Setting: close to present day. Aliens in ships about the size of the Pentagon are detected at about 10 AU, moving at about 100km/s, and will therefore take about 6 months to arrive.

Realistically at least one country will want to destroy them, which breaks the plot, so I need to know if they're told "no" by science or "no" for other reasons.

Nukes aren't anything like so destructive in space, for various reasons. The space ships have "sufficient" radiation shielding, so nobody on board would die (or even get sick) from the radiation of a merely "close" detonation. Alien ships also have armour plating designed for typical solar system material moving at ~100km/s. So, a nuke would only destroy one of these ships if it went off inside.

At 100km/s it takes 4.31 milliseconds to go through the entire thing and come out the other side. I don't know if nuclear explosions can be timed that well. Also I'm sure the bomb won't survive contact with the hull due to massive deceleration well before that point, so {mystery solution, if it exists} would have to make a hole in the hull and the front-facing Whipple shield first.

For reasons, assume the aliens don't have any sensible point defence weapons of note at this scale, or even vaguely good radar. Our existing stealth is more than good enough to stop them from knowing the danger until too late.

So, if this happened today-ish, could we hit these ships before they reach the Van Allen belts and the nuclear option turns into a footgun?

So, for example, any one of these being "humans fail" is sufficient to say "no" (but other reasons for "no" are also welcome):

  1. If nuclear warhead detonation is less precise than 4ms. (I'm assuming it is ~µs from the vague descriptions I've read about how precisely correlated the implosions have to be to avoid a fizzle but I see no specific numbers).
  2. There's nothing to fit the role of hole-puncher. (I don't know if a nuclear shaped charge do much more than a (series of) dumb rock(s) to a Whipple shield nor if either is sufficient).
  3. Can't be built fast enough. (So if this kind of thing takes >6 months then the aliens would have already landed).
  4. Can't be aimed or timed well enough, or would never get through the hull without damaging itself to the point of non-detonation.
  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Commented Dec 9, 2023 at 3:26

7 Answers 7


1) If nuclear warhead detonation is less precise than 4ms

Hitting a 4ms target window is chump change for modern semiconductor tech.

Consider an average desktop computer might have a CPU speed of 3GHz. This means a single "cycle" takes approximately 0.333 nanoseconds or 0.000333 milliseconds, and that's just consumer grade hardware. Even with a hobbyist grade $5 microcontroller, comfortably operates on microsecond scale, with the limiting factor probably being the ADC which reads out a sensor value taking a couple hundred microseconds.

For a military system, like that which you'd find in a missile, doing things (reading sensors, computing things, making decisions) on the nanosecond scale is a relaxing stroll in the park. For example, the military has missiles that are accurate enough to fly through windows and detonate once they've traveled a certain amount of centimeters through the now-broken window (and fill the room with shrapnel). This takes good timing.

2) There's nothing to fit the role of hole-puncher.


Aliens capable of interstellar travel and casually bringing their ship to a blistering 100km/s and withstanding impacts from debris and space material at these speeds is going to shrug off basically everything we can toss at it.

Consider that if they are traveling at 100km/s they, at minimum, are able to withstand 200km/s impactors due to relative velocity, and no sane designer would sign off on a "safety margin = zero" design. Realistically, they are able to shrug off much faster impacts, and if they lack "sensible point defense" then they are taking it all on the armor.

For a bit of mathematical backing, a 1kg impact that collides at a relative 100km/s has the energy of 1 kT TNT equivalent energy, which is a lot.

Maybe a staggered series of shaped nuclear detonations would do the trick. This is already how modern anti-reactive armor missiles work. They have two shaped detonators essentially stacked behind eachother: the first fires a lance to detonate the reactive armor panels on the target, and then the second fires into that now armor-less hole. Perhaps a similar strategy, but nuclear, could do the trick, especially if you used a long line or chain of many nuclear shaped charges, followed by a "conventional" bomb to detonate once inside.

3) Can't be built fast enough.

I'm going to put a solid maybe on this.

Fortunately, SpaceX exists, and this exact scenario of "needing to get something into space now" is something the US military has been playing with and awarding contracts to for a couple years now (keyword "Responsive Space").

A Falcon 9 rocket is "available enough" that provided the political and financial lubrication, it certainly would be on hand in an "alien defense" scenario. I think they could scramble together a launch in under a week.

