How would a nuclear explosion look in space?

Dim, brief light. Almost nothing more.

The vast majority of the effects you describe would not occur.

There'd be little heat because the heat is a result of the radiation interacting with the atmosphere around it. Ditto for the mushroom cloud and the shock wave. (You need some medium for a shock wave to travel in). There'd be some visible light from the collisions of the radiation with the nuclear ash ("ash" as in the product of the fission, in the case of Uranium the most common are Barium & Krypton nucleii), as the ash absorbs and then re-emits, occasionally at visible frequencies, photons. Not very much, however, compared to the first-generation fission products, such as the photons produced from fission which are gamma rays, invisible to the naked eye. Keep in mind at the end of the day you don't really need that much material actually undergoing fission: U-235 will explode if it's in a ball not a whole lot larger than your head. At the end of the day, that's not very much material to provide much cross-section for the gamma rays to scatter off of, and the gamma rays are going to outpace all the other matter in the explosion: They move at the speed of light, and nothing else does.

Still, there'd be a ton of gamma rays, beta particles (naked electrons), and neutrons emitted, enough to cause plenty of death and destruction to anything nearby. It just wouldn't be immediately visible - Think the inside of a microwave oven, only more so.

Keep in mind everything I just described occurs with the detonation of a nuke on earth, only nobody notices, because in part it's attenuated by the atmosphere, and in part because the attenuation produces the mushroom cloud, the heat, and the shock wave which destroys anything affected by the radiation, only a fraction of a second later. The first-generation radiation itself pretty much attentuates to a vanishingly low amount after about a mile-and-a-half from the atmosphere.

The air force did some test with high-altitude nuclear explosions, but they weren't really in space. They were in the kind of space that John Glenn orbited, which still has a fair amount (on the order of 1%) of atmosphere. What I'm talking about is SPACE space. Vacuum.

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Oh, it would NOT look like the sun, by the way. NONE of the light we see from the sun is a first-generation product of fusion. Sure, the fusion reaction produces photons (also gamma rays, actually) but they take an average of 100,000 YEARS to go from the core, where fusion actually takes place, all the way to the surface.

The light we see from the sun is just garden-variety black body radiation. A ginormous incandescent light bulb, only hotter. Light bulbs burn yellow because they're cooler than the surface of the sun, but the sun actually appears roughly the same color because it's depleted of violet, indigo, and to a lesser extent, blue light because of the absorption and scattering from the atmosphere.

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Is VISIBLE light emitted from nuclear reactions? No, not usually. The photons emitted from uranium and plutonium fission have some wiggle room in their energies, but not enough for many of them to end up in the visible spectrum.

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Nuclear Explosion From Space

Awesome, it would look awesome.

The shape of the explosion would depend on the weapon. But, in general, unless engineered to go off differently, the shape would be spherical. There would be no mushroom cloud, as you indicated, because there would be no cold air to trap the explosion and cause it to expand out like the top of a mushroom.

There would be no traditional shock wave because gas, like air, is required for a shock wave. There would be a different sort of shock caused by the impact of the heat and radiation flux emitted by the explosion.

There would still be a lot of heat and some of that would be in the visible range like infrared. There would also be a lot of light as the intense energy would be manifest over the entire spectrum of visible light depending on the energy levels. And as the explosion is in a vacuum, there would be no ejecta and debris from the ground to obscure the light.

This would be a spectacular and relatively long lasting flash because it takes time to deplete all that energy; it would not be a puff.

BTW: It does not take matter to have light. Light is energy; all it takes is energy in the visible light range. And a nuke has plenty of that.

ABTW: Nuclear and atomic explosions are the same thing. When an atom splits, it's the nucleus of that atom that does it.

There would be no mushroom cloud. That is due to heat rising, gravity, pressure, and some other variables.

It would be a very fast, perfectly spherical explosion of light.

However, there would be no sound at all.

There would definitely be light though. Nuclear explosions occur due to "fission": large atoms splitting, releasing energy, and creating smaller atoms.

This is similar to how the sun operates (however the sun uses Fusion, not fission, though they are similar). Therefore, the explosion would look like a mini-sun, but it would burn out very quickly

You see that big ball of glowing matter in the sky during the day? Or, those millions of little dots at night? Every star is one big nuclear fusion reaction. If you are talking about one big atomic bomb... It would spread out in an almost spherical shape, but would be oblong as it is shaped by the nearest source of gravity.

Simple- like the sun. the sun is a nuclear engine- energy released in all forms of radiation.

A supernova is a nuclear explosion in space, and it looks like a bright, expanding cloud of plasma and gas.

Space is a continue um. So how long do you think the effects of such a blast would take to get to where you are standing.

Who cares?