Is It Time to Reconsider Anti-Nuclear Laws?

[Andrew Mason]

Andrew, the energy from a nuclear bomb is omnidirectional, so sure, it isn't as "concentrated" as a laser. Still, if you detonate the bomb near an astroid, almost half of the energy will be captured by the asteroid. Maybe that's how you define efficiency (50% vs 100%), but since there is no laser that comes anywhere close to the power output of a nuclear bomb, you still get much more oomph from the nuclear bomb - efficiency really isn't a relavant concept here. [and that's without considering the efficiency of generating the laser]

But my point is that it is not the efficiency by which the asteroid absorbs energy. That should be very high. It is the efficiency by which it converts energy into asteroid momentum.

Let's take an example. I fire a bullet into a soccerball. Let's assume that the soccerball is designed so that it absorbs 100% of the bullet's energy (and momentum).

In the first case, it stops the bullet by heating up a clump of matter inside the ball and it simply converts the energy into warming the ball. The momentum of the ball will be equal to the loss of momentum of the bullet.

In the second case, the bullet pushes on a plate that comresses the air in the ball. The compressed air is allowed to build up pressure until the bullet is stopped. Then the compressed air is released in a rearward direction through the hole made by the bullet. In this case, the momentum of the ball is equal to the momentum of the bullet + the momentum of the air that is directed rearward through the hole. The momentum of that air has nothing to do with the bullet momentum. It is a function of the energy of the bullet and the "efficiency" by which that energy is converted into momentum of the escaping air.

To get an idea of the scale difference, the worlds largest laser, (actually a collection of 192 lasers) will have a total energy capacity of 346mj (it is still under construction). That's .000008256 kilotons. So you'd need to fire it one hundred twenty thousand times to impart the same amount of energy on an asteroid as a small nuclear bomb. [someone check my math...]

http://www.specialtyphotonics.com/knowledge_base/newsletter/0707/largest_laser.html

Given the logistics of getting a nuclear bomb near an incoming asteroid, perhaps developing a laser to move asteroids might be a better investment.

If a laser is significantly more "efficient" in converting photon energy into asteroid momentum - as I suggest that it may be but I don't know that - you would not need to deliver nearly as much energyas a nuclear bomb in order to achieve the needed asteroid momentum change.

AM

 

[quetzalcoatl9]

WRONG! WRONG! WRONG!

Where do you get the idea that the momentum of the radiation is some kind of limit?

if i may butt in here, why exactly is Andrew wrong on this?

assume a perfect transfer of energy from the radiation to the material of the asteroid. then the molecules of the asteroid will be kicked into an excited state. let's assume that all of that energy is kinetic, and so by conservation of energy the surface molecules of the asteroid (the ones that were resonant with the radiation) will then be blasted off of the asteroid (that is, "asteroid" being the part of the material that is still in the ground state). the kinetic energy of the leaving molecules will be equal to the amount of energy transferred from the radiation.

the energy of the radiation will be be related to it's (in this case rather high) frequency, or by the de Broglie relation, the photon's momentum. This momentum, unless I am missing something, will be a limiting factor in the momentum imparted to the asteroid.

What is wrong with this?

Q

 

[NeoDevin]

The ENERGY is the limiting factor, not MOMENTUM. Any material vaporized from the asteroid will have momentum in the opposite direction, allowing for a push of arbitrarily large momentum, given sufficient energy, even though the bomb is not actually touching the surface (if there even is one) of the asteroid. This method works whether the asteroid is a single solid piece, or a large collection of small bits of debris.

Putting the bomb inside the asteroid would be less effective at imparting momentum (ie. almost no net momentum change at all), plus there would be the chance that you would break the asteroid into pieces and then have an even bigger problem heading towards the earth.

 

[Andrew Mason]

if i may butt in here, why exactly is Andrew wrong on this?

assume a perfect transfer of energy from the radiation to the material of the asteroid. then the molecules of the asteroid will be kicked into an excited state. let's assume that all of that energy is kinetic, and so by conservation of energy the surface molecules of the asteroid (the ones that were resonant with the radiation) will then be blasted off of the asteroid (that is, "asteroid" being the part of the material that is still in the ground state). the kinetic energy of the leaving molecules will be equal to the amount of energy transferred from the radiation.

the energy of the radiation will be be related to it's (in this case rather high) frequency, or by the de Broglie relation, the photon's momentum. This momentum, unless I am missing something, will be a limiting factor in the momentum imparted to the asteroid.

