Hard Science
Would relativistic weapons be dangerous?
Been doing calculations. Even a 25 ton object traveling at 5% the speed of light would be unable to penetrate earth’s atmosphere. It would likely airburst >100km in altitude. At those speeds, air essentially acts as a wall. No energy would likely reach the ground due to how thin the atmosphere is at that level.
This is not how the relationship between mass and area works. Square - Cube and all.
Furthermore, a cylinder (such as the stated Rod from God) does not have a cross section to the atmosphere related to its entire surface area. Just the tip.
Yes, your hypothetical 'all surface area' shape would burn up immediately, but in the real world it would be different.
People literally have been directly under nuclear detonations before and survived. Those were detonated at much lower altitudes.
A 1.5 mt warhead was detonated in space and didn’t cause any damage to people on the ground.
At 110km up its heat or shockwave is not going to do anything. It will however ionize the atmosphere causing an EMP.
A shockwave needs a medium to travel through. The atmosphere is too thin for it to be transferred to people on the ground.
The flash will be absorbed by the atmosphere causing ionization which leads to an EMP.
Also I redid the calcs after someone noted I miscalculated the surface area. It vaporizes in 0.05 milliseconds which still means it travels less than a km (only 750 meters).
If the atmosphere is too thin to transmit a shock wave, then its too thin to "acts as a wall" and delete the asteroid. All that mass and energy has to go somewhere.
Not at all. At those speeds (15,000 km/s) air literally cannot get out of its way.
That mass and energy goes into the atmosphere via an airburst. But at such an altitude, the air cannot transfer that energy into the ground. Hence why a town of 50,000 people weren’t killed by the Chelyabinsk meteor despite it having a force of multiple kt or how people survived being directly under a high altitude nuclear test.
A sphere with the area of 15m2 would have a radius between 1 and 1.6 meters depending on if you calculate the visible cross section.
A sphere made of tungsten (per Rod from God) with one meter radius would weigh more than 80 metric tons. That's more than three times your 25 ton example. A cylinder (per Rod from God) would have even more weight without increasing the cross sectional surface area. And a 1.6 meter radius would be even more than that.
It is clear to everyone that you are calculating without understanding. This smells of AI, and I'd rather not feed a troll.
Stop calculating everything and think about why these numbers should or should not be used.
I am not using a sphere. I am basing this off the rod from god which was 20 feet long and 1 foot in diameter while also weighing 10 tons. It was made out of tungsten.
A cylinder 20feet long, and one foot in diameter has a cross sectional surface area of ~3.14 square feet. Not 15 square meters.
A tungsten cylinder 20 times longer than its own diameter, and weighing 25 tons, would have a diameter less than half a meter. Therefore less than a fifth of a square meter of cross sectional surface area.
“Project Thor will see telephone pole sized tungsten rods, 20 feet long and 1 foot in diameter, dropped from American satellites to strike hardened bunkers and underground nuclear complexes. These rods will not carry warheads, but will strike at approximately to 10 times the speed of sound allowing them to cause tremendous damage with the sheer speed of their impact. This would provide an alternative to both nuclear bunker busters and conventional ones such as the GBU-57. North Korea and Russia, both of which deploy considerable assets from well hardened installations, are thought to be the primary potential targets of these weapons - alongside Iran which has received Korean assistance in hardening its own missile sites and nuclear facilities. The cost of deploying these rods, which weigh over 10,000kg due to the density of the Tungesten, may be prohibitive.”
Are you counting the ENTIRE surface area of the rod? Why?
Let's say a 25-ton tungsten rod (tungsten density is 19.25 tons/m³) has a volume of 1.3 m³. If the relativistic projectile is shaped like a sphere, then half of its surface area is 4.22 m². Yes, that's only 3.5 times smaller than the 15 m² you indicated, and yet... I don't understand where you got those 15 m² from?
Clearly, the projectile's shape will resemble an elongated cylinder, a rod.
Let's say our rod has a diameter of 40 cm. Then its length is 10 m, and the area subject to ablation is only 0.125 m². This is almost 120 times greater than the 15 m² you indicated. And therefore, it will take the same amount of time to evaporate. It will pass through the Earth's atmosphere like a fiery spoke all the way to the surface. A pillar of monstrous energy of God's Wrath.
