What does an A bomb explosion look like...

…when detonated deep under water? I know they did some tests in the Bikini Atoll using nuclear weapons at sea, but I couldn’t tell how deep underwater the thing was when it went off. If we were to set one off in the Marianas trench, mad-scientist style, would the flash be visible on the surface? I know the blast is extremely bright, but water absorbs a lot of light (not to mention heat and energy in general). I’m wondering if you could see a bright glow from the surface when it went off.

…from space? What does an A bomb look like on the surface if you were in space when it went off? Have these kinds of observations ever been done?

IN space? Without gravity, or air for that matter, I imagine the blast would look very different. Would it simply be an expanding, glowing ball that gradually fades out? How much bigger (or smaller?) would the blast effects be without any atmosphere to push around or provide a shock wave?

No light would be visible from the surface of a bomb exploding in the deepest parts of the ocean. Maybe a little flash at the instant of explosion. You’ll even have a little bit to move out of the way of the huge ball of steam that going to leap through the surface. By the time it gets to the surface it may only be a mile or two across. But everything for several miles at the ocean floor would be killed by the shockwave. The shockwave should beat the steam to the surface - meaning a dome of water before it erupts. There are other mechanisms at play here too. I’m sure someone will give better details.

From space. it looks like a flash of light surrounded by a puff of smoke. the light doesn’t last very long.

A nuclear explosion in space would be much both smaller and bigger. The effects would be lesser in any given direction but would be spherical. Again the light wouldn’t last long. I don’t know how the shockwave would propogate without atmosphere. It would, space is not empty, its just sparse,so I don’t know the magnitude. The actual atoms spreading from the explosion would travel a great distance, far more than if it were in an atmosphere. And there would be no dust cloud - just the debris from the non fissile portion of the bomb - mostly in the form of vapor.

It’d look something like this. Purdy.

I can dig up relevant technical details later, if needed.

Operation Wigwam, a US Navy nuclear weapon test, detonated a 30kton device at a depth of 2000 feet.

http://www.dtra.mil/newsservices/fact_sheets/fs_includes/pdf/Wigwam.pdf (3 page PDF)

http://en.wikipedia.org/wiki/Operation_Wigwam (Has photo of air bubble rising to surface.)

No statements in either source as to how it looked at the moment of explosion. The water does not appear “lit”. The PDF states that it took ten seconds for the steam bubble from the explosion to reach the surface, and gives dimensions for the water plumes.

It’s possible that at the moment of explosion, there might have been a small flash bulb effect underneath the suspended cable, but that is a completely wild guess on my part.

I guess water really absorbs light. Sunlight only penetrates the top 200 meters or so, I think.

The following is a passage from a story written by a nuclear-Navy veteran describing a subsurface nuke explosion (ASROC nuclear depth charge) used by the U.S.S. Long Beach against a flotilla of Soviet subs that had damaged her when the Cold War turned hot in the story’s universe:

I don’t guarantee the accuracy of it, but it’s probably relatively close to an accurate description of what might happen in such a situation.

If one were detonated in the Marianas Trench at depths of around 35,000 feet, I have some doubts as to whether the steam bubble would survive the ascent. The initial large steam bubble would break up into smaller bubbles, and in fairly short order would cool enough to condense back into liquid water at those high pressures (~15,000 psi at depth). Hot liquid water might continue to rise, but over the course of the seven-mile ascent it’s likely a lot of mixing would take place that might cool the whole mess down enough so that no steam bubbles would appear at sea level.

Things might change if one moves from a 50-kiloton device (Hiroshima was I think 10-20 kT) to a 50-megaton device (largest ever tested was a USSR-made device that was 58 MT).

From Trinity and Beyond: The Atomic Bomb Movie (which I heartily recommend for anyone following this thread), we have this footage.

No flash that I can see there, and I don’t remember seeing one in the similar French nuclear tests back in the 90s, but if you look carefully at the footage of Bikini Baker—only the third test and fifth nuclear detonation overall—you can see what looks like a flash of light above the surface. However, considering how shallow that test was conducted (only 90 ft underwater), that’s really not surprising, and I would assume it’s the fireball itself breaching the surface.

This makes me doubt the writer’s technical knowledge. The effect of an underwater explosion should be measured in cubic units.

The lagoon was pretty shallow, too. I’m sure that has an effect as well. Wiki says that a lot of irradiated seabed sand got dumped on the test/target ships that were not capsized by the blast. Decontaminating the ships after the shallow water balst proved to be more difficult than anticipated.