Kinetic bombardment - how destructive?

[adam_grif]

I wasn't sure whether to put this in SciFi or SLAM, but I opted for the former because the latter is less active.

When dealing with railguns and the like in fiction, they're often running very impressive numbers in KE/Momentum terms. Because I've got it on the brain, the dreadnoughts in Mass Effect fire a 20 KG iron slug @ .013 C, which is reportedly (according to the game, I haven't crunched the numbers myself) equivelant to 38 KT's of TNT in energy terms. That said, it seems overly simplistic to assume that all energy is equal, and that a city getting hit by it would be just as screwed as one getting hit by a 38 KT nuke.

So my questios are as follows: what are the approximate effects of getting hit by a hypervelocity impactor such as this for, say, a city. How would the widespread destruction compare to an equivelant yield nuclear device? Would it just punch a clean hole in the ground, continue down quite a way then get stopped by the ground, without making all that much of a mess (other than kicking up a lot of dust maybe)?

[Ford Prefect]

At higher velocities, much of the kinetic energy does get converted into heat. As I recall it's not going to exactly equivalent to a bomb of similar energy going off, but it will be largely close enough. Certainly once you start measuring in percentages of the speed of light, the fact that it's kinetic energy is sort of irrelevant.

[Simon_Jester]

Let's take an example of real life kinetic bombardment- a relatively low velocity (for science fiction), high mass impactor It happened in prehistoric times, but it's a high profile case that has been the subject of substantial computer modeling.

Take a nickel-iron mass of about three hundred thousand metric tons and drop it on a planet at a speed of about... twelve kilometers per second, maybe fifteen or so. According to computer models, half of it burns up in the atmosphere, so you've got 1.5E8 kilograms hitting at 1.2E4 meters per second... about 1.1*1016 joules, or 2.6 megatons. Momentum is 1.8 trillion newton-seconds.

The result is this crater in Arizona, about 1200 meters wide and 170 meters deep- and the actual floor is twice that; the bottom is half full of random debris that fell down off the sides over the intervening fifty thousand years. Over and above the actual crater, the rock strata for one or two kilometers away from the crater in all directions are "inverted," meaning that the impact physically picked them up and flipped them over, dropping what had formerly been deep-buried rocks onto what was once the surface.

Now, this isn't going to be the same as the results you'd get from a higher velocity, less massive impactor with the same kinetic energy, but it's a start.

[The Yosemite Bear]

depends on the mass of the kenetic projectile, all things being equal, aluminum foil projectiles will tear apart a ship in vacumme won't do jack for planetary bombardment because the atmosphere will burn it up....

still like Sufficiant explosives, there the point that a projectile of suffiancient bore, traveling and sufficiant velocity, will jack someone's day up reguardless.

just don't get caught on the wrong side of conservation of momentum though....

[adam_grif]

three hundred thousand metric tons and drop it on a planet at a speed of about... twelve kilometers per second, maybe fifteen or so.

That's sort of the exact opposite of what I'm trying to find out

I'm well aware that massive objects going at orbital or greater speeds are terribly destructive.

Now, this isn't going to be the same as the results you'd get from a higher velocity, less massive impactor with the same kinetic energy, but it's a start.

Well obviously with a singly particle with the same energy isn't going to do much of anything, and the meteor you gave is what happens at the opposite end of the spectrum. I'm interested in somewhere in the middle.

[Simon_Jester]

Well, we don't have practical experience of impacts in that range; hence the problem. Realistic studies of the problem are mostly going to be looking at massive orbital-velocity objects, because that's the plausible threat. Meteor strikes or kinetic projectiles that somebody put in orbit on a chemical rocket could hit Earth in the next fifty years; relativistic mass driver rounds aren't a threat unless the Martians land.

At a guess, I'd expect high-energy, low mass projectiles to bear a much closer resemblance to an explosion: less cratering, less scattering of debris, more energy going into heating the vicinity of the impact.

A hypervelocity impactor probably won't just punch a hole in the ground, not unless it is extremely well designed using improbably tough materials. Making something that won't pancake when it hits the ground at those speeds is going to be tricky. The impactor is carrying far more than the energy needed to vaporize itself, after all.

I'm not qualified to comment on damage mechanisms, I'm afraid.

[DrStrangelove]

20 KG iron slug @ .013 C, which is reportedly (according to the game, I haven't crunched the numbers myself) equivelant to 38 KT's of TNT in energy terms. That said, it seems overly simplistic to assume that all energy is equal, and that a city getting hit by it would be just as screwed as one getting hit by a 38 KT nuke.

So my questios are as follows: what are the approximate effects of getting hit by a hypervelocity impactor such as this for, say, a city. How would the widespread destruction compare to an equivelant yield nuclear device?