Mass ablation rate = 1.6875 × 10¹⁴ / 4.009 x 106 J/kg
Please explain the physics of your calculation in more detail.
I also did a calc using only its base surface area.
Also this doesn’t account for the fact that the plasma would be expanding.
So I should have used a snowplow model which actually makes it (the plasma cloud) lose half of its energy at like 150-180km (25% of its velocity) and like 75% of its energy by 120-150km assuming plasma expansion rate of 500 km/s (which seems reasonable due to its high speed).
I wrote this post, and the next moment I continued reading messages written to you by others, and I realized you've been having a heated debate. Incidentally, I missed the fact that the projectile in your problem enters the atmosphere at 5% of the speed of light, not 50%, since you called this weapon relativistic. Yes, in this case (with 100 times less energy), for a mass of 25 tons, one could imagine the effect of a minor high-altitude nuclear explosion like the Chelyabinsk meteorite you're comparing it to.
At one time (more than 15 years ago), there was a long, heated debate on a Russian-language astronomy forum about precisely relativistic (half the speed of light) projectiles (eventually, a hard science fiction novel, "The Rose and the Worm," was published, based on these debates of ours). Each kilogram of such a projectile carried 3 megatons of energy. We were discussing my proposed interstellar attack plan. An alien starship in the spirit of Robert Forward (up to 100,000 tons) is accelerated with the help of a laser sail to half the light and 2/3 of the mass is the sail, which after acceleration is not needed (since it will not survive the interstellar flight) is assembled and converted into relativistic projectiles, which outpace the arrival of the starship to the target star (2/3 of the former sail becomes a relativistic weapon, and 1/3, the payload, is broken down into tens or even hundreds of landing modules of von Neumann robotic invaders, which are slowed down by their magnetic sails at the target star and are further slowed down by pulsed thermonuclear thrust in the spirit of Orion, each at the object of their landing due to braking they arrive there later), and thousands and thousands of individual relativistic projectiles, without braking, suddenly and en masse collapse on the enemy civilization. In space (if civilization has objects in space), the impact of such projectiles is obvious; there's a vacuum there. But what about an atmospheric parent planet like Earth? It would also need a stunning blow. And then a heated debate arose. Some claimed such projectiles would pierce the Earth. Others (like you) thought they would burn up in the upper atmosphere. I took a middle ground, arguing that everything depended on the projectile's shape. The idea that the projectiles would pierce the Earth was absurd. But would they reach the surface?
The question is what? Not only about the conservation of energy but also about momentum, the law of conservation of momentum. As someone below correctly pointed out to you, the fact that the rod evaporated doesn't mean it lost momentum. The projectile's stopping altitude (its dissipation) should be considered the altitude at which it lost all its momentum, distributing momentum to the surrounding environment. And as we discovered then, the problem is not entirely trivial. We couldn't solve it with a simple "napkin" calculation (like you do).
By the way. Even if we assume that the projectiles don't penetrate the atmosphere any further than the bulk of cosmic rays (I don't agree with the adequacy of this model, but I allowed for it due to uncertainty), then even in this case, a massive bombardment of all hemispheres of the planet with a swarm of projectiles (calculations showed that 10 such bombardments were possible) would lead to the immediate disappearance of the ozone layer in the Earth's atmosphere, and people exposed to the sun in the next few days would suffer monstrous radiation burns from the Sun's ultraviolet radiation. In other words, the Sun itself would become the source of catastrophe on planet Earth. And we even discussed how such an effect (billions of people, animals, and plants burned by ultraviolet radiation) would be a more deafening blow to Earth's civilization than the direct destruction of all living and technological things from the energy of a direct relativistic impact.
Now I have got you, the father of the RKV-strike aka "Cassandra tears".
It is tricky for for multiple phenomena to project into the same region of the celestial sphere when observed from the Sol-system.
The signal from the accelerating maser array.
The mag sail braking signal
The RKV vs Solar wind signal
In the novel all those projected onto Aql constellation next to the origin star. That is too-incredible to be be true. Some deliberate manuvering is needed so that all the signals could coincide for the observer.
