Nuclear explosions in deep space

[essenmein]

Trying to understand the effects of a deep space nuclear explosion.

Starfish prime was detonated at 400km, ie still within Earth's atmosphere, and the explosion effects are well described.

https://en.wikipedia.org/wiki/High-altitude_nuclear_explosion
A significant outcome is the high MeV gamma, hits the (thin) atmosphere creating a bunch of charged particles which then wizz along our magnetic field lines and create various EMP effects.

So what would happen in deep space, away from any atmosphere and magnetic fields?

My current thought is that basically its a bright blink of light, assuming the weapon itself is comparatively not a large amount of mass, all you have is the energy output in the form of radiation, neutrons, and electromagnetic, I imagine from IR out to Gamma.

From reading the wiki, the EMP is due to charged particles interacting with the Earth's magnetic field, neither of which exist in deep space, how bad would the EMP be in deep space?

Basically my thoughts are that a "space burst", ie detonating near a target, has minimal effect, nothing like an air burst in atmosphere.

So to get any real destructive power, the war head must penetrate the target, and detonate inside it.

Reasonable or way off the mark?

 

[scottdave]

So what will happen when the gamma rays or other particles strike a spacecraft . What about living beings inside the spacecraft . Will the skin of the spacecraft protect them? If this is sci-fi, does the spacecraft have shields? There was an episode from the original Star Trek series, which a nuclear warhead detonated near the Enterprise. You may get some context from that.

 

[stefan r]

My current thought is that basically its a bright blink of light, assuming the weapon itself is comparatively not a large amount of mass, all you have is the energy output in the form of radiation, neutrons, and electromagnetic, I imagine from IR out to Gamma.

It should have the fission fragments. Atoms of the physics package, the bomb casing, and unfissioned uranium/plutonium will also fly out. Inside an atmosphere the fission fragments strip electrons from gas molecules. The gas molecules then radiate light similar to what you see in lightning flashes.

...
Basically my thoughts are that a "space burst", ie detonating near a target, has minimal effect, nothing like an air burst in atmosphere.

So to get any real destructive power, the war head must penetrate the target, and detonate inside it.

Reasonable or way off the mark?

Way off. The energy will be proportional to distance squared. Inside an atmosphere some energy gets dumped into the gas so the amount of energy on target should be lower. A rising fireball/mushroom cloud carries energy with it.

Space is big. That distance squared can make a very significant distance. There are in effect many nuclear explosions sweeping past the solar system right now. Cosmic rays are normal. You certainly would not need to get the nuke "inside". Armor plating can deflect a nuclear blast on Earth. It is hard move around armor in space. The ships used in operation crossroads give some indications. Missing an ocean ship by a large distance would fail to sink it. Detonating nearby usually does the job.

Project Orion makes for fun reading. The pusher plate design will give you some ideas on how far away you can be from the blast. With no shock adsorber a pressure wave will be like a crash rather than like a gradual acceleration. The nuclear explosions clearly give a pressure wave. You can boost the damage using nuclear shaped charges.

The project rho website has enough information to get lost for a few days. Here is the nukes in space article. Project rho is not peer reviewed or up to physics forums standards. For science fiction it is superb. It goes into some details on what to expect.

 

[essenmein]

It should have the fission fragments. Atoms of the physics package, the bomb casing, and unfissioned uranium/plutonium will also fly out. Inside an atmosphere the fission fragments strip electrons from gas molecules. The gas molecules then radiate light similar to what you see in lightning flashes.Way off. The energy will be proportional to distance squared. Inside an atmosphere some energy gets dumped into the gas so the amount of energy on target should be lower. A rising fireball/mushroom cloud carries energy with it.

Space is big. That distance squared can make a very significant distance. There are in effect many nuclear explosions sweeping past the solar system right now. Cosmic rays are normal. You certainly would not need to get the nuke "inside". Armor plating can deflect a nuclear blast on Earth. It is hard move around armor in space. The ships used in operation crossroads give some indications. Missing an ocean ship by a large distance would fail to sink it. Detonating nearby usually does the job.

Project Orion makes for fun reading. The pusher plate design will give you some ideas on how far away you can be from the blast. With no shock adsorber a pressure wave will be like a crash rather than like a gradual acceleration. The nuclear explosions clearly give a pressure wave. You can boost the damage using nuclear shaped charges.

