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In a hypothetical spacefaring civilization, there is a belligerent militant group that has acquired access to nuclear armaments, with the intent of using them on a densely populated urban world. If this spacefaring civilization was unaware of this plan, would it be possible to detect a nuclear bomb aboard a starship if space travel is commercialized enough for the average civilian to acquire a ship for interplanetary and intersystem travel, or would this terrorist organization be able to covertly transport the weapon onto the planet undetected to detonate on the surface, as opposed to being dropped from a bomber?

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    $\begingroup$ As an answerer has already put it, if you can casually engage in interplanetary and interstellar travel, why would anyone be concerned about a nuclear bomb? $\endgroup$
    – jdunlop
    Feb 28 at 23:14
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    $\begingroup$ The threat of nuclear weapon/s being smuggled into a country by either a nation state or terrorists has long been recognised as a threat, especially by the US. For obvious reasons the countermeasures that are available and in use are not going to be widely publicised. Arguably it is also unanswerable without more information from you about the trade volumes/protocols (standardised cargo containers?), customs and border protection protocols of the world that may be attacked. $\endgroup$ Feb 29 at 0:22
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    $\begingroup$ Of course, if you have space ships that are so common that the average civilian can acquire one, you have far bigger problems than mere nuclear weapons. Each of these privately-owned space ships could become an extinction-level hazard without needing to contain an atomic weapon. Mass and velocity are quite dangerous enough without requiring atomic fission or fusion weapons. $\endgroup$
    – Monty Wild
    Feb 29 at 4:55
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    $\begingroup$ Not sure why this is closed? Seems like a fairly straight forward question... $\endgroup$
    – komodosp
    Feb 29 at 16:43
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    $\begingroup$ @komodosp as I pointed out already, we need the travel parameters - if privately owned spaceships can 1) fly to a planet and land anywhere it is a very different problem to if 2) all traffic is routed through particular spaceports on the surface, which is a different problem again to if 3) all traffic is routed to orbital spaceports and passengers/cargo must then go to the surface by shuttle or beanstalk. Detection is extremely difficult for the first scenario, non-trivial but less difficult for the second and trivial for the third. $\endgroup$ Mar 3 at 10:11

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I love the tag

Considering how much nuclear material would be involved in some phase of the development of regular space flight, it's no stretch of the imagination at all to have nuclear material detection. After all, we have metal detectors in schools and bomb detecting dogs in airport cargo areas and plenty of other "chemical detection" procedures in the world today. And in the future, it would be just as necessary (IMHO) to worry about lugging who knows how many tons of fissile material around. You know what I mean?

He and his research team revisited the idea after 9/11, in the hopes of creating something to help detect explosives as part of border security. After years of work, they came up with a handheld detector that could recognize 50 grams of explosives when being swept over a person at two meters a second. It could look into the body from a distance, and since it used low levels of radio waves, was completely safe.

...

Cutmore and his CSIRO team scaled up their explosive detecting wand to a scanner that fits over a two-meter-wide conveyor belt, creating the spinoff company NextOre in the process. The scanner has a sampling period of just tens of milliseconds, allowing it to peer into 10,000 tons of rock an hour to figure out which blocks have the right minerals. Rocks that contain nothing of value or are too impure don’t get crushed. It’s good for the environment and for profits. (Source)

That solution uses magnetic resonance to detect explosives and minerals... aka chemicals! What's nuclear material but just another chemical? Let's refine the idea by taking a step back and asking ourselves how an MRI works in the first place.

The strong magnetic field created by the MRI scanner causes the atoms in your body to align in the same direction. Radio waves are then sent from the MRI machine and move these atoms out of the original position. As the radio waves are turned off, the atoms return to their original position and send back radio signals. (Source)

How strong must that magent be to detect something in a space ship?

If we're thinking about this from the point of view of the magnet, whomping strong. As in reduce-your-tech-society-to-the-middle-ages strong. But when you think about this from the perspective of the detector, it's a whole new story. An enormous amount of the technical innovation in the last 20 years has been improvements in detection. It's therefore no stretch at all to believe that your space faring civilization (and others) might have improved detection technology — and while we're at it, emission technology.

Magic happens here!

As with any good story, there comes a time when you need to stop explaining the gory details and let the reader believe that you have a great solution. So we combine Magnetic Resonance Imaging with advancements in explosives and mineral detection at high speeds and link in our current deep space advanced radar capability and hook it all into a perfectly respectable satelite network. And what do you get?

A believable solution to the problem of detecting incoming nuclear armaments by detecting the fissile material in the bomb. A bit like detecting fluorescing minerals with a black light... only way cooler.

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Not likely

There are two big aspects to this.

The first is that an interstellar craft will out of necessity already have pretty significant measures to deflect radiation. Also, any viable means of producing significant thrust for interstellar travel would likely produce a significant amount of various types of radiation. There's basically no reason to believe a nuclear weapon could be distinguished from a normal spacecraft unless you basically handwave a reason why they would.