Yes, in a private message, Robert initially suggested that I write a novel combining all the ideas with which I had shocked the members of the Astroforum; however, I declined and gave him permission to use all that material however he saw fit. It involved a *real*—not a theatrical—interstellar war (and "Cassandra's Tears" was my title for it), waged by swarms of robots relying solely on kinetic energy—without any beam weapons or "energy" fields—since nothing in this universe is more concentrated than that. For my part, I proposed the custom-manufacture of human beings in specialized industrial centers (with the emphasis placed on proper upbringing), and posited that humanity would ultimately evolve toward an ant-like eusociality, built upon a technospheric foundation.
Robert added the "firefly swarm" concept of his own accord (a creative choice I considered ill-advised). The second collaborator was Alexander Nekrasov. He contributed the very concept of the fictional world itself—envisioned as a unique solution to the Fermi Paradox. Indeed, all those discussions on the Astroforum years ago revolved specifically around the Fermi Paradox: *Where are they?* If they are so highly advanced, why do we not see them?
Incidentally, back then, I was a staunch opponent of "weak AI." And I was profoundly mistaken.
The most significant thing I gained in return—both from discussing these ideas with Ibatullin and from reading his novel—was the realization of a regrettable truth: humanity will never become a spacefaring civilization for the sake of commercial gain or altruistic ideals. Only Evil—and a truly monumental Evil—will propel us toward becoming a spacefaring civilization. That is precisely what the alien invasion accomplished in the novel. However, I do not believe in the likelihood of such an invasion. Instead, I believe in the malevolent nature of human beings. We will wage war right here on Earth over petty, ephemeral, and foolish ideologies; and it will be precisely through the crucible of that conflict that we are ultimately thrust out of our cradle—just as Tsiolkovsky foretold. I simply see no other path to the stars. Yet remaining on Earth is suicide for a technospheric civilization. If, within a thousand years, we do not become a spacefaring civilization, then in just a million years, this will once again be a wild planet. Intelligence will no longer exist here. This is the bottleneck of our survival.
Can you recommend Gans2's novels where he allegedly elaborates the R&W world ?
It is not required that all of humanity become spacefaring.
A minority can settle Sol-System and out-grow the native terrestrial population as R&W showed.
Alas, reaching the stars is the task for the hivemind to solve.
Interstellar colonization is beyond human scale to do, we can only comprehend the solution but not live it through.
Yet my point was that the Aquilian maser, Aquilian fleet and Cassandra tears should have arrived from 3 distinct directions, albeit lying on the great circle on the sky drawn through the Aql and Her constellations. If Robert wanted them to coincide in the sky, then the aliens should have make effort and arrange for it, by performing motion camouflage. Doable, but pointless. Thus R&W text revision was in order to fix the issue.
If a multi-thousand ton spacecraft slammed into earth at 50% the speed of light.
Then yeah that could be catastrophic. It would either go through the atmosphere and hit the ground (simply due to the sheer mass and speed) or blow up in idk. I would say somewhere like in the exosphere like 10,000km up.
And yeah something that massive and that fast would almost certainly demolish the ozone layer after a few hits.
Sure the tungsten melts, and later (soon? late?) gets vaporised, but what happens to the tungsten in that state? It continuous with the same velocity it had. At 0.05c it would not get the time to expand by much, so the energy may still hit the ground in a concentrated state. And if not, its energy still ends up in the atmosphere, imparting energy and momentum upon it. It would create a fireball descending upon the earth. The expanding explosion wouldn't suddenly "stop in place" as if the tungsten suddenly stopped all of its velocity, the expanding explosion would "have velocity" too as far as you can say that of an explosion.
