Nukes… in… space…

September 26, 2008 2:25 PM

To what extent would the lack of an atmosphere reduce the effectiveness of nuclear weapons?
posted by Artw to technology (17 comments total)

For one thing, overpressure effects would be quite different, if present at all.
posted by Cool Papa Bell at 2:35 PM on September 26, 2008

Well, if it was floating in space there'd be no shockwave. Just a flash of radiation.

On a moon or something I guess you'd have vaporized rock giving you some sort of a pressure wave.
posted by codswallop at 2:35 PM on September 26, 2008

From the Federation of American Scientists:

For a low altitude atmospheric detonation of a moderate sized weapon in the kiloton range, the energy is distributed roughly as follows:

50% as blast;

35% as thermal radiation; made up of a wide range of the electromagnetic spectrum, including infrared, visible, and ultraviolet light and some soft x-ray emitted at the time of the explosion; and

15% as nuclear radiation; including 5% as initial ionizing radiation consisting chiefly of neutrons and gamma rays emitted within the first minute after detonation, and 10% as residual nuclear radiation. Residual nuclear radiation is the hazard in fallout.

Considerable variation from this distribution will occur with changes in yield or location of the detonation.
...
In an atmospheric detonation, this electromagnetic radiation, consisting chiefly of soft x-ray, is absorbed within a few meters of the point of detonation by the surrounding atmosphere, heating it to extremely high temperatures and forming a brilliantly hot sphere of air and gaseous weapon residues, the so-called fireball.

Note that they define 'blast' as "ground shock, water shock, “blueout,” cratering, and large amounts of dust and radioactive fallout."

So without an atmosphere, there would be no blast at all. Furthermore, the soft x-rays would not be absorbed, which would eliminate the fireball. Basically there would be a large amount of EM and nuclear radiation. In deep space, the effectiveness of a nuclear weapon would be limited by the cross-section of the target.

Here's a video of a report on the Argus series of high altitude (200km-540km) detonations. The last test in that series was the highest nuclear test ever performed, according to the Wikipedia article on high altitude nuclear explosions. The theory being tested by Argus was whether a very high altitude detonation could create a lingering radiation belt that would disrupt radio transmissions, radar, and ICBM arming and fuzing mechanisms.
posted by jedicus at 2:41 PM on September 26, 2008

This depends on whether you're considering something like the moon or whether you're just off in space somewhere.

No conductive heat transfer. So as long as you're in the shadow of a big rock or something you would be mostly protected from the blast. Also, no shockwave from expanding superheated air. On the moon it would throw some dust around, but...

No breathing of radioactive dust. This removes a lot of the secondary damage from a nuclear blast. On the moon it would throw up some dust, but it would settle a lot faster than in an atmosphere and it wouldn't tend to travel as far since there's no wind.

On the downside, no atmosphere for humans generally means sealed vehicles or suits which have electronic systems. EMP is going to cause problems for those - as in, you would expect most life support systems to fail. Same with computers unless they're shielded or somehow hardened against EMP.

If you're not talking about sci-fi scenarios but are instead asking about more realistic scenarios like hitting a satellite in orbit with a nuke, you'd pretty much have to hit it dead on with a missile to do anything other than EMP damage. I think most of the "star wars"-era nukes where lasers that were pumped with a nuclear explosion which made it a bit easier to hit something at a distance.

From the NY Times, in 1983:

"Known as an X-ray laser, the device takes the power of a nuclear explosion and channels it into laser rods that emit lethal bursts of radiation in space. According to scientists at the Lawrence Livermore Laboratory in California, the birthplace of the device, clusters of X-ray lasers could stop the entire force of strategic Soviet missiles, currently some 1,500 strong. The lasers would literally pound the rockets apart with powerful X-ray beams."

Google up "nuclear pumped space laser" to find it and a number of patents on the process.

The only damage caused by a nuclear device in space will be:
- radiation fairly near the blast
- shrapnel from the explosive device, again fairly near the blast
- EMP which induces high-voltage currents in metal objects, especially wires and circuit boards
- heat from 1 & 2 and some radiant heat, again only really close to the blast

(This is from memory and having played waaaay to many sci-fi RPGs in my teen years)
posted by GuyZero at 2:41 PM on September 26, 2008 [1 favorite has favorites]

Do you mean in empty space or do you mean on a surface with no atmosphere? What is your metric of to measure how effective the blast is (i.e. are you wondering about the amount of energy released, the physical destruction, or the effect on people (in spacesuits, presumably)?
posted by ssg at 2:43 PM on September 26, 2008

A nuke doesn't depend on oxygen (or any other gas) to explode. You'd still have the awesome explosion, incredible heat, and deadly radiation. But without an atmosphere, you'd lack the mushroom cloud and overpressure, which require some kind of gas. Probably the biggest difference would be radioactive fallout. With an atmosphere and gravity, the radioactive particles from the nuke eventually make their way back to ground level, making it radioactive there for a long time. But without an atmosphere and gravity, the little radioactive particles would keep on going, far, far out into space.
posted by exphysicist345 at 2:43 PM on September 26, 2008

In deep space, the effectiveness of a nuclear weapon would be limited by the cross-section of the target.