Realistically a 20kg projectile moving that fast would vaporize as soon as it hit the atmosphere. There is a reason there are very few kiloton range asteroid impacts that actually hit the ground.

[takemeout_totheblack]

I wasn't sure whether to put this in SciFi or SLAM, but I opted for the former because the latter is less active.

When dealing with railguns and the like in fiction, they're often running very impressive numbers in KE/Momentum terms. Because I've got it on the brain, the dreadnoughts in Mass Effect fire a 20 KG iron slug @ .013 C, which is reportedly (according to the game, I haven't crunched the numbers myself) equivelant to 38 KT's of TNT in energy terms. That said, it seems overly simplistic to assume that all energy is equal, and that a city getting hit by it would be just as screwed as one getting hit by a 38 KT nuke.

So my questios are as follows: what are the approximate effects of getting hit by a hypervelocity impactor such as this for, say, a city. How would the widespread destruction compare to an equivelant yield nuclear device? Would it just punch a clean hole in the ground, continue down quite a way then get stopped by the ground, without making all that much of a mess (other than kicking up a lot of dust maybe)?

I crunched the numbers using DW's SDN KE-calculator and Accelware Unit conversion (not sure about this software, so if anyone has any questions I'll give you a link to see for yourself). I came up with 36.3 KT for a 20kg projectile going .013c or 3897270m/s.
As for the orbital bombardment potential for such a weapon, I'm under the assumption that a 20kg ball of boring old iron would burn up in the atmosphere of an earth like planet before it could hit the ground. An object made of material tough enough for it to survive the atmosphere would probably leave a good-sized crater, make an interesting fireball, and make every city going person's day miserable.

[adam_grif]

If it burns up in the atmosphere, the energy doesn't just go nowhere. I imagine at that velocity it would make it to the ground with much of it still in tact? Even if it didn't, wouldn't it still do some damage by dumping all that energy into the atmosphere?

[The Yosemite Bear]

what part of conservation of energy do you not get?

projectile, is too small, it's like bug hitting windshiled time, provided that at that mass/speed the force doesn't cause the projectile to skip off the atmosphere. despite what physcists might consider Ideal, frictionless vacumes don't exist.

[takemeout_totheblack]

If it burns up in the atmosphere, the energy doesn't just go nowhere. I imagine at that velocity it would make it to the ground with much of it still in tact? Even if it didn't, wouldn't it still do some damage by dumping all that energy into the atmosphere?

For the energies we're talking about, solid iron might as well be ice or wood in terms of surviving to the surface, regardless of velocity!
The earth is hit by and harmlessly vaporizes 20kt equivalent meteors every year (give or take) and they aren't warming the planet any! A planet's ability to absorb and radiate excess heat is more than enough to disperse under 40 kt without it effecting the surface, we do sit right next to fusion furnace a million km across after all!
However, if you're talking continuous and sustained bombardment for, oh say, a week or a month? Yeah, there'd probably be consequences. But one shot at one city, you'd might as well wait for the 4th of July, that way you'd give them a light show and feel like you've accomplished something!

[Simon_Jester]

Energy that bleeds off into the air at high altitude is effectively lost, unless we're talking about quantities of energy large enough to physically boil off the atmosphere... which we aren't, in this case. The atmosphere can disperse a great deal of energy without harming the ground, as high-altitude nuclear airbursts illustrate.

Remember that going faster doesn't really help you penetrate deeper into the atmosphere before burning up, because the faster you go, the denser a barrier the air presents to your progress. From the projectile's point of view, as it flies through the atmosphere, it's getting hit by its own mass or more of air molecules, all of which are traveling at over 0.01c relative to itself.

To make matters worse, all those hits occur in the time it takes the projectile to pass through the atmosphere at those speeds: less than a millisecond. And flying through tens or hundreds of kilograms of air in under a millisecond isn't going to seem all that different from trying to fly through a wall.

The faster the projectile goes, the more energetic and damaging its collisions with air molecules will be, and the shorter the time interval those collisions are compressed into.

[adam_grif]

what part of conservation of energy do you not get?

projectile, is too small, it's like bug hitting windshiled time, provided that at that mass/speed the force doesn't cause the projectile to skip off the atmosphere. despite what physcists might consider Ideal, frictionless vacumes don't exist.

I get conservation of energy just fine, and have no idea how that relates to the friction of the atmosphere burning up a projectile before it hits the ground. At a sufficient velocity (i.e. 0.999 C) would not this 20 kilo slug make it to ground before it had been burned off by the atmosphere? I don't know, maybe not. But it seems intuitive to me, and so I'm asking questions for people who know more about it.