What is wrong with this?

It is kinematically possible for vapourized matter from the surface to provide a greater impulse to the rest of the asteroid than the momentum received from of the incident radiation. In fact, it is possible for that vapour to provide an impulse in a completely different direction. In the soccer ball example I gave, if the pressurized air is released out the front of the ball, the ball could recoil toward the incident bullet with greater momentum than the incident bullet. The limiting factor is the energy of the incident bullet.

My point is that whether this occurs depends on the actual mechanism. There has to be a build-up of vapour pressure within the asteroid before the vapour escapes from the the surface. Now there may be a mechanism by which pressure within the vapourizing surface will build up. One would have to study the actual physical process.

AM

 

[Andrew Mason]

The ENERGY is the limiting factor, not MOMENTUM. Any material vaporized from the asteroid will have momentum in the opposite direction, allowing for a push of arbitrarily large momentum, given sufficient energy, even though the bomb is not actually touching the surface (if there even is one) of the asteroid. This method works whether the asteroid is a single solid piece, or a large collection of small bits of debris.

The direction of the kick provided by the vapour does not have to be away from the bomb. You could have an asteroid in which the far surface (ie opposite to the surface facing the nuclear bomb) was covered in ice. If the asteroid was thin enough, it might be possible for the heat from the radiation to evaporate water under the ice and build up vapour pressure until the ice burst causing a sudden release of vapour away from the far surface and pushing the asteroid toward the nuclear bomb.

Putting the bomb inside the asteroid would be less effective at imparting momentum (ie. almost no net momentum change at all),

There can be no change of the total momentum, but that is the case whether the bomb is inside or outside the asteroid. You are only interested in the bulk of the asteroid. An exploding bomb could easily propel some of the asteroid matter. If the bomb were to propel large chunks of asteroid outward into space, the rest of the asteriod would have to recoil in the opposite direction.

plus there would be the chance that you would break the asteroid into pieces and then have an even bigger problem heading towards the earth.

Or more smaller ones.

AM

 

[russ_watters]

But my point is that it is not the efficiency by which the asteroid absorbs energy. That should be very high. It is the efficiency by which it converts energy into asteroid momentum.

[snip] If a laser is significantly more "efficient" in converting photon energy into asteroid momentum - as I suggest that it may be but I don't know that - you would not need to deliver nearly as much energyas a nuclear bomb in order to achieve the needed asteroid momentum change.

But both the nuke and the laser primarily fire EM radiation at the asteroid and both will mostly just heat it. So what's the difference?

I understand what you mean about the laser being more efficient (a refleced photon imparts twice momentum as an absorbed photon, for example), but we're not talking order of magnitude differences so the laser would still need to be on the same order of magnitude in energy as the nuclear bomb. And at that energy, the laser is still going to be vaporizing part of the asteroid anyway, so I'm not convinced the mechanism would even be fundamentally different.

Bottom line, it would be more efficient energywise to use the laser (at the very least, more than half the energy of the nuke is going in the wrong direction), but certainly not more than a single order of magnitude more efficient and that just isn't enough to be worthwhile.

Given the logistics of getting a nuclear bomb near an incoming asteroid, perhaps developing a laser to move asteroids might be a better investment.

The logistics of getting a nuke to an asteroid were perfected in the early 1960s. Today, we can pretty much use off-the-shelf components to send a nuclear bomb to anywhere in the solar system we want on short notice (under a year to launch, certainly) - you could almost literally rip the payload off a rocket on the pad in Florida and replace it with an ICBM warhead and guidance package. And since and lasers powerful enough to match don't exist, I'd bet on the nuke getting there first!

The NIF is costing $1.2 billion to build and I don't know how much it would cost to fire it a hundred thousand times. The second (ie, no development costs, just construction and launch costs) rover we sent to Mars cost $200 million to build and send to Mars.

So with choice A, we need technology that doesn't exist, a lot more money, and a lot more time to make it happen than choice B. It just doesn't make sense.