Unfortunately, nukes are heavy and the F9 rocket isn't a heavy lift vehicle. The payload that it can shoot out of the Earth/Moon system is maybe 4 tons, which isn't much. Falcon Heavy is significantly better, being able to send almost 17 tons, but FH launch vehicles would take more than a week to scramble, although it could probably still be done within a month.

Similarly, NASA's DART mission demonstrated that we have the technology to hit the target. They managed to hit a 160 meter-sized target in deep space at about 7km/s relative velocity. Still, this was quite the feat, and this isn't some equipment that NASA just has lying around on the shelf. I'm unsure if we could crash-develop a similar guidance package and spacecraft within the time window, and hitting it at 100km/s relative would be significantly trickier (although the aliens might want to start to slow down before they reach the Earth/Luna system).

For storytelling purposes, maybe NASA has one or multiple DART prototypes or testing duplicates lying about that they could repurpose?

The trickiest part is probably the set of, preferably many, staggered shaped-charge nuclear warheads. Shaped charge nuclear weapons were analyzed during the Star Wars era of US military strategy, however as far as the public knows, none were ever tested or developed--the most theory work was probably done on this topic by the team behind Project ORION. Regardless, this isn't an "off the shelf" solution like conventional nuclear weapons.

For storytelling purposes though, I don't think it's unrealistic that the US developed these in secret some time during the late cold war as some 'end-all be-all' bunker buster to destroy even the hardiest of Soviet nuclear silos.

4) Can't be aimed or timed well enough...get through the hull without damaging itself..

Doable. Maybe.

See previous response on DART. NASA managed to nail a 160m target (of which the position had only been estimated by the wobble of the larger body) at a relative 6.6 km/s on a budget of merely $300 million out in deep space. While hitting target that's an order of magnitude faster would doubtlessly be more difficult, provided that the equipment can be built in time, I think it could be done. The target is bigger, and there are 'fate of humanity' concerns that would motivate the engineers and politicians.

As for surviving entry into the craft, I think this is also doable. We have bunker-buster bombs today that are designed to first penetrate many meters of reinforced concrete and Earth. Then, once they detect open air around them and assume they've reached the human-inhabited parts of the bunker (again, nanosecond speeds), they blow up.

Using a staggered-shaped-charge nuclear lance to first punch a hole that the follow-up bomb(s) then "fly" into the now-open space before detonating should be doable.


If sold properly, I think it might be possible to "realistically" shoot down the alien craft provided the parameters in your question. Regardless though, this relies on rather "stupid" aliens and a lot of luck/crunch on the side of the humans.

Therefore, to answer the secondary question about saying "no", I think that you'll need to find a reason other than "technically impossible". Based on the various points I've outlined, I don't think it's unreasonable that the USA specifically comes to the conclusion that they have a realistic "shot" at success, or realistic enough that it's worth attempting if they decided doing so was the right course of action.

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    $\begingroup$ "(although the aliens might want to start to slow down before they reach the Earth/Luna system)": note that this would only make it easier if they start the deceleration early enough for us to account for it, and continue the deceleration without any change. If we launch on an intercept trajectory and they change the amount or direction of their braking for any reason, we miss. Lacking a detailed flight plan from the aliens, our only hope for hitting them is either zero maneuvering on their part, or one big perfectly-predictable maneuver. $\endgroup$ Commented Dec 5, 2023 at 17:44
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    $\begingroup$ @ChristopherJamesHuff Yeah, predicting this would be difficult, however it's not impossible that, because of their velocity, they'd already need to start decelerating many months out. In fact, maybe that's why they were noticed (drive plume?). Regardless, I suspect that any real attempt would involve as many launches as possible to not only account for misses, but also changes in trajectory $\endgroup$
    – Dragongeek
    Commented Dec 5, 2023 at 17:47
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    $\begingroup$ @ChristopherJamesHuff That said, very little dv is required to adjust the trajectory quite a lot if the target is far enough away. Obviously, as the craft gets closer, this dv requirement increases, however if they are heading directly for Earth, this also simplifies the problem somewhat as, again depending on the exact orbital mechanics, we might be able to launch our interceptor on a direct head-on course which would require minimal adjustment unless they explicitly dodge. Something coming straight at us is easier to hit than something at an oblique angle eg DART $\endgroup$
    – Dragongeek
    Commented Dec 5, 2023 at 17:51
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    $\begingroup$ Nuclear weapons aren't that heavy. A Falcon Heavy could throw the upper two stages of a Minuteman missile at an interplanetary target. If you don't need terminal guidance, it could throw 60+ W87 warheads instead. $\endgroup$
    – Mark
    Commented Dec 5, 2023 at 22:50
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    $\begingroup$ It's a bit speculative, but if the immediate survival of all of Earth was at stake, then it's likely that the USA, Europe, Russia, and China, each would be capable of quickly building the means to get an unmanned device in orbit and shooting it within 6 months; this kind of destructive mission is a lot easier than all the delicate space missions we have accomplished so far. And that's today; if the OP's novel is set in the future, even the near future, then it's entirely plausible. $\endgroup$
    – Stef
    Commented Dec 8, 2023 at 21:57