Secondly, as stated in a comment above, there likely wouldn't be much reason for them to use nuclear weapons if they have access to FTL technology. Gravity alone is enough to flatten cities by simply dropping significantly dense material from orbit. Likewise, if the fallout is the objective rather than the explosion, chemical or biological weapons would probably be a more feasible option.

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    $\begingroup$ Not necessarily valid objections, depending on the terrorists' budget. Obtaining a nuclear weapon, putting it in a shipping container with some office supplies and paying for it to be shipped to a densely populated area would be much, much less expensive than getting an entire spacecraft of their own and the expertise to use it. $\endgroup$ Feb 29 at 1:25
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    $\begingroup$ @KerrAvon2055 The point being made here is very much that under the conditions the questions OP has laid out space ships and the expertise to fly them are presumed to be both common and easy to come by for private citizens, so your objections to this answer would seem entirely invalid within the scenarios framework as outlined for the question. $\endgroup$
    – Pelinore
    Mar 2 at 17:29
  • $\begingroup$ @KerrAvon2055 I understand your point, but the question literally states "...if space travel is commercialized enough for the average civilian to acquire a ship for interplanetary and intersystem travel..." Therefore the assumption is that they already have access to all of that and the expense and expertise is not an issue. $\endgroup$ Mar 6 at 5:04
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If you care at all about bombs, rather than kiloton iron cones traveling at half the speed of light, and you have cheap interstellar travel, and people that want to kill you, you necessarily already have a way to hit incoming space objects very very hard. If you didn't, you'd already be dead. Use that on anything and everything that won't pull over into a deep orbit and let your customs officers onboard. As an added bonus, you can collect import duties and arrest ET-traffickers in addition to not going extinct. Worst case scenario, some terrorists blow up your customs boat.

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  • $\begingroup$ I believe that the OP is asking about nuclear smuggling, not point defence. $\endgroup$
    – Monty Wild
    Feb 29 at 1:12
  • $\begingroup$ @MontyWild So was I. At the energies involved, if you don't have good point defense, you don't have a problem with unchecked boats landing on your planet because you're dead. If you do have good point defense, you don't a problem with unchecked boats landing on your planet because they're dead. $\endgroup$
    – g s
    Feb 29 at 1:51
  • $\begingroup$ ...unless the question is really about how customs officials can look for suspicious big metal objects hidden in space ships, in which case I'd submit a different answer. $\endgroup$
    – g s
    Feb 29 at 2:00
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The issue of detecting nuclear smuggling is a troublesome one. While fissile materials such as Uranium and Plutonium emit radiation, it isn't very difficult to shield that radiation from detectors, and the people into whose country the nuclear materials - whether a bomb or the material to make a bomb - need to be looking for such materials, not just doing usual customs checks.

In the linked article above, it has been shown that it is quite possible to smuggle atomic materials past unalert customs officers. The next point is that we're talking about space ships. Space is an environment with high radiation levels already, so the radiation from a nuclear material or even an all-up bomb would be even more to detect against this radiation background. Space ships themselves may have legitimate nuclear materials that would mask the presence of atomic weapons.

It would probably take detailed x-ray scanning and neutron-beam activation to detect stimulated fissioning of fissionable materials on board a space ship or the luggage unloaded from it. Since these all require active irradiation of space ships or cargoes, it is unlikely that they would be employed casually unless the risks of smuggling were considered to be significant by the authorities.

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The very threat that they might do this would probably necessitate a sort of check-in or customs area when approaching the planet.

So all ships who want access to the planet have to pass through a giant ring device equipped with all sorts of detailed scanning equipment which could use things like spectroscopy or whatever to detect the materials used in nuclear weapons. This would be on the same lines as a metal detector at an airport.

While this doesn't exactly answer your question of how to detect nuclear weapons, it makes it a lot easier to detect threats than, say, the passive style "long range sensors" they have on Star Trek, which seem to rely on every different object giving off its own unmaskable "signature".

Anyone attempting to approach the planet without passing through is automatically regarded as hostile by the defence forces.

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The explosions during testing and developing nuclear bombs emit neutrino shockwaves.

Which can be detected, if a neutrino collides with a molecule and pumps out light in a large body of water. https://en.wikipedia.org/wiki/Neutrino_detector

If you put such detectors in a 3dimensional grid in space, and register those shockwaves moving at lightspeed, and then when the data is fused (with again lightspeed delay), you can pinpoint pretty accuratly were a nuclear test happened.

If we had several such "sensors" throughout the solar system, we might be able to even detect directional catastrophes, like major nuclear exchanges in other solar systems. Although neutrinos interact very rarely, the shockwave is distributed over ever larger areas and eventually blends into the background noise. So such a detector grid would only work on "local" systems.

Then again, might not waste a warhead on a species, that makes a lot of neutrino noise and then falls silent.

So the formula for detection is sensor-distance + signal to sensor hub distances for all nodes.

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  • $\begingroup$ A moon with a water ocean, far outside a system would be perfect for such a sensor $\endgroup$
    – Pica
    Mar 1 at 16:08

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