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Originally Posted by ericthered
One thing worth considering is that unless the vectors line up just right, the bulk of the energy will blow right past the planet the moon was orbiting.
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A majority of the energy, maybe, but any fragments that are in the right trajectory to de-orbit will be dangerous.
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Originally Posted by Anthony
Depends on distance. Debris from the moon is mostly uninteresting, unless the average expansion velocity of the moon fragments exceeds the delta-V required to deorbit the moon most of the fragments just stay in orbit creating a new ring system, and if you have enough energy it's a pretty small moon and most of the energy will be wasted in fragments that miss the planet, so likely better off just hitting the planet directly.
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"Most" covers a range of values, though, from 0.5 + epsilon to 1 - epsilon. Which is a big range when you're talking about a proportion of millions of megatons of TNT equivalent.
If it helps to clarify, though, I'm interested in situations with relatively close-in moons, like Mars' phobos. Couldn't the debris from that do a lot of damage?
I should also clarify that I wasn't necessarily assuming the goal is to damage the planet directly. Maybe there's an important space fortress on the moon, or maybe (as in THS' Mars) it's anchoring a space elevator.
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Gamma rays are also irrelevant, they'll get stopped by the atmosphere.
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Right, I forgot about that.
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However, the thermal flash will be dangerous at 100,000 km or so, so if the moon is relatively low altitude and the explosion is visible from the ground you might set a hemisphere on fire.
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Ack, good point. I was confused about this until I checked just now, but thermal effects decline with the square root of distance, so when they dominate the optional "cube root" explosion damage rules become the less realistic ones. So for a low moon, that would be very bad.