Frame challenge / "no" for other reasons

Realistically at least one country will want to destroy them

This is false. If an alien ship reaches our solar system, our scientists will tell our leaders that these aliens are necessarily far more advanced than us.

You do not start a fight with somebody who is 10x bigger and stronger than you. Even if you blow up this ship, you have to assume they can send another -- and the second ship will be coming here to annihilate us.

None of the countries on Earth that have access to nuclear weapons will be reckless enough to unilaterally start a war with advanced aliens.

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    $\begingroup$ "You do not start a fight with somebody who is 10x bigger and stronger than you" are you sure? Greek poleis vs Persian Empire, just to name a counterexample $\endgroup$
    – L.Dutch
    Commented Dec 5, 2023 at 17:22
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    $\begingroup$ @L.Dutch - 1,000% agree - many many instances in history of a militarily weaker side starting something with a militarily stronger side. $\endgroup$ Commented Dec 5, 2023 at 19:55
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    $\begingroup$ but the technology level at the time is different. At the time of Ancient Greek and Persians, the main fighting method is cutting and stabbing people with sharpened metals, collect battlefield status with eyesight, and perform battle control by shouting and flag waving. Even 30 years ago, our fighting method is already by GPS guided missiles and tactical air, recon by radio wave and digital intrusion, and perform battle control by datalink. In a head to head battle the one with weaker tech always lose. Taliban and other forces only won the war by fighting US off the battlefield, not on it. $\endgroup$
    – Faito Dayo
    Commented Dec 6, 2023 at 3:39
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    $\begingroup$ So in any head to head battle between Earth and anybody who can came to our system we would get trashed. Whether we can bloody them before getting a relativitistic kill vehicle that shatter our planet is another issue, but the wise move would be NOT firing the first shot--as demonstrated by Space Battleship Yamato 2199. Of course, if they fire the first shot then fighting would still be a better idea--bloody them first, show them we got guts and are not afraid of dying, and then sue for a more favorable surrender (then ursup the empire from within) $\endgroup$
    – Faito Dayo
    Commented Dec 6, 2023 at 3:44
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    $\begingroup$ "None of the countries on Earth that have access to nuclear weapons will be reckless enough to unilaterally start a war with advanced aliens." you're underestimating Murica $\endgroup$
    – Hobbamok
    Commented Dec 7, 2023 at 11:44

"Can we hit the ship" - yes, reliably, provided they don't change course (which I frankly expect them to be able to, anyway they are to shed off 100km/s delta-V to stick to Earth orbit) and don't actually launch anti-missiles. "Can we damage the ship" - yes, probably, and we don't even need a nuke, just a good old kinetic impactor on the counter course, 105 km/s delta would provide more energy than a nuke would! "Can we destroy the ship" - unlikely, even if it would be hit dead center, the area of damage would only expand about 0.05 radians each side (speed of sound in metal divided by speed of impact), meaning that a 200-meter long hole left from the impact would expand to about 10 meters from the center line, leaving the majority of the ship unharmed, although maybe thrown off course. And now remember, there are several ships out there and close enough, and should even one reach us in a capable condition, we're dead.

So, the proper answer would be "No, for other reasons", namely we are unable to destroy all of those ships reliably to prevent direct conflict. And there is another reason to not touch them - they might be plain flying past, just happening to visit a promising star system, that somehow was sending a weird spectrum of radio waves several bongs ago. In case they are hostile, we could do better reading their power capacity before delivering a decisive strike, be it a nuke or something else, so that they won't escape, and in case they are at least neutral, we still are better not doing any premature hostilities while they are yet in interplanetary space.