The project rho website has enough information to get lost for a few days. Here is the nukes in space article. Project rho is not peer reviewed or up to physics forums standards. For science fiction it is superb. It goes into some details on what to expect.

I was doing bunch of research about the physics of nuclear explosions, and it was there I realized that they would behave very differently in space.

Basically in air, I can't remember the exact proportion, but its large, like 70-80% of the energy out put of the weapon, IR upto soft X ray, is spontaneously emitted, and absorbed in the immediate surrounding air, 20-30m radius or something, gets super heated to 10's or 100's of thousand K can't remember exact numbers. This then rapidly expands into the low pressure atmosphere and causes the "explosion", pressure wave, mushroom cloud etc. This is why you see the characteristic double flash, and why an air burst will level a city, Hiroshima was detonated at ~600m altitude, and was only 16kt or something.

So now if I just consider energy per m2, I keep forgetting I should just run the numbers!

Anyway, assume 1Mt weapon (ignore type, in the story they are predominantly antimatter, and some fusion, but I think it makes sense to just look at energy density).

Lets say the weapon is 1m radius, this gives surface density of ~80kt per m2 if it penetrates, at 300m away (half Hiroshima) its down to 0.8t per m2.

The armor on the enemy ships is predominantly metal oxides, alumina, zirconia, already operating at an elevated temperature, ~1000K.

So if 1Mt nuke was detonated 300m away from large ship with thick layer of alumina as outer shell, the surface energy imparted per square m, if it was put into the 1m below, just to make the math easy, so that per m2 number gets put into one cubic m of material. At 300m, that 0.8t is 3.7GJ, this energy would heat 1cu m of alumina by a mere 1100K, if starting at 1000K, is not enough to even melt this (2900K mp).

However at point blank, the temperature increase is 100e6 K, this now creates a pocket of super heated gas that wants to expand rapidly, and then I think the effect is far more severe.

Then there is the problem of detonating near the ship means at least half the yeild is lost to space, inside 100% is imparted to the target.

 

[essenmein]

Reading about the orion... pretty cool stuff. Some exerts from wiki:

"The plasma would cool to 14,000 °C as it traversed the 25 m distance to the pusher plate and then reheat to 67,000 °C as, at about 300 microseconds, it hits the pusher plate and is recompressed. This temperature emits ultraviolet light, which is poorly transmitted through most plasmas. This helps keep the pusher plate cool. The cigar shaped distribution profile and low density of the plasma reduces the instantaneous shock to the pusher plate. "

Not sure here if they are talking about the bigger 1Mt pulse units, or the smaller ones, but it sounds like the blasts were 25m from the pusher plate.

"Exposure to repeated nuclear blasts raises the problem of ablation (erosion) of the pusher plate. Calculations and experiments indicated that a steel pusher plate would ablate less than 1 mm, if unprotected. If sprayed with an oil it would not ablate at all (this was discovered by accident; a test plate had oily fingerprints on it and the fingerprints suffered no ablation). The absorption spectra of carbon and hydrogen minimize heating. The design temperature of the shockwave, 67,000 °C, emits ultraviolet light. Most materials and elements are opaque to ultraviolet especially at the 340 MPa pressures the plate experiences. This prevents the plate from melting or ablating. "

ablate 1mm each time 1Mt explodes 25m away?

Hard to know what size pulse units they are talking about here, but let's turn this around, the orion would quite happily live through many thousand 1Mt blasts just mere meters away, it really does sound like nukes are quite toothless in the vacuum of space. The "momentum limited" design had a capacity of 300,000 1Mt pulse units...

Compare 1Mt detonated in the air over a city would level all buildings to about 3km radius.

 

[stefan r]

Hard to know what size pulse units they are talking about here, but let's turn this around, the orion would quite happily live through many thousand 1Mt blasts just mere meters away, it really does sound like nukes are quite toothless in the vacuum of space. The "momentum limited" design had a capacity of 300,000 1Mt pulse units...

Compare 1Mt detonated in the air over a city would level all buildings to about 3km radius.