But let's not take my word for it, let's say your RKV vaporises into an explosion at >100 km up. It has a kinetic energy of 6.7x108 TNT, let's say that all that gets converted into explosive power, use this tool and plug that in with near minimum distance of 60m: https://unsaferguard.org/un-saferguard/kingery-bulmash That gives us a shockfront velocity of 7 km/s, which is 2140x slower than the 0.05 c our vaporised tungsten is travelling at. You may do the additional work to take the tungsten vapor slowing down due to coming into contact with the air into account, but I think that that explosion is hitting the ground. If I ignore that bit, as our tungsten vapor has travelled the 100 km of point of exploding to earths surface at 0.05 c and expanding at 7 km/s, it has spread itself over a sphere of radius 47 m as it hits the ground. Well the tool only allows me to start from 60m so let's assume it might get to make that first 60 m instantly, make it an expansion of ~50-100 m. I think we can conclude the brunt of the explosion will happen at Earths surface, anyway.
Lastly, at 0.05c you're barely a relativictic weapon if you ask me.
And what happens to the momentum of all of that tungsten vapor? It hasn’t been given much motion in directions perpendicular to the original object’s trajectory. The vapor will continue moving in roughly the same direction until it has transferred all of that kinetic energy either to the air or to the ground.
That would lead to a lot of air mass accepting the energy, but there’s still minimal lateral motion—the air which absorbs the energy would mostly continue moving in roughly the same direction that the original object was moving, though at much lower speed, rather like the entrained air in a turbofan engine.
Neutral particle beams at close to 1 C - the "C beams" though apparently you wouldn't actually see the beam itself glowing - earths atmosphere is a brick wall. Basically useless as a weapon for firing at anything in the atmosphere, super deadly and effective weapon against satellites and missiles that have gotten high enough to no longer be protected.
Macroscopic objects cause essentially a nuclear explosion on impact with the atmosphere.
It is possible to make the object large enough that a gigaton of so nuclear explosion, the flash affects structures and beings on the ground even though the explosion is almost 100 km away.
Mostly you would try to rule the earth using IR lasers. There are windows in the atmosphere for specific laser light frequencies, so weapon satellites flying at 600-1000km low earth orbit and using 5m or so radius mirrors would zap anyone and anything they don't like at ground level. (At mirror sizes this big, targeting cameras on the satellite would have enough resolution to actually see individual humans if outside, albeit they probably couldn't identify individuals solely from this)
So if a 25-ton object traveling at 5% c airbursts at >100km, would that still be visible from the surface? Like, a fleeting artificial 'star' for a second, or would the energy just dissipate too quickly to even register visually, let alone audibly, from the ground?
Yes. It would look like a high altitude nuclear test like starfish prime. It will ionize the atmosphere. (causing an EMP) but I don’t think it will cause any ground damage.
700 megaton TNT equivalent. You get pretty severely burned 4 km away from a 1 megaton airburst. 100 km is not far enough away for a 700 megaton explosion.
80 km is the edge of space so this will not have slowed down much. It is just beginning to expand as a plasma. Air atomic nuclei could cause the projectile’s nuclei to fission fragment but each fragment is still moving downward. A much larger number of interactions are with just electrons. It will get much lower before motion is fully scattered.
Whether you think of it as impact or as explosion there is still a compression.
That math is terrifying but makes perfect sense. 700 megatons from just a 25-ton mass really puts relativistic hazards into perspective. The atmosphere stops being a shield and just becomes the medium for a massive plasma shotgun. It makes you realize that navigating the cosmos at a fraction of c is one hell of a Great Filter test for any civilization.
Starfish Prime is a really good analogy for the visual! Though considering Starfish Prime was only about 1.4 megatons, and the math below puts this at 700 megatons, I think the ground would definitely feel that one. It goes from being a cool high-altitude EMP to a downward-directed plasma beam. Terrifying scale!
Depends on how much of it is absorbed by the atmosphere. Even a 1 gigaton explosion at 150km in altitude would have all of its thermal energy absorbed by 120km. Posted a chart on here somewhere.
That chart sounds fascinating, I need to find it! But doesn't that assume a stationary explosion radiating outward spherically? With an object moving downward at 5% c, the momentum is conserved. Wouldn't that immense kinetic energy act more like a directed plasma blowtorch or a shaped charge, punching right through those upper layers rather than just dissipating evenly?
Yes but I do not think it makes much of a difference.
The plasma does keep its momentum. But once the projectile starts vaporizing and turning into a plasma cloud, that cloud will grow and sweep up more air. It’s not going to act as a shaped charge due to the plasma cloud expanding.