Frankly, the only way we could detect those ships that far away is if they would themselves emit at least infrared radiation, which is expected by hard science, then say James Webb would be able to find an unexpected dot during its sky survey routine. The levels of emission say enough of the technology used, as the emission is effectively blackbody radiation with a determined temperature and a red/blue shift depicting their motion towards or away frm the observer. Assuming the scientists would decipher those emissions as having a too big blackbody temperature, they would determine that such emission is beyond our own technology, and possibly (plot reasons) beyond our knowledge of physics. So if the ETs know more than us, they might also have ways to stop our best attacks that we can't imagine, which is a "no by science" way to answer this question.

PS: your proposed reasons of "humans fail" are all false. A kinetic impactor really does better than a nuke at this delta-V; building a rocket in time is now decently easy, just ask kindly off SpaceX to launch a Super Heavy; and other reasons do not apply if the weapon is purely kinetic.

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    $\begingroup$ What if we send a lot of marbles rather than a single impactor? $\endgroup$ Commented Dec 6, 2023 at 12:12
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    $\begingroup$ "..should even one reach us in a capable condition, we're dead". We can always say it was a typical human welcoming ceremony, and that we never anticipated that outcome. $\endgroup$
    – Joachim
    Commented Dec 6, 2023 at 13:02
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    $\begingroup$ "just ask kindly off SpaceX to launch a Super Heavy" I think you overestimate the civic-mindedness of Mr. Musk. Someone's gonna have to pay for that thing! $\endgroup$
    – Bear
    Commented Dec 6, 2023 at 14:43
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    $\begingroup$ @TheSquare-CubeLaw space shrapnel is the way to go, but vs human-built ships. Alien ships are expected to withstand shrapnel, as they had to pass through several parsecs worth of space, meeting micrometeorites and similar stuff. But if we'd somehow know that shrapnel of certain size will penetrate their armor, then yes why not - except for the case when they dodge the entire warhead. $\endgroup$
    – Vesper
    Commented Dec 6, 2023 at 14:53
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    $\begingroup$ @Bear asking kindly does not necessarily mean "for free", anyway launching a rocket has a price, and a state would gladly pay it in such an emergency. I mean asking as in "please allow us to launch sooner, because in 6 months they will reach here aaand", which would infer additional cost but looks doable from my inept perspective. SpaceX does have a schedule of when they would build or repair another rocket for the next launch, that schedule is already tight enough IMHO. $\endgroup$
    – Vesper
    Commented Dec 6, 2023 at 15:00

Your best bet is to rely on the "can't be built fast enough" side of things. There are only two countries that can launch an interplanetary payload on six months' notice: the United States, and China. The only other countries with more than two successful launches in 2023 are India, which doesn't have a heavy-lift vehicle that can launch on short notice, and Russia, which hasn't successfully operated a spacecraft outside low Earth orbit since 1988.

This isn't an absolute guarantee, but expecting two countries to be sensible is far more reasonable than expecting a dozen or more.


Can We hit it? Yes.

There's a multitude of ways such an object could be hit. Let's go through your points:

  • 4 ms response time - Easy with modern electronics, Next.
  • Nothing to fit the role of holepuncher - I'll come back to this.
  • Can't be built fast enough - Humans can build stuff exceptionally quickly if sufficiently motivated. I'm reminded of the BunkerBuster bombs made in 28 days for the Gulf War or taking 42 days (eventually) to build a liberty ship in WW2
  • Can't be aimed/Timed - This is either easy if they aren't able to jink/change direction easily or a deal breaker if they can.
  • Never get through the Hull to cause damage - I'll also come back to this.

Right - you'll notice a bunch of things where I said I'd come back to it - well now it's time for a....

Frame Challenge!

So, you have stated your goal to have the Governments with this sort of capability to not attack it. Realistically - that means the US, UK/Europe, China, Russia, India, Pakistan and Maybe North Korea.

You have an unknown object of unknown capability and no information about it - How does your government know it's armored? How do they know it's radiation shielded? How do they know it has no short-range detection and point defense systems?

Consider the Cargo Cults of the Pacific - how they would build mock-ups of ships/aircraft in hopes to invite the others back, without any appreciation of what those things are and their capabilities.

