It's 20 years into the future, and humanity is (trying to) reaching for the stars. And congratulation, we have met a lot of quirky alien civilization, as per SF tradition. Unfortunately, this isn't Star Trek, so conflicts do happen. Luckily, this isn't 40k either, so none of the star-faring nations are going for total war here. So long as human ships can prove themselves capable of fighting in a skirmish, the media (human and alien alike) is going to blow it up to the point everyone thinks we can handle a space war, and ironically, no full-scale war is going to happen.

The problem, of course, is that the majority of alien crafts are protected using energy shields, which render many types of weapon inefficient, however. As far as that future scientific understanding goes, the best way to beat them is with a large dose of neutron radiation to penetrate and destroy the shield mechanism underneath, which weapons would be the best to do so?

We are assuming that with futuristic technologies, anything modern piece of technology on Earth can be mass-produced reliably, at record-level performance, in space. The neutron source is standardized, the same amount of reaction material create the same impulse of radiation, regardless of weapon. Costs, weight, and other factors will be considered, but only as secondary, after 2 main criteria:

  1. Damage: The amount of radiation delivered to the target
  2. Combat usability: How good that bundle of radiation is at getting to a moving (though still limited to dogfight combat speed, since the enemy also want to shoot, too, and their targeting system is not noticeably better than ours) target in space

The 2 ideas I have in mind are missiles and beam weapons. Of the 2, missiles with neutron bomb warheads seem like it should do more damage (it is a modern weapon, after all), but also seem a little bit too easy to counter (speed, point-defense weapon, etc.). Whereas it is impossible to dodge a neutron beam, but people keep them in labs, and not battlefields for a reason, right? Is there any other way to deliver radiation accurately to a distant spot in a vacuum?

  • $\begingroup$ Can you make bullets that deliver neutron radiation on impact at relativistic speeds? $\endgroup$ Oct 9, 2019 at 22:17
  • $\begingroup$ Or, like, really small (~1cm) neutron bombs. $\endgroup$ Oct 9, 2019 at 22:19
  • $\begingroup$ This is a framing issue, as far as I'm concerned, but why neutron radiation? Ionizing radiation is a major concern in space, so shielding from neutron radiation and gamma radiation would be desirable in a normal spaceship, let alone a warship. (It just seems like a weird type of radiation for their ships to be vulnerable to.) Ultimately, it's your world, but it doesn't really make sense. $\endgroup$
    – jdunlop
    Oct 9, 2019 at 22:58
  • $\begingroup$ @jdunlop To be fair, the question is more about the best way to chuck radiation in space. I chose neutron mostly because it is available in both bombs and beam form. If it make any difference, just say that the shield is EM based which affect any charged particles heavily, but neutrons have no charges, so they can penetrate better. $\endgroup$ Oct 10, 2019 at 0:31
  • $\begingroup$ @NamNguyenHoang the difference is important; neutrons are awkward things to use in weapons, whereas lasers and charged particle beams are quite straightfoward. If you allow them, it'll be a very different question and one much easier to answer. $\endgroup$ Oct 10, 2019 at 18:26

3 Answers 3


I'd discard the idea of neutron beams, unfortunately. Whilst a working neutron beam would be an extremely effective weapon regardless of whether the target is shielded or not, making such a thing seems somewhat implausible outwidth a magical way to accelerate neutrons.

Firstly, generating such a beam is problematic. Once you've generated it, focussing it is problematic... modern spallation sources, for example, aren't particularly collimated, eg. they fire the damn things out in all sorts of directions, and fancy grazing-incidence mirrors can only really work on neutrons that are already heading in roughly the right direction so you'll lose a lot of neutrons and generate a lot of waste heat. Even once you've focussed it, you'll still have limited range due to thermal blooming (basically, neutrons in the beam bumping into each other causing the beam to spread out). No handwavium neutron accelerator, no neutron beam.

Now, lets talk about neutron bombs. An excellent way to generate an awful lot of neutrons, in a military context, is to use a fusion explosion. D-T fusion, one of the easiest reactions to spark off, generates over 15 times more neutrons for a given mass of fuel than U-235, and the neutrons it generates are much higher energy (about 15 times more again).

Problem: fusion reactions are difficult to kick off. Modern nuclear weapons do this with the aid of a fission primary, but the size of the fission primary is limited by the critical mass of fissile material which can often be many kilos. You also need a very carefully controlled detonation of an implosion-type fission device, which just increases the hassle.

How do you make a fission explosion with a tiny subcritical mass? Well, one way to do this is to use a very small amount of antimatter. Have a look at the ICAN-II antimatter-catalysed microfission rocket design. A tiny pulse of antiprotons (less than a femtogram) is fired into a 3 gram uranium pellet, causing it to fission and release a healthy number of gigajoules of energy. This can be used as the primary in a more conventional (but miniaturised) 2-stage Teller-Ulam thermonuclear weapon, with a radiation case chosen to be as neutron-transparent as possible. Now you have the basis for a tiny neutron source, maybe only a kilo. You don't need access to massive amounts of exotic materials, like plutonium or antimatter... a few kilos of uranium and a nanogram of antimatter will supply thousands of micronukes. You just need to supply the tritium to get the party going.

