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The sheer speed of an object at orbital velocity (8 km/s) is like the kinetic energy of 7.5 kg of TNT for every kg of mass. At the speed of an object falling from near the Moon, it's like 14.5 kg of TNT for every kg of mass. Meteors normally enter the atmosphere at even higher speeds, and are famously destructive if they don't burn up in the atmosphere enough for their energy to be dissipated.

The idea of turning that into a weapon has been pondered, but at the moment is not practical and so plays little role in strategic thinking. The near-term idea was to use tungsten rods dropped from orbit. That would seem to be designed to penetrate hardened defenses, not to do general damage, so a bit specialized.

At some point operations in space will make it possible to take material being mined from an asteroid or on the Moon and do something like this. It's still pretty far off, but considering the potential destruction, it seems to me it would be good for people to be aware of what could be done and what would be involved to do it. Surely our analysis of the value of space exploration, and how best to do it, needs to include this.

Since you could aim objects to enter at a near-vertical angle you could minimize how much the atmosphere slows them down and burns them away. So maybe the shortest path involves determining how to select appropriate boulders and shape them enough to be missiles. Is this an appropriate short list of necessary qualities?

  • a conical shape,
  • Solid enough to not break up before being close to the ground, but fractured enough to air burst once it is
  • Perhaps coated with an ablative material, also to help it get close to the ground
  • As dense as possible, so a bit of an iron meteor maybe, or a chunk of ilmenite if you are doing it from the Moon. If you have managed to start mining platinum group metals, they would work wonderfully but you probably wouldn't want to waste them.

And then there is the question of how much stuff would be needed to do big damage, say city-destroying level. The kinetic energy of a ton of material in the vicinity of the Moon is equal to the Hiroshima bomb, but much of that would be lost during atmospheric entry. If the material was prepared as in the list above, about how much of it would result in an explosion like Hiroshima?

Notes: TNT equivalent considers a ton of TNT to be 4.2 x 109 J of energy, and speed can be considered as kinetic energy with the formula $E_k=0.5mv^2$. The Hiroshima bomb exploded at a height of about 600 m.

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  • $\begingroup$ This will perhaps seem like a provocative question, but I genuinely feel it is a discussion that needs to happen much more broadly now. Answers are likely to be referred to during my interview tomorrow on The Space Show. $\endgroup$ – kim holder Jun 2 '16 at 19:41
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    $\begingroup$ "What would be the most efficient.." Efficient in terms of what? Time, money, propellant, the amount of energy expended versus that released in the final impact, technical expertise you'd need to gain before attempting such a thing..? $\endgroup$ – Andrew Thompson Jun 2 '16 at 20:02
  • $\begingroup$ @AndrewThompson The title change i think clears that up? The question is actually looking at a specific scenario, and sticks to asking what sort of treatment of bulk material would allow the least amount to be needed for the effect, and how much that amount would be. $\endgroup$ – kim holder Jun 2 '16 at 20:19
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"Solid enough not to break up" is what you need. Elongated shape (possibly spin-stabilized) to minimize the attack surface would be a boon, but not essetial. They are their own ablator, and the solid enough ones will be dense enough too.

Breaking up is the demise of great most meteoroids as turning into swarm of rocks and pebbles increases the attack surface dramatically, square-cube law making their area much larger in proportion to mass and allowing heat to penetrate the cross-section either leading to evaporation, breaking up into harmless gravel or slowing down enough to be harmless.

Besides hardening them though, your number one enemy will be delta-V. You need to accelerate them into a collision trajectory, by ejecting enough reaction mass (propellant) - likely amount comparable to the mass of the asteroid, maybe an order or two smaller - but with these things weighing thousands tons, that's still aplenty - and you need to bring that propellant there somehow. Probably by burning some 30x as much or more propellant on launch.

Economically speaking, you could nuke Earth to oblivion ten times over for the same price.

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  • $\begingroup$ This idea was looking far enough into the future that propellant launch from Earth wouldn't be an issue, there would be infrastructure in space for that. Perhaps if you have that much infrastructure you would be better off refining uranium and making nuclear bombs anyhow, i don't know. Then again, any infrastructure designed to move bulk materials around in space should also be capable of this, so again, it is a shorter path. $\endgroup$ – kim holder Jun 2 '16 at 20:12
  • $\begingroup$ @kim: yes, if asteroid mining is considered viable... sure orbital bombardment would be of comparable cost. The problem is the up-front costs of building that infrastructure. If it was an economically sustainable endeavor (mining) then the cost would be swallowable. If it was only meant to provide an inferior alternative to nukes for many times their price... not really. $\endgroup$ – SF. Jun 2 '16 at 20:25
  • $\begingroup$ Fair enough, but short of feeling that humanity will never settle the solar system, there is no industrial scale activity in space that wouldn't yield the capability to do this. If you can build a large space station, or a settlement on a celestial body, you can also do this. $\endgroup$ – kim holder Jun 2 '16 at 20:29
  • $\begingroup$ The classic "Moon Is A Harsh Mistress" scenario presumes a moon-based power source (nuclear or solar) launching iron-cased projectiles toward earth via electromagnetic railgun, rather than chemically propelled. $\endgroup$ – Russell Borogove Jun 2 '16 at 20:46
  • $\begingroup$ @RussellBorogove Yep. I rather like the tether or sling-launcher approach to the problem. More efficient. Some orbital assembly would be required... I picture railguns more as defensive artillery pieces launching small projectiles with high frequency... $\endgroup$ – kim holder Jun 2 '16 at 20:50

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