That's the whole point of this thread.

Energy that bleeds off into the air at high altitude is effectively lost, unless we're talking about quantities of energy large enough to physically boil off the atmosphere... which we aren't, in this case. The atmosphere can disperse a great deal of energy without harming the ground, as high-altitude nuclear airbursts illustrate.

No, I don't mean to imply that it's going to catch the air on fire and kill everybody or anything silly like that. My thought processes were going as follows:

- Nuclear weapons cause a blast by rapidly heating the air surrounding the device.
- If a slug is going to deliver 38kt equivelant of energy and get burned up in a very short period of time, this will rapidly heat the surrounding air.

Ergo it seems intuitive that it would create a blast around the area it enters the atmosphere. I have no idea how deep it would penetrate, or how spread out the energy would be, and so on.

Remember that going faster doesn't really help you penetrate deeper into the atmosphere before burning up, because the faster you go, the denser a barrier the air presents to your progress. From the projectile's point of view, as it flies through the atmosphere, it's getting hit by its own mass or more of air molecules, all of which are traveling at over 0.01c relative to itself.

Right, but the slug doesn't have trouble punching through walls

Is there some formula for working out how far it would get into the atmosphere?

I'm seeing a lot of "Well it wouldn't make it very far" but not many numbers so far in this thread. Is there some way to quantify it? We've established that we're regularly struck by kilotonne level objects, but are these similar to the projectiles in question, with low mass and high velocity? Or are they low velocity high mass?

[Nyrath]

From THE KILLING STAR by Dr. Charles Pellegrino and George Zebrowski. Relativistic weapons with a velocity of 0.92c bombard the Earth.

Then, looking east, she saw it coming -- at least her eyes began to register it -- but her optic nerves did not last long enough to transmit what the eyes had seen.

It was quite small for what it could do -- small enough to fit into an average-sized living room -- but it was moving at 92 percent of light speed when it touched Earth's atmosphere. A spear point of light appeared, so intense that the air below snapped away from it, creating a low-density tunnel through which the object descended. The walls of the tunnel were a plasma boundary layer, six and a half kilometers wide and more than 60 deep -- the flaming spear that Virginia's eyes began to register -- with every square foot of its surface radiating a trillion watts, and still its destructive potential was but fractionally spent.

Thirty-three kilometers above the Indian Ocean, the point began to encounter too much air. It tunneled down only eight kilometers more, then stalled and detonated, less than two-thousanths of a second after crossing the orbits of Earth's nearest artificial satellites.

Virgina was more than three hundred kilometers away when the light burst towards her. Every nerve ending in her body began to record a strange, prickling sensation -- the sheer pressure of photons trying to push her backward. No shadows were cast anywhere in the tower, so bright was the glare. It pierced walls, ceramic beams, notepads, and people -- four hundred thousand people. The maglev terminal connecting Siri Lanka Tower to London and Sydney, the waste treatment centers that sustained the lakes and farms, all the shops, theaters, and apartments liquefied instantly. The structure began to slip and crash like a giant waterfall, but gravity could not yank it down fast enough. The Tower became vapor before it could fall half a meter. At the vanished city's feet, the trees of the forest were no longer able to cast shadows; they had themselves become long shadows of carbonized dust on the ground.

In Kandy and Columbo, where sidewalks steamed, the relativistic onslaught was unfinished. The electromagnetic pulse alone killed every living thing as far way as Bombay and the Maldives. All of India south of the Godavari River became an instant microwave oven. Nearer the epicenter, Demon Rock glowed with a fierce red heat, then fractured down its center, as if to herald the second coming of the tyrant it memorialized.

The air blast that followed, surging out of the Indian Ocean -- faster than sound -- flattening whatever still stood. As it slashed north through Jaffna and Madurai, the wave front was met and overpowered by shock rushing out from strikes in central and southern India.

Across the face of the planet, without warning, thousands of flaming swords pierced the sky.

[takemeout_totheblack]

Well, that's all well and good, but those are multi-ton high grade R-bombs! What is being discussed here is a ball of iron being accelerated to .013c at a planet. The Killing Stars are gigaton, or even teraton level 'spears of fire', this thread is about kiloton pebbles. compare an atom bomb to a stick of dynamite.

[The Yosemite Bear]

force=mass times excelleration

so chunk of metal the size of living room traveling at near C, vs ball berring traveling at .013 C, that's no comparison...

[adam_grif]

Well, other examples are welcome in the thread too. But of course Nyrath's wasn't related to the ME example.

