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The United Nations Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies explicitly states that

States shall not place nuclear weapons or other weapons of mass destruction in orbit or on celestial bodies or station them in outer space in any other manner

Citation: UNOOSA.

I am more an expert on the technology and reality of space than the legality of it, but suppose an enterprising space agency invented a nuclear-fusion engine that has an extremely high specific impulse achieved by ejecting fusion products at significant fractions of $c$.

All well and good, reaching Mars in days or weeks, reaching Jupiter in due time as well, but of course the Kzinti Lesson comes up: "A reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive”. I can’t imagine a torch drive that isn’t functionally just a slow-burning nuclear fusion reactor, and of course if it can be used as a reactor it can be modified to be used as a weapon, and being a fusion weapon it is categorized as a weapon of mass destruction explicitly “shall not”’d by the UN.

Would there be any precedent as to the torch drive’s legality if it were invented?

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  • $\begingroup$ I mean the best answer I could give myself was just "we haven't enough information" because who knows if such an engine could ever be done. I figured I might as well try and find as much info on the topic as possible since I'm interested. Certainly isn't my best question but it's something I'm curious about either way $\endgroup$
    – controlgroup
    Commented Oct 30 at 18:35
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    $\begingroup$ I don't think I agree with the close votes currently put forth, particularly the "opinion-based" ones. I think there is a fact-based answer about if the treaty obligations apply to the torchship as described. I just don't know if I trust us to come up with that kind of answer versus, say, Law SE interpreting international law. $\endgroup$
    – Erin Anne
    Commented Oct 31 at 2:40
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    $\begingroup$ @nobody As someone who also has the close-vote privilege, I can confirm that they did in fact VTC, and I also don't think it's reasonable to decide which questions are 4AM dorm room stoner questions because of course that's subjective. $\endgroup$
    – controlgroup
    Commented Nov 1 at 16:59
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    $\begingroup$ Given that this is, strictly, a legal question, and, presumably, you're actually looking for a legal-based answer, it would have been more appropriate to ask this on Law, or, potentially, Politics, as it's a question about treaties. It's not off-topic here, but the focus here on Space Exploration isn't specifically the law or treaties. In addition, any "is it legal" question requires the country, and sometimes the subsection of the country, in which that legality will be evaluated (yes, even for legal in space questions), because "legality" is something that's different by jurisdiction. $\endgroup$
    – Makyen
    Commented Nov 2 at 13:28
  • $\begingroup$ It's worth noting that a "torchship" usually implies not just high specific impulse, but extremely high thrust power. Very low thrust, high impulse thrusters are not so remarkable. $\endgroup$
    – ikrase
    Commented Nov 3 at 13:58

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Would there be any precedent as to the torch drive’s legality if it were invented?

Any orbital rocket would be quite potent weapon in wrong hands. Even low-tech terrorists could explode it. In fact, many definitions for WMD include rockets.

So the line drawn by the treaty is not so clear. It is reasonable to expect that a fusion reactor designed to be safe wouldn't be considered a weapon. It is also conceivable that there could be inherently safe fusion reactor designs. The current research reactors would be much more difficult to turn into a bomb than fission reactors, because if the plasma containment is disrupted the reaction quickly stops.

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    $\begingroup$ NASA and DARPA are currently developing a flight demonstrator nuclear fission rocket engine intended to fly in 2027 (see link). One of the key benefits of the project will be establishing regulatory precedent on launching, flying, and deorbiting something with the potential for criticality. I figure the efforts will be relevant to this discussion once they happen in a few years. ntrs.nasa.gov/api/citations/20230018491/downloads/… $\endgroup$
    – A McKelvy
    Commented Oct 31 at 13:10
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    $\begingroup$ Note that it was the prohibitions on above-ground nuclear testing and on nukes in space that put an end to the original research on nuclear propulsion. But fusion without needing a fission trigger does have a lot more potential for using safely. $\endgroup$
    – Perkins
    Commented Oct 31 at 18:13
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    $\begingroup$ note that fission reactors can NOT be turned into nuclear weapons. At worst, a large enough conventional explosion inside a fission reactor's containment structure would cause the spread of radioactive contamination akin to a dirty bomb (which is actually what happened at Chernobyl, a very large conventional hydrogen explosion caused the lid to blow off of the reactor vessel and pulverised material from the reactor to disperse on the wind). $\endgroup$
    – jwenting
    Commented Nov 1 at 12:09
  • $\begingroup$ @jwenting Yeah, not into nuclear explosion. But dirty bombs definitely count as WMD. $\endgroup$
    – jpa
    Commented Nov 1 at 13:01
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    $\begingroup$ @jpa possibly, I don't know the details of the classifications. Of course in space a dirty bomb is pretty much useless as those depend on contaminating the environment, making it harder or impossible for people (civilians or enemy combatants) to operate in it and during a space battle you'd just create a small cloud of radioactive particles for enemy ships to fly through, the radiation being completely blocked by the same shielding that stops all other radiation hitting that ship. $\endgroup$
    – jwenting
    Commented Nov 1 at 13:37
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Your main flaw here is thinking that a nuclear reactor (fusion of fission) is a nuclear weapon. It's not.

