# Antimatter weaponry issues

In my fictional universe antimatter projectiles are regularly used in space warfare. I'm not asking, whether it is practical or not, but have three specific questions:

1. Impulse shaped charge: The idea is, that a warhead containing antimatter in it (like a photon torpedo) would approach the target at high velocity, and detonate. The conservation of impulse dictates, that the photons resulting from the annihilation, and traveling forward, have to have higher frequency, than those traveling backwards, thus the majority of the energy hits the target, instead of being dispersed in the space. By detonation at 0.1c how many percent of the energy would go into the forward hemisphere, in a case of pure electron-positron annihilation, and how many in the case of antihydrogen annihilation?

2. Naked antimatter projectile. The second question concerns a macroscopic sized projectile, constructed from solid antimatter, as suggested in the comments. How long could an 10 kg mass of antiwater ice/anti-iron/anti-depleted uranium travel in the interplanetary medium at the orbit of the Earth (which is not full vacuum) before losing half of it's mass, assuming 10000m/s velocity? And if I fire it in te interstellar medium? Or in the upper atmosphere of the Earth on 300 km altitude?

3. It seems to me, that it is quite hard to make up physically plausible forcefield shields. But I can imagine using magnetic field for a specific defensive purpose: Could a strong magnetic field be utilized to break the antimatter containment on enemy missiles, and thus made them explode far away? Or could they be easily coated?

4. If naked antimatter projectiles described in question 2. would be used regularly, would 'gas shields' mean a viable countermeasure? The idea is, that if the computer senses an incoming antimatter projectile, it emits gas from dedicated vents (or the maneuver thrusters in emergency cases) to annihilate and evaporate it before it hits.

• Dumb question: why detonate it en-route? Why not just let it It your made-out-of-normal-matter target and let it annihilate some of their matter, while also ensuring the sphere of resulting energy is absorbed almost completely by their craft? – Dan Bron Jan 26 '17 at 11:54
• @DanBron: Detonation's a good idea. A shaped charge behind the magnetic bottle containing the antimatter gets triggered by the compression of the nose of the warhead, pushing the bottle into contact with the broken tip, which breaks containment causing an annihilation reaction. – nzaman Jan 26 '17 at 11:59
• @nzaman That explains how a shaped charge works, but not why it's better than just using the antimatter "ballistically", which is what I was asking. Why is pretty-contact detonation superior to to contact detonation for the sake disabling enemy craft? – Dan Bron Jan 26 '17 at 12:02
• @DanBron In the case of bombing cities the bombs are detonated above ground so the damage can spread further (rather than creating a lot of damage in one place). So I suppose, in the same way, an explosion on the surface of a ship would cause a lot of damage to that point in the hull but if that area can be sealed off it isn't such a problem, causing widespread damage would breach more areas. – Lio Elbammalf Jan 26 '17 at 12:14
• @nzaman The ballistic approach literally only explodes when in contact. In fact, it actually uses the enemy craft's matter to produce half of the resulting energy damage. You cannot get any closer than that. Anything other than that, is, in fact, further away. The analogy to TNT on a rockface is inapt. – Dan Bron Jan 26 '17 at 12:15

There is a distressing lack of math among these answers, especially concerning question number 2.

The solar wind is about $400 km/s$, so a speed of $10 km/s$ is negligible in this calculation. The solar wind/interplanetary medium has a density of about 5 ions per $cm^3$. Suppose our projectile is roughly spherical, with a cross section of $10cm^2$, making it about the same diameter as an artillery shell. The we use Avagadro's number and hydrogen's atomic mass () to convert from ions to grams. Now we have everything we need:

$\dfrac{400\frac{km}s\cdot100000\frac{cm}{km}\cdot5\frac{ions}{cm^3}\cdot10cm^2}{6\times10^{23}\frac{AU}{g}\cdot1\frac{ion}{AU}}\approx3\times10^{-16}\frac{g}{s}$

If it were only mass we were worried about, that'd be just about nothing. But that mass turns into energy, too; converting to $kg$ and multiplying by $c^2$, that's $0.3W$, a surprisingly everyday quantity. Assuming the projectile is made of water (weighing about a kg), and half the energy is turned into heat, that would heat the sphere by...$0.00004\frac{^\circ C}s$. So, tl;dr, your projectile is relatively safe in the interplanetary medium! Gunners would probably need to take into account the kinetic energy created, which would essentially turn the projectile into a weak rocket engine pointing away from the sun, but presumably future computers can compensate for that pretty easily. It's also still iffy enough that its effectiveness at long ranges may be reduced during bad space weather, which can increase the density of the solar wind by up to twenty times.

Obviously, this means that the projectiles would also be fine in interstellar space. However, the same cannot be said of upper earth atmosphere, which even at 300km is about 2 billion times as dense as the interplanetary medium. The atmosphere doesn't move as fast as the solar wind, so the temperature would still only go up at about, er, $2000\frac{^\circ C}s$. The rocket effect would also be much more powerful, and pointed backwards instead of toward the sun. So such a projectile launched that close to the earth would instantly vaporize, probably sending a shotgun blast of antimatter right back into the ship that fired it.

Answering points 2 and 3 only:

Naked antimatter projectile.

