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I just spotted this article about a metal foam that stops armor-piercing bullets as well as radiation. It's a press article, not a scholarly paper, so it may be complete bunk. And rifle bullets are amazingly slow relative to just about any object wandering around in orbit. However, metal foam seems like something that could be "easily" manufactured in a Lunar environment.

Is making a habitat shell out of material like this going to be significantly helpful, or are meteorites just going to punch through it anyway?

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    $\begingroup$ You might be interested in a technology that sort of works off of the same idea, called the Whipple Shield. $\endgroup$ – Lacklub Apr 11 '16 at 17:26
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    $\begingroup$ Objects orbiting in space are moving so much faster than a bullet that conventional ideas of shielding and armour no longer apply. The Whipple shield is a perfect example; it would be useless against a bullet on Earth, but protects the hull of a spacecraft in orbit. $\endgroup$ – Thucydides Apr 12 '16 at 0:09
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    $\begingroup$ @Thucydides Why would it be useless against a bullet? Mass is mass. F=mv^2. $\endgroup$ – Chloe Apr 12 '16 at 6:09
  • $\begingroup$ "Are meteorites just going to punch through it anyway?" -- Depending on size and speed of a single meteorite, it might or might not. Lots of meteorites will ablate the protection provided by any physical armor. Either by sheer numbers, or over time. $\endgroup$ – DevSolar Apr 12 '16 at 8:57
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    $\begingroup$ A Whipple shield uses some rather elegant physics (as described by Tristan), while 5+ metres of solid lunar regolith will need a lot of kinetic energy to punch through. An object at orbital velocity is moving at kilometres per second, rather than hundreds of metres per second, so the kinetic energy of a bullet sized object will be greater than even a hypervelocity 120mm tank round. Foamed metal can absorb the energy of a bullet, but not a thousandfold increase of energy from orbiting debris $\endgroup$ – Thucydides Apr 13 '16 at 3:14
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It would work, but would also need constant repairs.

The basis of the armor mentioned in your attached article is not simply to stop the bullet with a metal foam, but to use the metal foam as the middle layer in a new form of composite armor plating, in which an outer layer breaks up the bullet and distributes shock, a middle layer absorbs kinetic energy, and a third layer prevents passage of the projectile. In this configuration, the foam layer offers better energy dissipation and better protection than a simple layer of solid metal. A similar set up could be used to armor a lunar base, though you'd need significantly thicker armor to stop an asteroid.

However, your armor will take significant damage whenever a projectile hits it. Meteors have lots of kinetic energy, and the method of energy absorption in most such compounds is through mechanical deformation, a.k.a. damage. As even small meteors hit your shielding, it will slowly break down and become useless, requiring constant repairs to maintain effectiveness.

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You probably need metal foam as a form of "spacer" in a series of spaced armor plates. although most people tend to use kevlar to stuff spaced armor gaps. The hard plates would deform the incoming projectile as it passes through and the metal foam absorbs the fragments.

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

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

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As the technology is not really proven I think any answer would be primarily opinion based.

You might build your base out of a certain material and then armor it with this sort of shielding. However, when considering any sort of shielding you have to also consider the mass and acceleration of the object impacting it. Basically, if a meteor the size of a house impacts your base no amount of foam is going to save it.

By far the easiest approach is to simply build the base underground. That way the surface of the moon is your "shield".

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It could be workable but the layered shell technology they tested for next generation spacesuits works better and requires both less metal and less maintenance so foam is probably not going to be the go-to unless the metal being used is a waste product, or otherwise exceedingly cheap, and the location is a stationary base that doesn't have to consider mass too carefully.

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