The energy beam coming from the torch drive could be used as a weapon (indeed, as stated by Niven), but that's no different from any rocket engine being a weapon (it just works at longer distances). Heck, the rocket itself is a potential kinetic energy projectile. That doesn't mean that space probes are considered weapons under the outer space treaty, if they were all satellites currently in orbit would be in violation of that treaty!

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In a real sense, the OST is more of a 'declaration of intent' rather than an actual law, because for something to be a law, there needs to be an enforcement component, and the UN struggling with enforcing anything is a perennial problem.

In a less charitable sense, the OST is nearly worthless and only hasn't been "broken" because no country has arrived at the point where making a choice that violates it is a good decision from a strategic or tactical perspective irrespective of the OST.

For example, if the USA military command came to the decision that it is of vital strategic importance to put a military base on the Moon and fill it with space marines, they wouldn't just say "aw shucks, the OST says that's illegal, so we can't do that!"

When they go ahead and do it anyways, what would happen? Maybe the UN would send them a strongly worded letter and other nations might grumble about it, but they'd presumably also be in the planning stages for their lunar military outpost because the broader situation applies--the same factors that influenced the US military command to make this choice are likely also present for eg. Chinese military command.

For your torchship, this likely means that so long as the intent of the system does not violate the spirit of the OST (essentially peaceful use of space for all mankind) then you'll likely have no more than the usual level of background oppositionalism.

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  • $\begingroup$ While legally this is true, politically it is not. There's always value in not being the first one (seen) breaking a treaty, and in appearing trustworthy and upholding your treaty requirements. The first country to station troops on the Moon would probably get away with it, but they'd have a heck of a time convincing anyone to negotiate future space treaties with them. $\endgroup$
    – Cadence
    Commented Oct 31 at 10:27
  • $\begingroup$ Even politically it does not make sense. It depends on which country breaks the treaty and which countries are its friends. $\endgroup$
    – Johannes Linkels
    Commented Oct 31 at 21:49
  • $\begingroup$ It depends upon whether the party considering breaking the treaty regards the UN as a useful entity to keep empowered. Its basically just a debating society that can be replaced at a whim, so eventually it will fall by the wayside. $\endgroup$
    – SafeFastExpressive
    Commented Nov 2 at 20:10
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You seem to misunderstand how a thermonuclear weapon works (I promise this will be relevant, just bear with me). It’s not just a fusion weapon with nothing else involved, it uses a fission-based primary stage to trigger a much bigger fission-based detonator which in turn generates the conditions required for fusion to happen. The secondary stage is only dangerous in that it’s really radioactive because of that fission-based detonator assembly, it’s not a case like a ‘conventional’ fission weapon where things are just teetering on the edge of criticality. Fusion quite simply does not ‘just happen’ like fission does, at least not in conditions where anybody would be dealing with it.

The upshot of this is that a fusion reactor is not a viable nuclear weapon on it’s own. Period. End of story. It still needs to generate the conditions required for fusion to happen, but that’s done by the containment and regulation system itself. Containment loss mostly just means you lose fuel and the reaction stops, but that’s an economic issue and not a safety issue, because the fuel for a fusion reaction is not really dangerous in and of itself (well, aside from possibly burning well). Regulation loss means the reaction stops. You don’t end up with situations like Chernobyl, Fukushima, Windscale, Three Mile Island, or SL-1, and you definitely don’t get a nuclear explosion (you may get a conventional non-nuclear explosion, but it is unlikely to be anywhere near high enough energy to be considered a WMD).

This does not mean that such an engine couldn’t be part of a WMD. A simple kinetic impactor accelerated by such an engine would almost certainly qualify, and it could also be used as propulsion for some other ‘traditional’ WMD

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  • $\begingroup$ I'd certainly see situations like Windscale and SL-1, where the major impacts were (a) release of radioactive materials to the atmosphere and (b) deaths of people on the premises and a large amount of irradiated debris. $\endgroup$
    – prosfilaes
    Commented Oct 31 at 22:09
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    $\begingroup$ @prosfilaes In the case of SL-1 specifically, the issue was a prompt criticality incident, and that quite simply cannot happen with fusion unless you’re dealing with a reactor the size of a star. The end effect definitely is possible (though I would be surprised to see something launch the reaction vessel in a similar manner in a fusion reactor), but the root cause is what prompted my use as an example. As far as Windscale, that was indeed probably not the best example, but OTOH I would not expect atmospheric release like that to be anywhere near as dangerous for a fusion reactor. $\endgroup$
    – Austin Hemmelgarn
    Commented Oct 31 at 23:22

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