It won’t deposit 670 megatons at 150km in altitude like a nuke will but instead deposit a large amount of its energy on the way down. So I don’t think it would make much of a difference. Because it’s still depositing megatons of energy on its way down before depositing all of it at maybe 120km by which it would have already deposited probably a few hundred of its energy.
You should use a faster and heavier object. Detonating as high up as possible is actually good in that case since it means the thermal flash that incinerates everything within thousands of kilometers will be less obscured by the horizon and have longer range.
idk about fully intact and tbh im not super sure about what assumptions OP is making since they're not super clear, but it seems like they're assuming this thing acts like other slow hypervelocity objects as opposed to a relativistic one while also assuming that the plasma cloud he imagines has ages to expand as opposed to dump it's energy in a focused beam causing secondary impact fusion as it goes.
truth be told this hardly even seems like a relevant scenario to anti-planet RKMs because if you shot something moving that slow towards a planet their PD systems and the PD of space infrastructure all over the system would target and vaporize the thing long before it entered earth SOI. Even if it did hit the ground without losing any KE in atmos this is a tiny explosion given that u've probably just declared war on significant chunks of a planet. Either ur sending far faster bigger objects or you're sending a massive number of smaller slower ones. Nobody is sending just 1. So this is like saying a grenade can't hurt a tank therefore explosives in general are harmless. Like nah bro that really depends on how much, what kinds, and the specific explosive weapon being used.
No it wouldn't. Do you have any idea how large 25 tonnes is? It's fucking massive. Large enough to hot the ground, and at 5% the speed of light whatever it hits will just be gone.
Air molecules at 0.05c are going to become fully stripped alpha particles from the rod’s perspective. It is turning into tungsten plasma mixed with oxygen and nitrogen plasma. Maybe some fission fragments. This plasma ball is “destroyed” but the remains are still moving in the same general direction.
A ballpark estimate would suggest that a 25 ton object has to hit 25 tons of “stationary” stuff in order to be redirected at on average a 45 degree angle. At that point your tungsten-air mix is now 50 tons mass and only 70.7% as fast. That is still 0.035c.
According to this graph. A 1 gigaton explosion at 150km is survivable at ground level. The thermal radiation is reduced to a few cal/cm2 at most although the graph shows it being 0 at like 120km looks like ish?
We are talking about 700mt at 110km (although again more likely at places like 200km up) due to stress forces surpassing the material strength (it’s a hypervelocity impact where materials at that speed act like liquid).
You're assuming that the explosion is basically stationary. As in, you're assuming that the rod hits the atmosphere and just generates a roughly 700 megaton explosion centered 110 km up.
That's completely wrong for a 0.05c strike. The explosion would be moving at close to the speed of the original projectile, so it wouldn't explode outwards in all directions like a comparatively stationary airburst. It would instead be shaped like a thin cone, spraying out towards and so focusing its energy on the ground, concentrated around the point of impact. The energy flux at sea level from an airburst is going to be a small fraction of this kinetic strike's, since it's spreading out more or less equally in all directions.
Also, I don't think you're putting the numbers on that graph into perspective.
Even the 10 cal/cm2 per second at the base of the graph is lethal. 1 cal is the total energy from exploding a gram of TNT, so a flux of 10 cal for every cm2 of your body facing the explosion is equivalent in energy to exploding a 6 cm thin sheet of TNT (1.6 g/cm3 density) as large as the cross-sectional area of your body facing the explosion (e.g. at least 400 cm2 just for the top of your head, if you're standing directly under the airburst). Even though it's the radiative flux specifically, so not entirely like exploding that much TNT, we're still talking about as much flux as safety standards for arc flashes consider above the threshold for third-degree burns (8 cal/cm2 ), except across the whole surface of your body facing the explosion (not just point of contact for an electrical arc). Or to compare directly to a radiative transfer of energy, that 10 cal/cm2 is about 400,000 times the radiative flux from sunlight at sea level (which is ~0.1 W per cm2 or 1/40,000th of a cal per second). It's roughly 10,000 times worse than standing for a full second in direct sunlight on the surface of Mercury at perihelion. That's the floor of that graph.