Your antimatter will hit a hydrogen molecule, get partly annihilated and partly evaporate; hit another hydrogen molecule, get partly annihilated and partly evaporate.... until it's completely destroyed or until it hits its target. The total antimatter hitting the enemy vessel will be a small fraction of what you actually fired, like a meteor passing through the atmosphere. Just like a meteor, the amount of antimatter left and the damage it can cause depends on the initial mass and composition of the antimatter. Given the costs of producing antimatter and the difference in "bang for buck", it makes more sense to put in in a casing for delivery.

Magnetic field defence.

This is very easily countered by making the shell casing of magnetically permeable material. No part of the external field will get through. Of course, you'd then have to ensure that the containment field inside doesn't try to go through the casing instead of around the antimatter. Just make the shell bigger and your magnetic bottle smaller.

1) I'm making a few inferences here, but the detonation would be largely spherical no matter how fast it was going, since the resulting energetic reaction would propagate near to the speed of light and whatever you contained it in wouldn't put up significant resistance to the blast.

2) 10kg of antimatter? Wow. A gram would explode in the tens of kilotons range. 10kg would explode in the hundreds of megatons range.

3) The device could be shielded. The best way to take out the device would be to penetrate its containment with some sort of normal matter to force a detonation -- there would be the drawback of scattering unreacted antimatter all over the vicinity.

4) Probably not -- a gas would disperse extremely quickly in space, and it would be unlikely that you'd put enough mass in between you and the target to evaporate it.

• About question 1 : the shaped charge effect I am asking is not based on the containment, but from the velocity of the annihilated anti- and normal matter itself. If, for example one positronium atom is annihilated while traveling at 0.1c, it has an impulse of two electron masses * 30,000,000 m/s. This impulse have to be conserved during the annihilation. each photon carries E/c impulse, so one radiated paralel to the original velocity has to have more energy than the other. – b.Lorenz Jan 26 '17 at 20:29
• About question 4: Is it not possible to put out the gas immediately before impact? Or use magnetically directed plasma as 'flamethower' on the antimatter slug? – b.Lorenz Jan 26 '17 at 20:32
• @b.Lorenz I'm inferring, since I can't find any hard data on this, that the release of energy would be so much faster than 0.1c that the initial relative velocities wouldn't matter very much. Further, the effect of the explosion of a 10kg antimatter bomb would be so enormous that 'shaping' wouldn't be a major concern, since it would vaporize everything within 10-20km. – Epicedion Jan 26 '17 at 20:46
• @b.Lorenz To the other question, there's no safe way of reacting a large quantity of antimatter in a short amount of time. Each gram of antimatter you stripped away would release as much energy as several Hiroshima bombs. – Epicedion Jan 26 '17 at 20:49
• I didn't say that the first scenario uses 10 kg of antimatter. (It is the naked slug idea in the second question which plays with such large amounts) Probably a warhead will just contains some grams. And for the 4. question. Altough the annihilation of the antimatter slug is quite energetic, since there is no shockwave, the nukes are not very effective in space. Polished surfaces can reflect quite a lot radiated energy before getting vaporized. So it will matter whether the annihilation happens 1 or 100 meters away, since it will reduce the surface energy density by 10000 times. – b.Lorenz Jan 26 '17 at 21:10

Let me focus on question #2.

Interplanetary space near Earth is not only not vacuum - it's constantly affected by solar wind. My prediction is that antimatter's macro-mass in near-Earth space would overheat and disintegrate within few minutes. At 10 km/s it might be able to travel for a few 1000s km. And should any sizable dust particle cross its path, game's over.

300 km above Earth is relatively dense, so I would give this mass no more than a few seconds. It can't even hit the Earth as a solid body, although 10 kg detonated at that altitude can inflict serious damage on the ground.

Interstellar medium is another matter. It's density is very low, and more importantly, stellar wind is very thin out there. My prediction is that antimatter can travel there without overheating for many days and possibly years.

So, there are two "types" of antimater: plasma or charged subatomic particles (including positrons) and antielements (and antiatomic? compounds). The charged stuff would have to be contained either with magnetic (or electric) fields or with light. You can look up the physics of the colliders used around the world, make a couple of assumptions about future technological improvement in the magnetic fields and figure out how much is likely to be contained that way. It's not much. I forget how much mass the LHC has orbiting around, its on the order of a gram or so (iirc). So, you're not likely to get much into a missile that way. On the other hand, take some anti-iron. It's easy enough to compute E=mc² the energy released when one iron atom annihilates with 30 (say) hydrogen atoms (the largest component of the solar wind). It's easy enough to use the heat capacity of iron to calculate how many collisions would be enough to heat the (anti)iron to its boiling point (which will be the same as that of regular iron and it's easy enough to look up what the density of atoms is in space (at any given distance from the Sun (assuming sufficient distance from other atmospheres). So, given the density of iron, you can assign a shape of a bullet, calculate its mass, and then assuming 0.1c, calculate the "tube" it is making thru space and then calculate how far it would have to travel to create enough energy to vaporize itself. Pretty simple math, once you've done the legwork. As far as using a gas, uh no. If you've got a projectile coming at you at 0.1c, then you're not going to be able to deploy a meaningful amount of gas fast enough and get it far enough from you to make a difference. A magnetic field of stupendous (and currently technically impossible) strength could theoretically be used to deflect sub-atomic/microscopic 'bullets'.

• Clever. But the gas shield is not meant against the 0.1c type (1. question), but against the slower ones (2. question) – b.Lorenz Jan 27 '17 at 15:56