Without any information - a Government (let's say North Korea for example) is likely to go 'Sod it, let's try it!' - which, as you say, breaks your narrative.

In order to know the things which would make any current military intervention pointless, you have to know a thing or two about what you are facing. If I know the capabilities of an MBT and all I've got is a pistol - I know that I'm not going to pick a fight with it.

My Suggestion then would be to have a partially damaged scout craft from said Alien Civilization crash to earth - damaged enough that the occupant wasn't able to avoid crashing to earth, but not so much that it can't be reverse engineered/understood - from there make it so that the capabilities of such an intervention are known to not be sufficient.

You could even weave this into your narrative as to why the ships are arriving (could be an innocent rescue party... or an invasion force) - but you use the small scout vehicle as the basis to assess the Alien Tech so that the Governments know that our current weaponry is insufficient to damage it.

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    $\begingroup$ And that's how you reinvented the plot of Independence Day... $\endgroup$
    – Nzall
    Commented Dec 6, 2023 at 10:17
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    $\begingroup$ @Nzall - Heh - I wasn't consciously thinking of that - but now that you point it out... $\endgroup$ Commented Dec 6, 2023 at 19:53

Realistically at least one country will want to destroy them, which breaks the plot, so I need to know if they're told "no" by science or "no" for other reasons.

Tom already talked about this, so ill keep it brief. Even the simplest Interstellar spacecraft, such as Project Orion, carry a stockpile of nuclear warheads with them that could easily kill billions. Any even just slightly more advanced propulsion system, let alone the really good stuff like Antimatter, would be able to do an untold amount of damage by simply being turned on. So this is the first reason why you dont attack the Aliens, their drive can probably end the Human race in an afternoon.

Second, there is the first rule of Warfare. Never assume you know all the cards of the deck. Isnt it convenient that there is just a small number moderately big spacecraft moving at a very low velocity ? Perhaps we do not know the full extend of the Aliens might. Perhaps, this is a decoy, or a Test, and the moment we launch our missiles a god damn moon sized spacecraft leaves Warp and obliterates the planet. You dont know what the Aliens war capabilities are, and you would be very wise to not try and find out.

For reasons, assume the aliens don't have any sensible point defence weapons of note at this scale, or even vaguely good radar. Our existing stealth is more than good enough to stop them from knowing the danger until too late.

I would really like to know these reasons. Because there are non. You dont cross interstellar space without a radar system capable of detecting sand grain sized elements several light seconds out. Similarly, there is no stealth in space. And there is absolutly no reason to believe any Human stealth would be able to avoid detection by literal alien tech. Also, you mention todays tech. We dont have any stealth platforms for space, let alone nuclear tipped missiles capable of reaching mars in less than a couple of months.

  • $\begingroup$ "without a radar system capable of detecting sand grain sized elements several light seconds out" - man, the main radar equation states that range is proportional to fourth power of radar emission power! Several light seconds is more than 1e10m, so you need e40 W power to receive 1W back from a single 1m^2, and grain sized means add another e6. This starts reaching the famous FOE level of power aka supernova. If they'd have such radars, the Earth would get cooked long before they would reach the outskirts of Oort cloud! $\endgroup$
    – Vesper
    Commented Dec 7, 2023 at 5:59
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    $\begingroup$ More, a grain sized object receiving e10 W/m^2 would just vaporize in microseconds, so why using a radar of such power if you could use a frikking laser of the same power and not let anything reach you by using brute force. $\endgroup$
    – Vesper
    Commented Dec 7, 2023 at 6:01
  • $\begingroup$ @Vesper , i dont see the problem :D Your Radar simply is the point defense system. And i was obviously using hyperbolic language. $\endgroup$
    – ErikHall
    Commented Dec 7, 2023 at 7:46
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    $\begingroup$ Hmm I actually made a mistake of 1 order of magnitude, which resulted in 4 orders less power required for such a radar. Still, be careful with such hyperbolae XDDD $\endgroup$
    – Vesper
    Commented Dec 7, 2023 at 14:20

Nukes are actually more destructive in space than on Earth

(As an aside, first I'd like to tell you that a nuclear fission reaction typically is over within 100 "shakes," a shake being 10^-8 seconds, so that means the whole fission reaction takes about a microsecond. A few milliseconds is plenty of time for that.) Now, onto my main subject...