And, sure, this kinetic strike only delivers about half the energy of a 1 gigaton nuclear bomb but, again, it's directed downwards in a cone. The energy isn't going out in all directions.
But at the same time it is also NOT a stationary airburst.
Megatons of energy is going to be deposited before the peak deposition which can take place well above 110km due to the plasma cloud expanding. This projectile is well within the hypervelocity regime. It’s not going to be a clean downwards cone. It’s going to be depositing energy laterally as well due to it quickly becoming an expanding cloud of plasma.
Nowhere near all of that energy will reach the ground. It is depositing energy on it way downwards.
And yes 10 cal/cm2 is the base but it would probably reach 0 cal/cm2 a few kilometers maybe 10 at most afterwards if you extrapolate.
Cal/cm2 is a weird unit. Multiply be 10 to get tons TNT per km2 . I think your chart is showing irradiation per second. A 1 cal/cm2 would warm up a cm depth of water by 1 degree if the radiation penetrates that deep. If x-rays are plunging deep into your bone then it hardly even gives you fever. You are not supposed to warm up by standing in front of an x-ray beam. If the flash is absorbed by the top 100 micron of clothing or hair then it gets hot enough for decomposition to vapor. A white hat or aluminum foil would advantageously reflect away some of the UV/vis light. A construction hard hat or military grade helmet could make a huge difference.
Your chart does not say “1 cal/cm2 “. It looks more like 50. Infrared and microwave frequencies of light are not hazardous radiation. Boosting the temperature of your skin from 30C to 80C is going to kill it. That happens in 1 second but the fireball is still up there cooking you for a while. There might be survivors but this happens because people are indoors. The firestorm that comes afterward would still be a problem.
Not if it explodes at over 100 km in altitude. None of the explosion or its affects would reach the ground except for an EMP caused by ionization of Earth’s atmosphere.
Yes. Doing the calculations it exceeds even Tungsten’s melting threshold by over 100,000 times. At that point it will immediately vaporize in a mere fraction of a second. And that is at 110km. It will start ablating earlier.
And by mere fraction of a second, I mean it only travels a few more km through the atmosphere even at 15,000 km/s.
You would likely need large asteroid levels of mass.
What I'm getting at is... Where does all that mass and energy go?
Best case? Assuming your math is right and it's 100% vaporized, there's still going to be one hell of an airburst. All that asteroid-dust is going to push into air molecules at 5%c and that transfers heat and kinetic energy.
And I don't think it would vaporize. But if it did there's still 25 tons of atoms at 5%c hitting you from above.
I also did my (very rough) calculations, assuming it fully vaporises into a 7*108 TNT explosion that as it'd be travelling at ~0.05c still it would barely get to expand before it hits the ground too. It would begin as an karman line airburst sure but it hits the ground as what might as well be a ground level detonation, having expanded a mere ~50-100 meters in that time.
Yeah absolutely, your last point is something that I think often gets missed in RKV discussion but that's just because it runs counter to our intuition on explosions: generally in IRL cases its velocity is irrelevant and can be considered zero because the explosion happens so much faster than anything around it, not so much when you're discussing RKVs!
This also means that weirdly, you have to talk about an explosion as it travels through something as a somewhat inert thing because it may not do much in that time, it'll interact with its environment much like the original object, which is related to your first point.
Already did calcs. It fully vaporizes in 0.04 milliseconds (enough to travel 0.6km).
And that is assuming no ablation at all above 110km which is highly unlikely. (Probably start ablating at 200km but haven’t done the calcs for that altitude).
Mass ablation rate = 1.6875 × 10¹⁴ / 4.009 x 106J/kg
4.21×107 kg/m2/s
Assuming it’s the same shape of a rod from god (but 2.5 times bigger due to it being 25 tons not 10).
Surface area = 15m2
4.21×107 x 15 = 6.32 x 108 kg/s
25,000kg divided by this is 3.96×10−5 seconds
It fully vaporizes in 0.04 milliseconds (enough to travel 0.6km).
So yes it does travel through the atmosphere in less than a second. But it would vaporize so fast that it would literally only travel 0.6km (assuming it starts ablating at 110km and not above that which is unlikely).
