who wants to guess which side withdrew from it in 2002 under a war criminal leader?

It could not be one of the NATO states...i mean the nato is to preserve freedom...right? :)

Just searched for Hansen's book on Amazon. The only one available was 'U.S. Nuclear Weapons the Secret History'. Steep price, $159 for a used copy.

The Swords of Armageddon was only ever available digitally (first on CD-ROM, then as download). Hansen's widow used to sell it after his death but she is no longer responding to sales requests. Or at least mine was ignored early this year. You can find it on Library Genesis.

U.S. Nuclear Weapons: The Secret History is sadly missing from LibGen, but it appears to be available as a digital checkout from the Internet Archive. It's a good companion book to Swords because it went through a professional editing-publishing cycle. Swords is a lot longer and rawer, with a higher ratio of text to illustrations.

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