Earth has an atmosphere, which absorbs a great deal of the energy from a nuclear explosion, reducing the damage dealt. In space, you get hit harder because you don't have the atmosphere to shield you.

What you get hit with is neutrons and photons instead of an air blast, but when the neutrons and photons hit your ship, it goes boom. Specifically, the radiation penetrates into the hull of your ship and heats up the metal to the vaporization point instantaneously. It also instantly melts a lot of the metal that's too deep to actually vaporize. This vaporized and melted metal rapidly expands, causing a shockwave, which spreads through the ship, smashing things. It's as if someone had strapped a large amount of dynamite to every square cm of your hull on the side facing the nuke, and detonated it at once.

In addition to the shockwave, the vaporized metal explodes away from the ship, which causes a very sudden and powerful reaction force on the whole ship, like a rocket engine - indeed, like Project Orion. This extremely sudden acceleration breaks things all over the ship, if you don't have a huge shock absorber facing the nuke like Project Orion does.

Since you thought space nukes were less dangerous and linked Project Orion in support of that, I should tell you why the nukes used in Project Orion wouldn't destroy that ship, and it's not because the ship's in space. First note that Project Orion would use the nukes to climb out of the atmosphere as well as in space. Second, nukes are powerful, but a few meters of steel goes a long way to withstanding them. Project Orion has a huge pusher plate that protects the back of the ship. The other sides of Project Orion aren't so protected. Third, Project Orion has an enormous shock absorber between the pusher plate and the main ship, without which the extremely sudden acceleration would be breaking the ship even with the pusher plate. The shock absorber is a large fraction of the length of the ship. And fourth, the nukes used in Project Orion would be small fission devices, not the huge thermonuclear weapons I'll be talking about.

The DNA damage from radiation is also much, much more dangerous in space than on Earth. If Earth didn't have an atmosphere or the ground/buildings to protect us, the 50 MT Tsar Bomba would have irradiated every single human being on the planet; a person 12,742 km away, on the exact opposite side of the planet, would have received around 1 Sv dose, which is possibly fatal. That's what happens in space.

So I'd like to frame challenge by considering what happens if a big nuke does not penetrate, but just explodes nearby. It could easily destroy the alien ship or kill everyone inside if their ship is not sufficiently prepared for such an event.

We're serious about trying to stop those aliens and they do have a really big ship, so let's imagine hitting them with a "Big Nuke" detonated 1 km off to the side. The weapon we'll be thinking about is a 100 MT (4.184 * 10^17 J) thermonuclear weapon - the original design yield of the Tsar Bomba, if the Uranium tamper had been included. I like this because 100 MT is a nice round number. I'll go ahead and show you some example calculations so you can adapt them to your setting and decide whether the aliens can survive it.

First, the explosion damage. The ship will receive 3.3e6 J/cm^2 of neutron and gamma radiation. Converting, this is about 800 grams of TNT per cm^2, or 8000 kg TNT per m^2.

You indicated the ship is 431m long; let's suppose the hull area exposed to the blast is 400m * 200m = 80000m^2. This would be the equivalent of 640,000,000 kg TNT strapped directly to the hull and set off.

Could the alien ship survive this? This is more of a judgment call, but it's unlikely.

I've done a calculation indicating that if the ship has a steel hull, the top 10 cm of steel will turn into vapor, and the atoms in the vapor will have an average speed of 8000 m/s. All those atoms will practically instantly go flying off to the side, and from conservation of momentum, the rest of the ship is punched the opposite way. If (and we're being very generous here) the rest of the ship masses 100x more than the vaporized 10 cm of steel, that means the rest of the ship would instantly be moving at 80 m/s after the punch. Any humanlike aliens inside the ship would be killed by that, when the wall hits them at 80 m/s. Crash couches wouldn't save them. Plenty of equipment would be destroyed by the sudden g-forces as well. The hull, also, would cave in and cause massive damage, even if it was meters thick.

One possibly useful comparison is the Mark 48 torpedo. This torpedo has a 293 kg warhead, which it detonates under the enemy ship to break its keel. In other words, just like our nuke, the Mark 48 doesn't penetrate, which is why it is a possibly useful comparison. Clearly, the Mark 48 delivers many orders of magnitude less energy per m^2 than our Big Nuke does.

