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When asteroids crash into the Moon they give of huge amounts of energy resulting in a "bright" flash, as can be seen in this video. At the end of the video its also says that anyone looking at the Moon at that moment would have been able to observe it without a telescope.

But what kind of collision would be required to be able to be seen from Earth when it would involve more distant bodies? Would one of Jupiter's retrograde moons colliding with one of its prograde moons be visible?

A real example of a high energy collision would be the collisions of the fragments of comet Shoemaker-Levy-9 with Jupiter. However according to this video from ESA it happened on the opposite side of Jupiter, so not in our view.

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migrated from May 19 '14 at 0:44

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OK, so... you said the collision with Shoemaker-Levy 9 and Jupiter happened on the non-visible side. What if we'd seen it? The energy released by the largest fragment collision was estimated at 6 million megatons of TNT, which is 2.52e22 joules. The Sun's luminosity is 3.846e26 watts, by comparison. So obviously the collision won't rival the Sun. Using the luminosity-distance relation and taking the 6 million megatons as pure luminosity (an overestimate, but we're not going for precision here) gives the result of 0.0033 watts/square metre.

Relating flux to apparent magnitude, we get an apparent magnitude of the collision of... approximately -12. That's about the brightness of a full moon. Bear in mind this is a very rough calculation and it makes the assumption that all the energy was released over a second and all of it was light, so probably the real value would be a few magnitudes down But, assuming I've made no terrible calculation errors, it would definitely have been visible.

Given that was a relatively tiny comet, your example of two of Jupiter's moons colliding would release FAR more energy, therefore would be clearly visible, I'd imagine. All depends on how much energy gets emitted as light.

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