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Putting aside shrapnel effects, I believe that high-explosives cause damage by producing a shockwave. How do shockwaves work in space? I've managed to convince myself that a high-explosive shockwave would work "about the same" by producing a shell of high-speed gas that would arrive at the surface with pretty much the same amount of energy that you'd get in air. Is that right?

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What is a shock wave?

definition:

A shock wave (also called shock front or simply "shock") is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or in some cases in the absence of a material medium, through a field such as the electromagnetic field.

This about covers it.

Your thought experiment speaks of an explosive, and it can create a shock wave only if a medium exists to be shocked: sudden almost instantaneous creation of vacuum in air/medium and immediate collapse of that as air/medium rushes to fill it.

The answer to your question

that would arrive at the surface with pretty much the same amount of energy that you'd get in air. Is that right?

is no, it is not right.

To reply to the title question:

Would a high-explosive in a vacuum be less harmful?

one would have to know the number of shrapnel from the explosion, and also if there is dust from the explosive or gas for some reason. From conservation of energy the fragments in vacuum will have larger energy because they will not be pushing against a medium ( to create the shock wave) and if they hit would do more damage. Any dust and gas would also move faster but one would need a detailed model and objective for the explosion to know whether the damage would be the same as in non vacuum.

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He means that the bomb-fragments will carry the shock energy in space, so that they will impact with the same energy as the shock wave would have if the explosion were in air. –  Ron Maimon Dec 7 '11 at 5:58
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That would be shrapnel, in my vocabulary and the questioners, but still not a medium to transfer a shock wave, imo. The shock wave happens because the explosion fragments, large and small, push out the air and have therefore less velocity than they would have in vacuum. In vacuum they will have higher velocity, but that is another problem. –  anna v Dec 7 '11 at 6:57
    
it's a fair interpretation of the question--- I agree with the content of your answer--- I just am not sure if the OP wasn't thinking that shrapnel takes over for shock, which is true. But the shrapnel might have less impact, because it is more concetrated, less of a club, more of a bullet. –  Ron Maimon Dec 7 '11 at 9:44
    
I am thinking only of the vapor created from the explosive itself. My thinking was that vapor would spread in a rapid, dense sphere that would carry energy and maintain its integrity for a second or two. But if I read the answer correctly, any such shape would lose its integrity immediately. –  Larry OBrien Dec 7 '11 at 16:42
    
"The shock wave happens because the explosion fragments, large and small, push out the air and have therefore less velocity than they would have in vacuum. " This is actually the key to me: I thought the shock wave was caused by the vaporized material of the explosive itself. –  Larry OBrien Dec 7 '11 at 16:47
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