This is a very good point. The amount of damage will vary greatly based on how much radiation/radiant EM the target can soak up.
posted by GuyZero at 2:44 PM on September 26, 2008

you'd still have the awesome explosion,

Would you? It seems like a lot of it must come from the oxygen in the atmosphere combusting.
posted by smackfu at 3:23 PM on September 26, 2008

Can someone explain how an Orion drive works given there is no blast in vacuum? I've never quite figured that out. Orion drives work by dropping nuclear bombs behind a pusher plate attached to a payload. But obviously there must be more involved than that since I don't see how you generate thrust on the pusher plate in a vacuum.
posted by Justinian at 3:53 PM on September 26, 2008

An Orion drive presumably works the same way as a solar sail, except with more photons coming from closer up. Whether you could design a substance that reflects 100% of photons across such a wide range of the EM spectrum is presumably an engineering issue.
posted by GuyZero at 3:57 PM on September 26, 2008

It seems like a lot of it must come from the oxygen in the atmosphere combusting.

As noted above, not combusting but being hugely heated by xrays.
posted by ROU_Xenophobe at 4:00 PM on September 26, 2008

An Orion drive would also presumably work by capturing the kinetic energy of the bomb casing and the non-fissionable particles released by the bomb -- little bits of stuff flying out of the centerpoint at high energies.

Kinda like how an ion thruster works, only in reverse -- instead of shooting particles out the back of your craft, the particles are shooting back at you.

Which raises the question -- how do you make an Orion payload vehicle that could survive this repeated, high-energy bombardment?

This is why it's science fiction. ;-)
posted by Cool Papa Bell at 4:08 PM on September 26, 2008

The guys at General Atomics actually seemed pretty confident they could pull it off. Details:

Propellant (water or wax) surrounding the bombs would be transformed into high-energy plasma and bounce off a pusher plate at the rear of the rocket and push it forward. Shock absorbers would even out the ride. Although the plasma from the explosion would have a temperature of 80,000 deg K, the impulse would be brief and only a tiny layer of the ablative pusher plate would sublimate after each explosion. A method was developed of 'greasing' the plate between explosions to protect it.

This is a great video, part of a great BBC doc.
posted by smackfu at 4:25 PM on September 26, 2008

ablative pusher plate

I was about to say this so it's good to know that the big brains think the same thing. But I just expected that you'd have a plate thick enough to survive all the bombs you had packed. Like any vessel, you don't expect it to go forever and the pusher plate essentially becomes part of the fuel as it vaporizes.

if you want your ship to go forever, you speed it up close to c and use a ramscoop. Obviously.
posted by GuyZero at 4:32 PM on September 26, 2008

jedicus has the best write up so far.

However, the asker needs to clarify what exactly they mean by "lack of an atmosphere" to get a precise answer...

If we're talking about setting off a bomb in space than many of the aforementioned points are right on point, especially the comment regarding the cross-section of your target.

On the other hand, if you're setting off the bomb on Earth, and just happen to be positing a lack of atmosphere at ground level for some unknown reason then the results would be a lot more difficult to predict.

The thermal radiation that causes fires and/or firestorms with normal airburst explosions wouldn't do as much damage without oxygen to fuel actual fires, but at the same time the lack of air circulation might mean that some materials with high specific heat capacities would not cool off as quickly, leaving a hot surface both in terms of hard radiation and heat. Your average industrial city (steel and concrete) already acts as a sizeable heat sink during the day and radiator at night WITH normal air circulation, so I would expect that the thermal implications would be extensive for any sort of urban target.

In the same respect, you could expect fallout, the physically irradiated materials displaced by the actual energetic blast, such as the dirt that used to be the crater (for a near-ground blast) would spread over a much smaller area due to lack of primary blast wave effects and secondary wind effects. Additionally, much of the other energy released that is usually absorbed by the atmosphere would continue on the strike the ground, adding to the direct irradiation and heat effects. Without oxygen to burn things you wouldn't have fires, but you can still have a lot of windows and exposed metal structures melting outright, depending on the size of the weapon.

I do have to disagree with one thing mentioned above though; it's been suggested above that there would be no blast effect in space, this is incorrect. While most of the blast effect of a nuclear weapon is carried out by atmospheric shock waves, there would still be a hard-nuclear reaction carrying on and expanding within the limited sphere around the weapon itself, the reaction that consumes the nuclear fuel, heavy casing and delivery system would still qualify as a 'blast'. However, the force exerted per m² would geometrically decline as the distance increased from the 'space zero' since there would be nothing to carry the shockwave other than the originally energized plasma. That said, it would still be quite extreme at close range and a nuclear explosion occurring up against the side of your ship would take a sizeable slice out of any known material.
posted by tiamat at 4:54 PM on September 26, 2008

An orion drive would detonate "shaped" nuclear bombs near the pusher plate, meaning the plate would receive considerably radiation pressure. I they envision coating the pusher plat with uranium, being quite hard, and foresaw little erosion from the radiation.
posted by jeffburdges at 5:38 PM on September 26, 2008

Additionally, much of the other energy released that is usually absorbed by the atmosphere would continue on the strike the ground, adding to the direct irradiation and heat effects.

I would debate the term "much" here. If we assume the bomb sends energy equally in all directions, then the only difference in vacuum would be ground that was previously "shadowed" by the atmosphere - from a strictly geometric point of view the same amount of ground is exposed to the bomb regardless of the atmosphere. For a ground-level blast the ground will absorb the same amount of energy. At least 50% of the energy goes away from the ground (and this goes up as the bomb is detonated higher off the ground and the horizon drops). In an atmosphere all this energy gets absorbed, in a vacuum it does nothing for the most part.

So my non-nuclear-scientist feeling is that the extra amount of energy absorbed by the ground is fairly negligible compared to the total energy released overall. The only time this would be useful is is you did an airburst above some sort of vacuum-planet settlement to cause radiation damage instead of setting it off at ground level where more direct physical damage would be done. Plus EMP as always. IMO, EMP is the primary issue as it remains dangerous at ranges much farther than the nuclear radiation or radiant heat.
posted by GuyZero at 5:41 PM on September 26, 2008