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Let's suppose that a fusion bomb device with a yield of 200 megatons (4x Tsar Bomba) is detonated in Mare Tranquillitatis on the Moon.

My question is:

  • Would the explosion be visible from Earth at night? I imagine that it would be a flash followed by the covering of the explosion site in dust clouds.
  • Would there be any immediate or long-term effects observable from Earth?
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Let's run a few numbers. We'll assume that 10% of the energy from the explosion is radiated as visible light -- that's a good enough estimate for this purpose. 10% of 200 megatons is 84,000,000,000,000,000 joules, or 8.4E16. Figure it lasts for ten seconds, and that's 8.4E15 watts.

The Moon (and Earth) get about 1,360 watts/sq.m of solar radiation. Let's assume 50% of that is visible light, 680 watts/sq.m. The Moon's albedo is 0.12, so 12% of that gets reflected, 82 watts/sq.m. We'll call it 84, or 8.4E1, because it makes the sums ever so much easier.

So, the bomb will emit about as much light as 1E14 sq.m of lunar surface. The radius of the Moon is 1,700,000 metres, so the area of the disc of the full Moon is 9E12 square metres. Call it 1E13.

Therefore, for a few seconds the bomb will be something like 10 times as bright as the full Moon, and will thus be easily visible, but not immensely dramatic.

There won't be any dust clouds (for more than a minute or two), because there's no atmosphere to hold them up. The dust will fall back to the surface as quickly as any larger ejecta.

There would be no long-term effects other than a small (not visible without a pretty good telescope) new crater. There have been many thousands of bigger meteorite impacts on the Moon.

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  • $\begingroup$ Is it not possible that such an enormous explosion would have the capability of knocking the moon off its axis by a degree (maybe a bit less) or at least move the moon a couple of metres? I'm not certain of the magnitude required to have such an effect. $\endgroup$
    – Garto
    Jun 11, 2017 at 17:09
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    $\begingroup$ @Garto No, it's not an enormous explosion at all by astronomical standards, it's a tiny and barely noticeable explosion. $\endgroup$
    – Mike Scott
    Jun 11, 2017 at 17:10
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    $\begingroup$ "The dust will fall back [..]" ... hm ... with no atmosphere removing speed from particles I guess many will not fall back, but rather reach (moon) escape velocity, and possibly start orbiting earth? Though these particles are probably not much more than a bit of dust ... $\endgroup$
    – Daniel Jour
    Jun 11, 2017 at 19:33
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    $\begingroup$ @Garto, see this question and answers. True, the calculations are for Little Boy rather than Tsar Bomba, but the effects are so small that a factor of 10,000 difference in bomb size isn't meaningful. (In short, the expected movement isn't meters, it's nanometers.) $\endgroup$
    – Mark
    Jun 11, 2017 at 19:55
  • $\begingroup$ If I remember correctly, a handful of asteroid impacts have been observed as visible flashes on the Moon, although I'm not necessarily sure they've been verified. $\endgroup$
    – Andon
    Jun 12, 2017 at 2:29

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