 

[quetzalcoatl9]

The ENERGY is the limiting factor, not MOMENTUM. Any material vaporized from the asteroid will have momentum in the opposite direction, allowing for a push of arbitrarily large momentum, given sufficient energy, even though the bomb is not actually touching the surface (if there even is one) of the asteroid. This method works whether the asteroid is a single solid piece, or a large collection of small bits of debris.

Putting the bomb inside the asteroid would be less effective at imparting momentum (ie. almost no net momentum change at all), plus there would be the chance that you would break the asteroid into pieces and then have an even bigger problem heading towards the earth.

i kindly disagree on all points.

1) whether we talk of momentum or energy here is mostly irrelavant. the energy imparted will be proportional to the square of the momentum so i could equally argue that it is the momentum of the radiation (i.e. the inverse wavelength weighted by a factor of Planck's constant)

2) if it were possible to blast the asteroid out from the center, that would not necessarily be detrimental since it is possible that those pieces will all "radiate" from their trajectory and miss the Earth completely.

 

[Andrew Mason]

But both the nuke and the laser primarily fire EM radiation at the asteroid and both will mostly just heat it. So what's the difference?

As I said, I am not sure that a laser would be any more efficient in converting energy into momentum. I just thought it would be quicker to deliver and more accurate, as well as delivering a more concentrated energy. I may be wrong on that last point as Morbius points out. The energy of the radiation in the xray/gamma ray part of the spectrum is much higher than light, energy/unit area might be higher even if the number of photons/unit area is lower.

I tend to agree with you that a nuclear weapon would actually be more effective. The radiation would heat the asteroid vapourizing the surface molecules, but the enormous shock wave from the blast would also compress the vapour, allowing for a buildup of vapour pressure that would then be released when the blast pressure subsided.

AM

 

[Morbius]

I didn't say that. I said that unless the energy is allowed to build up in the target, the kick delivered by the release of vaporised asteroid matter cannot exceed the change in momentum of the incident radiation and matter.

Andrew,

Which is where you are 100% WRONG!

Consider the following. Suppose the asteroid is traveling in the "Z" direction; and we
bury some explosive charges on the "X" side of the asteroid. Because the asteroid
direction defines the Z direction; the momentum in the X direction is ZERO.

Now we detonate the high explosive so that we propel a bunch of the matter that was
once a part of the asteroid into the +X direction. That matter represents momentum
in the +X direction. However, conservation of momentum requires that the total
momentum of the entire system, asteroid plus blow-off material has to be ZERO;
which is what it was before the detonation. Therefore, the asteroid has to recoil
in the -X direction. That's the ONLY way that the total momentum can be ZERO.

It seems to me that it depends on how energy is transferred from the incident photons to the atoms in the asteroid.

If the absorption of radiation energy produces heat in the asteroid and that causes a build-up of pressure of asteroid matter that is subsequently released as a burst of vapour from the asteroid, you are absolutely right that the incident momentum is immaterial.

YES - that is EXACTLY what happens. The absorption of radiation produces heat and
vaporizes the material of the asteroid. This material "blows-off" or "jets-off" and by
necessity to conserve momentum - the asteroid has to recoil in the opposite direction.

As I stated before - this is EXACTLY the mechanism that is being used in laser fusion
for example. The lasers are directed in a hohlraum where the build-up of laser energy
causes the hohraum to radiate X-rays. Those X-rays impinge on the surface of the
spherical fusion pellet and the ablation of the surface causes the pellet to be compressed.
The pellet is "recoiling" in response to the blow-off of the surface.

But if it is simply a matter of individual incident photons knocking off atoms from the surface, I don't see how the atoms can receive an impulse from an incident photon that exceeds the change in momentum of the incident photon.

As I have stated REPEATEDLY - that is NOT the mechanism. The asteroid is NOT
recoiling due to the momentum of the photons [ consider that to be zero, if you like ];
the recoil is produced by the ABLATION of the surface due to the absorbed energy.

I am not pooh-poohing it at all. I am just saying that it requires a build-up of heat in the asteroid followed by a release of vapour.

Which is EXACTLY what happens when you irradiate the material with intense X-rays!