[Darth Wong]

The quick and dirty rule of Earth impactors is that the impactor must mass more than the column of air it will displace on its way to the ground, otherwise it will just burn up in the atmosphere. I'm not sure if this rule of thumb must be modified if it is applied to something moving at much greater than typical NEO velocity, but the basic principle is still there: high velocities do not eliminate the basic problem of needing to push that air out of the way in order to hit the ground.

If something has been deliberately designed as a weapon, I would assume that it attempts to minimize this problem by giving the slug a cylindrical shape, although you would have to optimize the thermal capacitance of the shape (which decreases with thinner objects) relative to the reduction in frontal area.

In any case, if you're shooting at things with 0.13c objects, that's the first question I'd have: how have you determined that it won't just bloom in the atmosphere rather than arriving at the ground in solid form?

[adam_grif]

In any case, if you're shooting at things with 0.13c objects, that's the first question I'd have: how have you determined that it won't just bloom in the atmosphere rather than arriving at the ground in solid form?

I haven't determined anything; I have zero experience in such matters.

One of the reasons I decided to ask was that in the MEverse specifically, there is a treaty about the construction of the ships with mainguns firing these 20 KG @ .013 C slug, because they carry so much energy. It's not outright stated, but heavily implied, that it's referring to the bombardment of cities and planetary colonies.

Then there's a snippet of dialogue from here:

http://www.youtube.com/v/7GqqDCe4Yrs

Again, not outright stated, but heavily implied that it would do significant damage to a city.

I was curious as to whether this would be accurate, whether the 38kt equivelancy of energy in KE/Momentum would be a significant threat to an Earth like planet, or even to one with minimal atmosphere.

Since people seem fairly confident that it would burn up relatively harmlessly, new question:

- Dreadnought is a few hundred meters above the city, fires it's main guns at the city. What happens?

[Darth Wong]

Using the rule of thumb mentioned earlier, the column of air displaced by the projectile would probably be less massive than the projectile itself, so it would hit the ground intact. At the moment of impact, the extreme-velocity compression of projectile and surface material will create shockwaves which will vapourize material and heat it up to very high temperatures. Basically, it will look pretty much like an atomic bomb going off, except that there will be a low-pressure column leading back to the ship (the wake from the projectile), so the fireball won't be round like a normal A-bomb fireball. Instead, the column will be more cylindrical, and much of the plasma will shoot right back up to the ship.

In short, big A-bomb-style explosion but the fireball is shaped like a big middle finger, pointing back up to the ship and slamming it right down the launcher tube with a significant fraction of the energy released by the weapon.

It would be funny if the captain has no idea that this would happen.

[adam_grif]

???

You can ignore air resistance, or pretend that the slug was as large and fast as it needs to be in order to have a mass of exactly 20 KG and a velocity of 1.3% of C when it hits the ground. Big fireball? More or less devastating to a city than a nuke of equivelant yield?

[The Yosemite Bear]

like I said much earlier, conservation of energy, and equal and opposite reaction is going to be a royal bitch.

[adam_grif]

http://www.youtube.com/watch#v=UsBRH6YT ... re=related

I think that trailer is what gave me the mental image of KE impactors kicking up dust and not much else.

[Darth Wong]

You can ignore air resistance, or pretend that the slug was as large and fast as it needs to be in order to have a mass of exactly 20 KG and a velocity of 1.3% of C when it hits the ground. Big fireball? More or less devastating to a city than a nuke of equivelant yield?

By this badly worded line, I'll assume you mean "as large and fast as it needs to be in order to still have a mass of 20kg and velocity of 0.013c when it hits the ground." The problem is that anything so massive would do far more damage than the mere 36 kT figure derived from a simple ½mv² calculation.

So, let's suppose instead that the ship has some kind of magic forcefield technology to make a vacuum column through the atmosphere, all the way down to ground level. It can do this much more slowly than the projectile would, so the process is less violent and has no destructive side-effects. Now the object hits the city with no impediment, and goes off as I said: an explosion much like an A-bomb, but with a plasma column shooting up into the sky out of the fireball.

It should be less destructive than a ground-burst nuke of the same yield. Some of the fireball energy goes up the column and this means that less of it remains near ground level. Also, you won't have the same nasty nuclear material fallout.

[Darth Wong]

http://www.youtube.com/watch#v=UsBRH6YT ... re=related

I think that trailer is what gave me the mental image of KE impactors kicking up dust and not much else.

That looks like a downward-firing rocket. It wouldn't be going anywhere near 0.013c, and it could also fire its rockets the whole time, so its velocity wouldn't peak until its hits the ground. Still, such a device will have far greater energy per kg than a chemical explosive would, so at the least you should get a nice big explosion.

Getting a nuclear fireball, however, requires nuclear yield. At a much smaller yield, you'd get something more like a conventional explosion.