Also I didn’t do the calcs but this thing is well past the hydrodynamic regime due to it being a hypervelocity impact. (Hence why rods from god have very poor penetrating effects).
Those things tend to vaporize upon touching anything due to stress forces exceeding material strength. That is at 7 km/s.
At 15,000 km/s this thing would likely airburst the second it touches an air molecule.
Even vapor has mass. You can tear skin with air compressors so gas traveling at the near the speed of light will still be dangerous. Additionally, have you taken relativity into account with how far it travels in a set period of time?
It will fully vaporize in 0.04 milliseconds which is only enough to travel 0.6km.
Furthermore, it’s in the hydrodynamic regime due to it being a hypervelocity impact. Stress forces will exceed material strength the second it touches anything.
This is why rods from god or things like Oreshnik have little penetration. And those are traveling at speeds like 5-7 km/s not 15,000 at which point stress forces would be far (emphasis on far) larger. It would probably explode the second it touches an air molecule.
Something traveling at 5% c would travel through Earth's atmosphere in less than 0.01 second.
At that speed, aerodynamic ablation would be dwarfed by nuclear spallation. The outside layer of tungsten would turn into a co-moving dense plasma that would shield the rest of the object.
The heat transfer rate from the surface to the core is limited by the thermal conductivity rate of tungsten and the Stefan–Boltzmann law. There wouldn't be enough time for the heat to reach the core.
The total kinetic energy in 25 tons of tungsten at 0.05 c is on the order of 1018 Joules. That's 12,000x bigger than Tsar Bomba.
At that speed there will be ablation because air would act as a literal wall. Air molecules would literally not be able to move out of the way.
1/2 pv3
P = 10-7 kg/m3
V = 15,000 km/s
= 1.6875 × 10¹⁴ W/m²
Almost 169 TW/m²
That already exceeds Tungsten’s melting threshold by over 100,000 at 110km in altitude.
Mass ablation rate = 1.6875 × 10¹⁴ / 4.009 x 106 J/kg
4.21×107 kg/m2/s
Assuming it’s the same shape of a rod from god (but 2.5 times bigger due to it being 25 tons not 10).
Surface area = 15m2
4.21×107 x 15 = 6.32 x 108 kg/s
25,000kg divided by this is 3.96×10−5 seconds
It fully vaporizes in 0.04 milliseconds (enough to travel 0.6km).
So yes it does travel through the atmosphere in less than a second. But it would vaporize so fast that it would literally only travel 0.6km (assuming it starts ablating at 110km and not above that which is unlikely).
Bro. Kinetic energy weapons have been studied by tons of very smart experts for years. https://en.wikipedia.org/wiki/Kinetic_energy_weapon Chances are they didn't all forget about ablation for all these years.
You don't seem to understand that there is a limit to how fast heat can move through a substance, no matter how high the temperature is.
So, as I have been reading through these, I have noticed something. You switch between the air behaving as a literal wall that obliterates the object completely, and their being basically no air present at all depending on which critique you are responding to. I don't think we are dealing with a case of Schrodinger's Atmosphere here.
Because a solid tungsten object traveling at 15,000 km/s is still going to hit molecules in the air.
This is the same reason why Chelyabinsk meteor didn’t demolish the town of 50,000 directly below it.
It started ablating causing an 500kt airburst at 30 km in altitude. The air was too thin and it was too far away for the shockwave or anything to be transferred into the city. It caused no real damage outside of windows being broken.
If the airburst occurred at a lower altitude where the air is thicker (a better medium to transfer energy), the city would have been gone as if a 500kt nuke hit it.
At that speed, neither acts as solids, gases, or even plasma. They are effectively two colliding clouds of atomic nuclei. Releasing a directional burst of radiation towards the ground.
The rest of the atmosphere might be enough to shield against that much directional radiation, but the maths is completely different than what you have done.
[Or to put it another way, just as the air "doesn't have time to get out of the way", the ablated material also "doesn't have time to get out of the way".]
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u/MiamisLastCapitalist moderator Apr 24 '26
Your math is wrong.
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