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Comment by yock | Hacker News Reader

On the subject of privateers, why is the author so insistent on the danger of crashing ships into planets? What does the math look like when space craft like he describes hits the upper reaches of our atmosphere? At those speeds, could a space craft really penetrate far enough to cause terrestrial damage?

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A relativistic speeds, having airbags is not a free pass. Great, it hit your atmosphere and disintegrated. Now your atmosphere hits you.

An example: Project Thor / "Rods from God": http://en.wikipedia.org/wiki/Kinetic_bombardment

The linked article seems to mention re-entry friction as a problem yet-to-be-solved:

The system would also have to cope with atmospheric heating from re-entry, which could melt the weapon.

Then there's the question of whether or not heat is truly the most immediate issue. NASA and USAF had to actually fly an airplace near and past the speed of sound to see how the properties of the atmosphere changed when compressed. Is it logical to hypothesize that similar changes may be encountered as speed continues to increase?

I don't think melting is that big a deal. It's still the same amount of mass, only now it's molten steel instead of steel rods.

It'll start breaking into little blobs, though, which will have less and less power.

It doesn't matter. As long as each "blob" is heavy enough to resist deflection by the wind, the same total mass will impact at about the same speed. The only downside I can see is that the blobs might hit at slightly different times, which would reduce the instantaneous force on the target. But I think it would transfer the same total energy.

The same total energy would be transferred, but it would be spread out over a larger area. If the area is sufficiently large, then all you've done is raise the temperature of Germany by one degree for a while.

(Still, there are plenty of configurations that make good kinetic-kill weapons with a manageable amount of re-entry heat. Think large rods with good amounts of mass and small cross-sections. This is an engineering problem, not a show-stopper.)

Figuring Germany at 357021 sq. km, and "1 degree" to raise the equivalent of a meter of water by 1 Kelvin, gives 1.5E18 J.

If I did my math right, that's the energy in a 400 megaton bomb, or 17 kg of antimatter. The US nuclear arsenal is about 2500 Mt.

I don't think you'll be able to distribute that much energy so uniformly. Almost certainly you'll end up with all of it dumped into the surface, with people, land, and cities burned to a crisp. Only a few people in mines or deep valleys might survive.

The article mentioned tungsten, which has an absolutely preposterous melting point. I doubt it would melt moving at typical orbital velocities, but even if it were to be moving faster you could easily make the front of the projectile ablative.

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