Planetary Defense Experiment

[barbos]

100% efficiency means asteroid is split into 2 equal pieces and all bomb energy is then converted into giving them opposite momentum. There is no way standoff nuke can achieve 10% efficiency.

They were measuring it as bomb energy converted to movement of the asteroid.

In that case, you can have the same efficiency with vastly different movement.

 

[lpetrich]

I'll estimate the best case for vaporized asteroid material in a nuclear explosion. I'll take a 150-kt blast, with 628 TJ of energy.

The material is most likely stony, and as far as I can determine, stony asteroids have a composition similar to the Earth's mantle: silicates with lots of magnesium and iron, and not much aluminum. Thus being "mafic" or "ultramafic" by the standards of Earth rocks. A common mineral is olivine, which is (Mg,Fe)2SiO4. Another common one is pyroxene, XY(Si,Al)2O6 (X: Ca, Na, Fe, Mg, Zn, Mn, Li; Y: Cr, Al, Mg, Co, Mn, Sc, Ti, V, Fe). Mineralogy of meteorite groups - RUBIN - 1997 - Meteoritics & Planetary Science - Wiley Online Library Both olivine and pyroxene are common in the Earth's mantle.

The closest I can come is Properties of Molten Magnesium Oxide | PNAS - 3873.2 K (calculated)

I now have to find the specific heat, the heat of fusion (melting) and the heat of vaporization. From 10.10: Enthalpy of Fusion and Enthalpy of Vaporization - Chemistry LibreTexts (enthalpy is a way of stating heat energy) -- I notice that (heat of vaporization) / (boiling point) per mole (gram molecular weight) is approximately constant. I checked it against Fusion and Evaporation Heat of common Materials and  Enthalpy of fusion and  Enthalpy of vaporization

(heat of vaporization) / (temperature) in J / K / mol:

Acetone: 95, Aluminium: 105, Aluminum: 103, Aluminum: 109, Ammonia: 97, Bismuth: 139, Bromine: 86, Butane: 77, Carbon tetrachloride: 86, Chlorine: 85, Copper: 220, Diethyl ether: 85, Ethane: 80, Ethanol: 110, Ethyl Alcohol: 113, Gold: 106, Hydrogen: 45, Hydrogen (parahydrogen): 44, Iron: 108, Iron: 117, Isopropyl alcohol: 124, Lead: 88, Lead: 89, Lithium: 9, Mercury: 91, Mercury: 93, Methane: 73, Methane: 73, Methanol: 104, Neon: 67, Nitrogen: 73, n-Nonane: 82, Oxygen: 76, Oxygen: 76, Phosphine: 79, Propane: 68, Sodium: 85, Sulfuric acid: 84, Water: 109, Water: 109, Water: 109, Zinc: 109

It's a bit duplicative since I used multiple sources.

The heat of melting jumps around quite a bit, but it is usually much less than the heat of boiling, typically by a factor of 10 - 20.

 

[lpetrich]

I've been using Periodic Table - Ptable

 Table of specific heat capacities - the closest to stony-meteorite material is glass at 0.84 and granite at 0.790.

For the heat of vaporization, I will use a value of 90 J/K/mol, and I will assume dissociation. For half-half olivine MgFeSiO4, a mole is 172.231 g, and divided among 7 atoms is 24.6044 g.

This gives a heat of vaporization per unit temperature of 3.7 J/K/g, much larger than the specific heat. I'll assume 0.8 and add it in: 4.5 J/K/g. For a boiling point of 4000 K, this gives 18,000 joules/gram.

628 terajoules can thus vaporize 3.5*10¹⁰ grams, or 35,000 metric tons.

That's a best-case scenario, where all the bomb's energy goes into vaporizing asteroid material with none left over.

It's somewhat difficult for me to estimate how much of the vaporized bomb's heat energy goes into vaporizing asteroid material, because it may quickly move away without vaporizing very much.

 

[Bomb#20]

Assuming those numbers are right, we're answering the wrong question. Back of the envelope, the Trinity explosion had to blow away about 50,000 tons of rock to leave the crater it created, and that was only a 20-kt bomb. So my assumption was wrong -- most of the excavated material evidently wasn't vaporized. Liquid and particulate disintegrated rock is just as good reaction mass as vaporized rock. Better maybe, since it's probably moving slower so it carries more momentum per joule.

 

[barbos]

The is no need for going through heat of melting and evaporation.
Within the order of magnitude it's equal to typical chemical energy.
Which means 1MT explosion is able to "evaporate" 1 million tons asteroid.
1 million tons is about 100 meters in diameter asteroid.
10km asteroid is million times heavier than that.