The nukes were as small as possible. Wikipedia says 0.03 kt. You could use big nukes and get better results. Pulse propulsion is one of the rare things that scales up extremely well. The Orion Project ship is already too huge so designs tend toward minimum. The momentum limited ship says this:

the diameter and mass of the hemispherical pusher plate would have to be 20 kilometers and 5 million tonnes, respectively.

You could stand 10 km from a 1 mt explosion wearing welding PPE and you would be fine (at least until the fallout). An M1 tank could drive through an airburst. The crew might die from radiation poisoning but the engine, suspension system, and weapons would function.

The chart on the wikipedia page says that 800 detonations would be used to launch a Orion Project ship into orbit. So you get around 10 m/s per blast. The 10 m/s includes the entire ship and cargo. The dry mass (last detonations) would get accelerated more than the wet mass (first detonations). 10 m/s second is enough. Think of what your car would look like after a side collision that bounces it away at 36 kph. Low density areas will be accelerated more than high density areas.

Project Orion may have been a bad example. The charges were supposed to contain a tungsten plate that gets vaporized. The tungsten bounces off of the pusher plate. Fission fragments should penetrate deeper. I suspect rupturing the vacuum seals would be more serious than the effect of ablation. Space platforms are not usually equipped with extra mm of metal. That might be enough to cause failure.

 

[essenmein]

The nukes were as small as possible. Wikipedia says 0.03 kt. You could use big nukes and get better results. Pulse propulsion is one of the rare things that scales up extremely well. The Orion Project ship is already too huge so designs tend toward minimum. The momentum limited ship says this:

You could stand 10 km from a 1 mt explosion wearing welding PPE and you would be fine (at least until the fallout). An M1 tank could drive through an airburst. The crew might die from radiation poisoning but the engine, suspension system, and weapons would function.

The chart on the wikipedia page says that 800 detonations would be used to launch a Orion Project ship into orbit. So you get around 10 m/s per blast. The 10 m/s includes the entire ship and cargo. The dry mass (last detonations) would get accelerated more than the wet mass (first detonations). 10 m/s second is enough. Think of what your car would look like after a side collision that bounces it away at 36 kph. Low density areas will be accelerated more than high density areas.

Project Orion may have been a bad example. The charges were supposed to contain a tungsten plate that gets vaporized. The tungsten bounces off of the pusher plate. Fission fragments should penetrate deeper. I suspect rupturing the vacuum seals would be more serious than the effect of ablation. Space platforms are not usually equipped with extra mm of metal. That might be enough to cause failure.

The big diameter Cu plate was the energy limited design:
"The more conservative energy limited pusher plate design simply had to absorb all the thermal energy of each impinging explosion (4×1015 joules, half of which would be absorbed by the pusher plate) without melting. Dyson estimated that if the exposed surface consisted of copper with a thickness of 1 mm, then the diameter and mass of the hemispherical pusher plate would have to be 20 kilometers and 5 million tonnes, respectively. 100 seconds would be required to allow the copper to radiatively cool before the next explosion. It would then take on the order of 1000 years for the energy-limited heat sink Orion design to reach Alpha Centauri. "

Sounds to me like the size of the Cu plate was more for radiating. The numbers certainly roughly work for radiating half the total thermal energy of the weapon to space, either the Cu ~ reaches melting point and only radiate from one side, or if its radiating from both sides then it will maybe reach 500-600C. But that is with the big assumption that you can get all the heat from a single point source to be evenly spread over a 20km copper disk (not likely!).

Note that the number used in the wiki for total weapon yield is 4e15J, this is the 1Mt device, 1tonne TNT equivalent is 4.2e9J.

The momentum limited design had a sacrificial pusher plate, ie it was expected some would be lost to ablation. The momentum limited design had a vehicle weight 100,000t and to achieve 1G acceleration required one 1Mt detonation every three seconds.

Needless to say I think the calculations done around the orion project certainly make it sound plausible to design a spaceship that operates with nuclear explosions right next to it as the design intent. However I bet the orion would have serious problems if one of the pulse units were to detonate prematurely, say before it exiting the pusher plate.

So it would seem reasonable that if designing a ship of war where nukes are exchanged regularly that the "splash" from a weapon detonating near a vessel, should be survivable. However if that weapon is exploding another ship and now mass is involved, either via shrapnel or plasma, then the potential for damage is increased spectacularly.