Dr. Gregory Greenman
Physicist

 

[Morbius]

Andrew, the energy from a nuclear bomb is omnidirectional, so sure, it isn't as "concentrated" as a laser. Still, if you detonate the bomb near an astroid, almost half of the energy will be captured by the asteroid. Maybe that's how you define efficiency (50% vs 100%), but since there is no laser that comes anywhere close to the power output of a nuclear bomb, you still get much more oomph from the nuclear bomb - efficiency really isn't a relavant concept here. [and that's without considering the efficiency of generating the laser]

To get an idea of the scale difference, the worlds largest laser, (actually a collection of 192 lasers) will have a total energy capacity of 346mj (it is still under construction). That's .000008256 kilotons. So you'd need to fire it one hundred twenty thousand times to impart the same amount of energy on an asteroid as a small nuclear bomb. [someone check my math...]

Russ,

You are EXACTLY correct - "efficiency" means NOTHING here. [ You see this all the
time - people get "hung up" on efficiency when that's not the important metric ].

What you want is to impart a force, and hence momentum and energy to the asteroid.
That's ALL that counts.

If you impart 10 kilotons worth of energy to the asteroid using a 1 megaton nuclear bomb;
who cares if the process if 1% efficient?

Would imparting 0.00001 kilotons of energy with 100% efficiency be better? If that amount
of energy is insufficient to deflect the asteroid enough - who cares that it can be done
with 100% efficiency.

That's one of the hallmarks of a good scientist - to know what metrics are important; and
which are NOT! Here efficiency is meaningless; all that matters is how much momentum
and energy can be imparted to the asteroid, and will it produce sufficient deflection.

Dr. Gregory Greenman
Physicist

 

[Andrew Mason]

Andrew,

Which is where you are 100% WRONG!

Consider the following. Suppose the asteroid is traveling in the "Z" direction; and we bury some explosive charges on the "X" side of the asteroid. Because the asteroid direction defines the Z direction; the momentum in the X direction is ZERO.

Now we detonate the high explosive so that we propel a bunch of the matter that was once a part of the asteroid into the +X direction. That matter represents momentum in the +X direction. However, conservation of momentum requires that the total momentum of the entire system, asteroid plus blow-off material has to be ZERO; which is what it was before the detonation. Therefore, the asteroid has to recoil in the -X direction. That's the ONLY way that the total momentum can be ZERO.

Morbius, we seem to be talking at cross purposes. You are giving an example of what I have been saying is required.

If you blow off part of the asteroid, the rest of it must receive an impulse in the opposite direction. I agree. That is exactly what is required: an explosive release of matter from the asteroid.

You are saying that you don't need to do that directly. You can do it indirectly using radiation from the bomb to heat the asteroid to vaporize the surface to such a temperature that it blows off. Ok. It is not sufficient to just have radiation just knock atoms off the asteroid. I think we both agree on that.

All I am saying is that in order to provide the explosive impulse, there has to be some containment before the vapour is released. I am just not sure of the mechanism by which that occurs. One way to do that would be to produce a heat gradient within the asteroid with the greatest heat just below the surface. Another might be to have the pressure from the explosion shock wave contain the vapor as the heat is building up.

AM

 

[Morbius]

All I am saying is that in order to provide the explosive impulse, there has to be some containment before the vapour is released.

Andrew,

NOPE - you don't need to have any type of containment.

The radiation flux from a nuclear bomb is going to be much higher than if you took the
most powerful laser and contained it!

You don't need to concentrate the radiation from a nuclear bomb - it is so intense even
without containment or any type of concentration; that it will do the job of providing the
impulse to do the deflection.

That's all that matters - do you put enough radiation on the target to blow off enough
material. If you are thinking in terms of lasers or other radiation sources - I can see
where you would want to contain and concentrate the radiation.

However, for a nuclear bomb - that containment and concentration is just NOT
necessary.

Dr. Gregory Greenman
Physicist

 

[russ_watters]

All I am saying is that in order to provide the explosive impulse, there has to be some containment before the vapour is released.

I read once that it has been considered as a means to propel airplanes to spray raw fuel on their wings...

Anyway, I think we've beaten this thread to death.