Another thing to consider is, how much of the hull would directly melt or vaporize because of the heat from the radiation? It's a bit of an involved calculation; we may use the number that an inch of steel reduces radiation intensity by about 50%. The heat of melting for steel I take to be 700 kJ/kg, and the density of steel is 0.007859 kg/cm^3. The formula I came up with is: 0.272893 e^(-0.272893 x) * (energy deposited in units of J/cm^2) / .007859 = 700000, with x being the depth of steel hull in cm that will immediately melt. For our case x = 19 cm. So steel plating will be directly vaporizing or melting to a depth of 19 cm. Of course, most of the damage will probably be from the shockwave and the whole-hull stresses, not the direct melting.

Next, we can check how much radiation shielding the aliens would need to avoid dying from irradiation. Based on slide 15 of this slideshow, neutrons are far more damaging to the human body than photons for a given energy, so we can ignore the photons for a ballpark estimate. The neutrons that would deal most of the damage would be high-energy 14 MeV ones from the fusion reaction. These would be a fraction of the yield, certainly less than half (half of the Tsar Bomba yield is from fission). Let's say 10% of the yield is 14 MeV neutrons. Based on the chart from the slideshow, 14 MeV neutrons deliver about 450 pSv cm^2 of radiation dose to the human body. This means, if you are hit with N neutrons over an area of A square cm, you will receive 450 * N / A picoSieverts of radiation dose. If you get more than 1 Sv dose, you may die.

This radiation dose is in terms of number of neutrons, so we need to convert to Joules. Each neutron has 14 MeV = 2.243 * 10^(-12) J of energy, so the dose per J/cm^2 is 450 pSv cm^2 / (2.243 * 10^(-12) J) = 200 Sv cm^2 / J. Multiply this by 3.3e6 J/cm^2 * 10% and we find that an unshielded alien would receive 6.6e7 Sv.

To bring this down to a safe level we need to add radiation shielding. From this article, 30 cm of concrete will reduce the radiation damage by 90%. The alien would need 8 layers of that - 240cm of concrete between the alien and the blast - to bring the dose below 1 Sv. It would be more efficient to use lead. From this blog, the radiation halving thickness of lead is 0.4 inches, or about 1 cm. The alien would need about 26 cm of lead.

You can decide what this all means for you. Really the hardest part would be getting your nuke within 1 km of the enemy. Why wouldn't they avoid it? There are ways you can make the nuke stealthy - you can put it on a ballistic trajectory (no thrusting) with angular radar-reflective surfaces, like a stealth bomber. But this will only work if the enemy also travels in a predictable trajectory without thrusting at all, which they wouldn't do if they expect any trouble.

  • $\begingroup$ This answer could be improved by expressing TNT on the hull as a useful analogy for getting a ballpark estimate of the energy transfer rather than an equivalent. Hull albedo multiplies one end of the equation, and 1 minus the fraction of the interaction energy that goes into accelerating the TNT away from the ship (probably most of it) multiplies the other. $\endgroup$
    – g s
    Commented Dec 8, 2023 at 3:54
  • $\begingroup$ @gs Hull albedo doesn't matter because the radiation will penetrate directly into the hull, not reflect off the surface. By the way, I have calculated an analysis of how much momentum is transferred to the ship as a result of the hull vaporizing and it turns out it is enough to crush the ship and kill everyone inside! I guess I'll edit that into the post. $\endgroup$
    – causative
    Commented Dec 8, 2023 at 4:09
  • $\begingroup$ for the neutron radiation, yeah. Actually now that you reminded me of vaporization, it probably doesn't matter for the x-rays either, since I imagine whatever the hull is made of makes a pretty good blackbody once it's vaporized and possibly ionized. The TNT side of the equation definitely gets a multiplier, though. Most of the energy of all that TNT would just be spent pushing whatever hot gases TNT makes when it explodes away in all directions. (i.e. it's an underestimate; you probably need even more TNT to be as damaging as the nuke) $\endgroup$
    – g s
    Commented Dec 8, 2023 at 4:20
  • $\begingroup$ Although the same principle also affects your depth of melting etc calculations. Vaporized hull material will carry away most of its energy into space, not stick around to heat the metal under it. $\endgroup$
    – g s
    Commented Dec 8, 2023 at 4:28

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