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wedrifid comments on 60m Asteroid currently assigned a .022% chance of hitting Earth. - Less Wrong
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Where on earth did widely dispersed efficient-life-killing thermonuclear strikes come into it? Multi was considering a bomb (or cluster thereof) at a single location. The closest to 'dispersion' was when you brought Orion into it, with it's chain of bombs spread out over the launch distance.
Or, alternately, it would be overwhelmingly inefficient because the projectile is aimed away from the planet - or at best along the surface of it.
I incidentally dispute your efficiency claim anyway. I'd be willing to bet that if you collect every one of the bombs you were using for your Orion weapon and place them in single location then it would be more capable of slippiting the planet than the projectile would have been. Even if you managed to make it target the earth directly. If necessary you would of course use several years worth of the entire earth's production of steel (and maybe lead, gold and anything else hard or heavy) and use it to cover the bomb and keep the energy around a tad longer.
Why does it need to be aim along the planet? Use orbital mechanics: Send your spacecraft on an orbit such that it hits the planet it launched from at the fast point of a very long elliptical orbit. Or even just at the far side of the current planet's orbit, whatever. It can't be that hard to get an impact at whatever angle you'd prefer with most of the Orion vehicle's energy, launching direction barely seems to matter.
No particular reason. It's just that the arbitrary task of planetary self destruction that Multipartite specified happens to be that of destroying the planet with a bomb on the surface. If you were just trying to destroy the planet then doing so from the surface seems like a terrible idea.
(For thoroughness, noting that the other approach was also wondered about a little earlier. Surface action is an alternative to look at if projectile-launching would definitely be ineffective, but if the projectile approach would in fact be better then there'd no reason not to focus on it instead.)
As I already said, the scaling laws mean the larger the bomb, the less efficient it becomes. Piling together 200 bombs just means you get inefficiency and possible fratricide from the explosion themselves or neutron emission. (Looking briefly at my old notes, underwater explosions have an exponent of tonnage raised to 2/3 for the resultant overpressure wave.)
If we had some convenient test planets handy I would bet against you when it came to optimal planetary suicide methods.
Orion's is a great idea for a different task but just not the solution to this problem.
/shrug
I'm the one who has read in detail about basic equational descriptions of nukes by their tonnage, targeting concerns, the engineering involved in going to megatonnage or gigatonnage range, etc. (I wanted to make a nuke simulation game), so you'll pardon me if I put more weight on my opinion than yours.
And you must pardon me if I find that:
Whatever mass of theoretical expertise you may have behind you, you most decidedly haven't applied it coherently here. To the extent that if you had just made an assertion and appealed to your own authority I may have believed you but given that you explained details that are clearly wrong I have to dismiss it out of hand.
No they aren't.
Three is more than sufficient to undermine your proof of concept "not enough fissiles" declaration.
What the? Don't be absurd. "Bomb on the planet's surface." was exactly the problem specification. If I wanted to destroy the earth I would obviously not try to do so with a bomb on the planet's surface.
Any interstellar method is a fortiori a terrestrial method as well.
In what respect? The Tsar Bomba gives us an estimate on what the third state buys one, let's be generous roughly an order of magnitude (6mt to 60mt). The analysis I linked concluded that the existing nuclear stockpile was at least 100x too small to power an Orion, so a 10x increase is useful but not enough (and where's the plutonium for 65k stage threes coming from?). If we go with the link's world-wide estimate of 300,000 years of production and blindly apply the 10x estimate, that still leaves us 30,000 years short.
And where does the Orion vehicle hit?
(Would an ICBM be excluded as an answer because it enters space?)
It's a task with entirely different challenges and to which entirely different tactics are optimal. It is just plain not the case that all interstellar methods are a fortiori terrestrial methods - since the very method your declaration of impossibility assumes is sufficiently optimal to rule out any possibility is one that doesn't work when launched from the surface of the target planet.
The Tsar Bomba was deliberately crippled from it's original specifications so as to reduce fallout while still being a sufficiently excessive explosion (although this was only on the order of a twofold reduction). Regardless I am almost as willing to accept the Tsar Bomba as the theoretical best (or worst) case for what humankind can do with a three stage thermonuclear device as I am to declare that 'Little Boy' the most potent fissile device that humans could make.
Simply taking the calculations used for hypothesizing about Orion weapons and applying them to surface based doomsday device is a gross error. The approach taken and the limiting factors would be entirely different.
This is not to say that I believe we currently have the technology to destroy the planet itself. To the best of my knowledge we do not. I simply reject the notions that an Orion device would be the way to go about it and hence that the fission limitation found when hypothesizing about Orion still applies at the levels calculated. I don't present an alternate hypothesis that because these limitations no longer apply as calculated that we must be able to do the destruction - that would be reversing stupidity.
Not at the location where the bombs go off - that's exactly the problem.
I suppose Multi would allow it provided in returns and detonates at or near enough to the surface. My initial speculations about using Orion for self destruction speculated at finding a gravity well sufficient to slingshot it back to us - but that engineering feat strikes me as a tad difficult.
I'll rephrase it, then: any method planet A can use to attack planet B can also be used by planet A to attack planet A. In the case of an Orion object, some large hyperbolic orbit which intersects the Earth seems possible, but if not, it could always be slowly launched out to whatever distance necessary and turn around.
No doubt that's part of the explanation, but is it the whole explanation? From one of Wikipedia's refs:
Another part of the ref mentions those modifications were made due to bitter arguments about whether it would work at all. (A quote from a report to the Soviet politicians says the design allowed for bombs of 'unlimited power' - which given the context, is as trustworthy as a $3 bill.)
Reducing the yield and eliminating an entire tamper sounds like something one might do if one is not sure the instabilities have been dealt with. Finally, to quote from one of the rival Soviet bomb-makers:
(Unfortunately he doesn't say whether the complexity and size were solely an issue for delivery or intrinsic to scaling, but it's suggestive.)
Yes, it is a gross error. An Orion vehicle is probably more efficient than a giant nuke or set of nukes, so the lower bounds aren't going to be tight.