For your next trick, you will need a high-velocity projectile weapon, such as a railgun, coilgun or powerful rocket engine (though the previous two are likely to be more economical here). The projectiles won't travel at a significant speed of light, unlike a laser or neutron beam. Make up for this by firing Quite A Lot of them. Most of these projectiles will be dumb metal slugs, but some, say, one in every hundred, are your micronukes. Throw a lot of rounds down range. Ships have a limited ability to dodge, especially when the incoming fire is a big cloud of hypervelocity crud. They have a limited ability to intercept these weapons with point-defense fire... they're small, hard to see via any spectrum and coming in fast. And they have to shoot all of them, because the one they let though might be the micronuke.

After a few skirmishes with your new toys, maybe you'll be able to collect the weapons that the aliens use to oppose shields, and use something sensible instead...

  • $\begingroup$ Can magnetic weapons (railgun, coilgun...) even shoot something with its core insulated from outside EM source, like being put inside a Faraday cage for example? Remember, impact alone won't cut it, you have to deliver a nuclear bomb in one of those bullets. Don't want any premature detonation there $\endgroup$ Oct 10, 2019 at 21:46
  • 1
    $\begingroup$ @NamNguyenHoang of course they can. Most things aren't magnetic or conductive, as I'm sure you're aware. If you wanted to just fire non-magnetic, non-conductive projectiles then literally anything would suffice. You asked for neutrons, though, so that's what you've got. Also, I'm not sure you've thought about quite how much energy a hypervelocity projectile is capable of delivering, especially when fired in quantity. $\endgroup$ Oct 11, 2019 at 8:44
  • $\begingroup$ As this has been over a week from the last answer, and this is the most detailed answer so far, I will accept this $\endgroup$ Oct 21, 2019 at 11:23

A fission or fusion nuclear reactor or some form of high powered cyclotron or synchrotron aimed at a solid (spallation) target would be capable of producing a large neutron flux of the order of 10^15 neutrons / square cm for the reactors and in the case of the spallation target method a neutron flux in excess of 10^17 n/square cm is possible.

It is also possible although difficult to focus a neutron beam. So a large synchrotron projecting into a spallation target with a specialist Wolter optics for focusing neutron should do the trick. However is would be very heavy, very large, very energy intensive to run and very expensive as well.

  • $\begingroup$ Good thing the tech level is "anything modern piece of technology on Earth can be mass-produced reliably, at record-level performance, in space" then $\endgroup$ Oct 10, 2019 at 8:02
  • $\begingroup$ You can't focus a neutron beam better than a laser beam, and even those deteriorate rapidly with distance. $\endgroup$
    – Alice
    Oct 10, 2019 at 10:13
  • $\begingroup$ @Nam Nguyen Hoang And even better that these are secondary considerations. At the end of the day a compromise would need to be made between power and useability as with many weapons. A cyclotron could easily be fitted to a space craft with a focusing lens of some sort. The question is how powerful does it need to be and at what range? These issues were not addressed by the OP. $\endgroup$
    – Slarty
    Oct 10, 2019 at 13:25
  • $\begingroup$ @Slarty The fight is mostly important for a show to prove humanity have what it take to be in "the game", afterward the weapon is going to get replaced by whatever alien weapons we can scavenge from the enemy ships/trade for/get "generously donated" by the enemy of our enemy (i.e. plot). So the only considerations are going to be delivering as much neutron, to as small a surface, as accurately as possible. All others are secondary at best. $\endgroup$ Oct 10, 2019 at 21:36

It's time for my favourite space weapon.


A sandcaster missile is about as simple as it gets. Fill a regular missile with sand, put a conventional explosive in the middle so the sand disperses nicely, shoot it towards them at relativistic speeds, and blow it up outside of their point-defense range.

It doesn't even have to be sand. It can be lead ball bearings, for extra kinetic energy, or even...

Tiny neutron bombs...

Design a small neutron bomb that explodes on impact. Pack them into your sandcasters. follow above procedure.

You can't dodge them all. Or see it coming, since the light informing you of the missile's presence doesn't appear long before the payload. Send regular sandcasters after your neutron ones if you want to actually blow anything up.

In regards to "dogfight speeds", shoot the missiles from very far away.

  • 1
    $\begingroup$ The question asked for a way to deliver radiation, not kinetic damage. $\endgroup$
    – Muuski
    Oct 9, 2019 at 22:50
  • 2
    $\begingroup$ You can't actually make "tiny neutron bombs". A neutron bomb is a low-yield nuclear weapon, and nuclear weapons have a minimum mass for criticality. $\endgroup$
    – jdunlop
    Oct 9, 2019 at 22:51
  • $\begingroup$ The trouble is that the question says that it uses tech from 20 years in the future. I don't think that we'll be able to get missiles up to relativistic speeds by then. Also, it would be easy to dodge. The sand may not be visible but the rocket's thrust will be. You will know which direction it was going and how fast at the point the thrust stops. You could dump payload early and keep thrusting but that trades speed for deception. $\endgroup$
    – ShadoCat
    Oct 9, 2019 at 22:55
  • $\begingroup$ @ShadoCat Your assumptions are correct, no relativistic rocket just yet, but remember, the enemies are not going relativistic either, still trying to to get their own shot in. Still, the predicting missile path is correct. $\endgroup$ Oct 10, 2019 at 0:36
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    $\begingroup$ Probably best not to use the words "simple" alongside "relativistic" when describing a rocket or projectile weapon, except in combination with phrases like "not at all". $\endgroup$ Oct 10, 2019 at 14:45

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