So you will need million megaton bomb to evaporate it

 

[steve_bank]

An asteroid strike or nuke on the surface heats air which expands, and air is the how the shock wave is transmitted.

Look at impact craters on the moon or the Arizona crater.

Meteor Crater - Wikipedia

en.wikipedia.org

As to te heat thought experiment

q = m*c*dT

q heat in Joules
m mass in kg
T temperature Kelvins
c specific heat

Thermodynamics/Temperature And Heat - Wikiversity

en.wikiversity.org

Heat capacity - Wikipedia

en.wikipedia.org

Q=mcΔT Q = mc Δ T , where Q is the symbol for heat transfer, m is the mass of the substance, and ΔT is the change in temperature. The symbol c stands for specific heat and depends on the material and phase. The specific heat is the amount of heat necessary to change the temperature of 1.00 kg of mass by 1.00ºC.

Table of specific heat capacities - Wikipedia

en.wikipedia.org

 

[Loren Pechtel]

628 terajoules can thus vaporize 3.5*10¹⁰ grams, or 35,000 metric tons.

That's a best-case scenario, where all the bomb's energy goes into vaporizing asteroid material with none left over.

It's somewhat difficult for me to estimate how much of the vaporized bomb's heat energy goes into vaporizing asteroid material, because it may quickly move away without vaporizing very much.

Vaporizing rock is wasted energy, it gets you nothing. You want material blasted off the surface as fast as possible, energy spent heating it doesn't produce delta-v.

 

[steve_bank]

If it can be softened then i can be blown apart.

 

[barbos]

628 terajoules can thus vaporize 3.5*10¹⁰ grams, or 35,000 metric tons.

That's a best-case scenario, where all the bomb's energy goes into vaporizing asteroid material with none left over.

It's somewhat difficult for me to estimate how much of the vaporized bomb's heat energy goes into vaporizing asteroid material, because it may quickly move away without vaporizing very much.

Vaporizing rock is wasted energy, it gets you nothing. You want material blasted off the surface as fast as possible, energy spent heating it doesn't produce delta-v.

Vaporizing IS blasting material off with extremely high velocity, room temperature is about 300 m/sec.

 

[Loren Pechtel]

628 terajoules can thus vaporize 3.5*10¹⁰ grams, or 35,000 metric tons.

That's a best-case scenario, where all the bomb's energy goes into vaporizing asteroid material with none left over.

It's somewhat difficult for me to estimate how much of the vaporized bomb's heat energy goes into vaporizing asteroid material, because it may quickly move away without vaporizing very much.

Vaporizing rock is wasted energy, it gets you nothing. You want material blasted off the surface as fast as possible, energy spent heating it doesn't produce delta-v.

Vaporizing IS blasting material off with extremely high velocity, room temperature is about 300 m/sec.

I guess it would be in a vacuum.

 

[barbos]

628 terajoules can thus vaporize 3.5*10¹⁰ grams, or 35,000 metric tons.

That's a best-case scenario, where all the bomb's energy goes into vaporizing asteroid material with none left over.

It's somewhat difficult for me to estimate how much of the vaporized bomb's heat energy goes into vaporizing asteroid material, because it may quickly move away without vaporizing very much.

Vaporizing rock is wasted energy, it gets you nothing. You want material blasted off the surface as fast as possible, energy spent heating it doesn't produce delta-v.

Vaporizing IS blasting material off with extremely high velocity, room temperature is about 300 m/sec.

I guess it would be in a vacuum.

Yes, outer space is a vacuum.

 

[steve_bank]

You know what theyl, the best defense is a good offense.

 

[Loren Pechtel]

628 terajoules can thus vaporize 3.5*10¹⁰ grams, or 35,000 metric tons.

That's a best-case scenario, where all the bomb's energy goes into vaporizing asteroid material with none left over.

It's somewhat difficult for me to estimate how much of the vaporized bomb's heat energy goes into vaporizing asteroid material, because it may quickly move away without vaporizing very much.

Vaporizing rock is wasted energy, it gets you nothing. You want material blasted off the surface as fast as possible, energy spent heating it doesn't produce delta-v.

Vaporizing IS blasting material off with extremely high velocity, room temperature is about 300 m/sec.

I guess it would be in a vacuum.

Yes, outer space is a vacuum.

Boiling it wouldn't produce thrust if there was an atmosphere. I was forgetting that once you have a gas, temperature and speed are effectively the same thing.

 

[lpetrich]

With Its Single “Eye,” NASA’s DART Returns First Images from Space | NASA -- pictures of stars. There is not much in them, but these pictures will be useful for calibrating DART's camera, DRACO (Didymos Reconnaissance and Asteroid Camera for Optical navigation).