I was considering having the spacecraft in my setting use antimatter-matter spacecraft, but then I realised that at least some of the annihilation of protons and electrons with antiprotons and positrons will release gamma photons, as well as the charged pions being magnetically directed as exhaust. Putting aside the issue of antimatter exploding if storage failed, would this make these spacecraft dangerous to launch from ports based near settlements, or to be nearby when one is launching? Or would it be possible to shield the engine in such a way that it poses no risk to people nearby and doesn't cause environmental damage? Could you launch a craft like this from the centre of a city or would it be too energetic/radioactive?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Mar 7, 2021 at 15:22
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    $\begingroup$ @Starfish Prime There is no 'nature of the spaceship'. Nothing about this spaceship is defined. We have matter-antimatter annihilations on earth all the time - most thunderstorms produce them and then they are immediately annihilated. Certainly they are not an environmental hazard. It is the quantity of them that matters, and there is no indication of quantity. There can be no 'hard science' about this question. It is obvious any answer will be flagged for deletion because it does not meet the hard science criteria. Removal of the hard science tag would change this. $\endgroup$ Mar 7, 2021 at 15:22
  • $\begingroup$ @JustinThymetheSecond your argument is spurious and you seem to be ignoring my comments in favour of your own internal notion of "hard science" that doesn't seem to bear much resemblance to the standard as used elsewhere on this site. You are welcome to try and have the question closed, of course. $\endgroup$ Mar 7, 2021 at 15:56
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    $\begingroup$ I agree in principle with Justin, more details are needed to fit the hard-science tag. Else, downgrade the tag to science-based. $\endgroup$ Mar 7, 2021 at 16:26
  • $\begingroup$ @Starfish Prime If my interpretation of the hard science tag is spurious and not following the normal interpretation, then why is MolbOrg flagging answers for exactly the same reason I am presenting? I am not sure who is pushing their own internal notion over the standard. $\endgroup$ Mar 7, 2021 at 16:53

3 Answers 3


Certain events early in Larry Niven's Known Space series involve what's known as the "Kzinti Lesson", a lesson taught by a pacifist and demilitarized humanity to the invading Kzinti. The lesson is "a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive".

The Kzinti Lesson referred to giant laser propulsion stations and photon drives. A beam core antimatter rocket, pretty much the (theoretically) practical approach to building an antimatter rocket that operates as you describe, would be very nearly as efficient as these. One with enough thrust to be useful for launch from a planet would have a devastating effect on the surroundings. The atmosphere is actually pretty opaque to gamma radiation, so the radiation hazard would be limited, but in absorbing the radiation it would turn into something approximating a nuclear fireball.

You would want a much lower performance drive that heats and ejects propellant for such uses. Apart from the less-lethal exhaust, this also has the benefit of consuming far less antimatter, and not requiring the ship to handle near as much power. Possibly you could use antimatter to heat the propellant, but good luck absorbing the gamma and charged particles efficiently to do so.

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    $\begingroup$ @MolbOrg nothing about the tag forbids reference to relevant fiction. $\endgroup$ Mar 7, 2021 at 2:55
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    $\begingroup$ sure u may go with it, but the answer does not provide anything, not a bit of what the tag asks for. "Requires answers backed up by equations, empirical evidence, scientific papers, other citations, etc." - etc by the spirit does not mean fiction, etc. This is just bad q, u jumped in without noticing, I missed it too at the beginning. But could not rest thinking why one guy removed decent answer, better than yours, and then I noticed why. op needs a bit punishment, go and downvote him, lol $\endgroup$
    – MolbOrg
    Mar 7, 2021 at 3:01
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    $\begingroup$ science fiction doesn't represent a valid source for hard science questions, unless backed up by the same citation of real scientific paper, formulas and evidences $\endgroup$
    – L.Dutch
    Mar 7, 2021 at 6:09
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    $\begingroup$ If you were to edit this answer to provide evidence as required by the hard-science tag, then you could avoid its deletion. $\endgroup$ Mar 7, 2021 at 14:57
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    $\begingroup$ Larry Niven is a hard science fiction writer who always puts a lot of effort into making sure his stories are scientifically accurate. Especially in his Known Space stories. $\endgroup$
    – Philipp
    Mar 9, 2021 at 11:52

The answer is, of course, that it depends.

You can, very roughly, divide rocket engines into three types:

  • high thrust, low exhaust velocity, low running time, good for takeoff on a planet
  • low thrust, high exhaust velocity, high running time, good for efficient travel in space
  • high thrust, high exhaust velocity "torch drives".

Antimatter engines could be any one of the above.

An example of a modern day takeoff-type engine would be literally any rocket engine we've used over the past hundred years or more. A common type would be a liquid propellant rocket, such as those used by SpaceX.

An example of the second type would be an electric propulsion system such as an ion thruster or perhaps a VASIMR. The thrust is so low that it could not be used to lift a rocket off even from all but the most miniscule of moons, but the engine can keep running for years at a time making them ideal for deep space probes.

The third type has never been realised, but the closest anyone has come to it would be Project Orion, which would have used nuclear explosions to propel a spacecraft.

Now, note that I mentioned exhaust velocities above. The faster the exhaust velocity, the more efficient the rocket is at providing thrust over a long period of time.

Your suggested antimatter rocket is what might be known as a beam core rocket, or sometimes a pion drive. Its exhaust velocity is that of the annihilation products, which in turn depend on exactly how your engine works and who you ask. Early simulation suggest an exhaust velocity of ~.3c, more recent papers suggest .69c may be practical and others even suggest that >.9c might be achievable.

Lets go for the lowest estimate of .3c, though. This has been used by Forward and Frisbee amongst others, and comes about through the interaction of charged pions and the magnetic nozzle of your rocket. It gives us the most conservative estimate for how destructive your rocket is.

The force exerted by a reaction engine can be described by the following equation:

$$F = \dot{m}v_e$$

where $\dot{m}$ is the mass flowing out of the nozzle and $v_e$ is the exhaust velocity. Lets say you have a hundred tonne spacecraft... this is plucking a figure out of the air, but is a little over the maximum mass of an airbus A321 and a little under the mass of a loaded space shuttle. You did not supply a spacecraft weight, but I'm sure you can rerun the simple calculations below if you felt the need.

Anyway, in order to just lift off vertically from Earth's gravity with a 100 tonne craft, you need to generate at least a meganewton of thrust.

With an exhaust velocity of .3c, that gives you a mass flow of ~11g of pions a second.

This should already be ringing alarm bells!

Charged pions are unstable, and rapidly decay into muon/antimuon and sometimes electron/positon pairs. Those muons are also unstable, and decay into electron/positron pairs. Those positrons will in fairly short order meet a friendly electron, and then promptly annihilate releasing a pair of 511keV gamma rays.

That means that almost the entire mass-energy of your rocket exhaust will turn into gamma rays. Using good old $E = MC^2$ you can see that this will produce a gamma luminosity of the order of about 1 petawatt, which is approximately the equivalent of a 250kt nuclear weapon being detonated every second.

You can use the NIST mass-attenuation coefficients to work out how penetrating this radiation is, or use this handy calculator instead. You'll see the xray luminosity will be reduced by about 99.5% over a 500m radius. This will produce a bright plasma fireball reminiscent of the early phase of a nuclear explosion. Gamma rays will of course extend further from ground zero... after 1km 99.9975% of the radiation will have been absorbed into the air and converted into heat, but that still leaves 25GW of gamma rays. I won't work out the safe distance for an unshielded human in air here, but it will be quite a way out!

This is city-sterilising levels of bad.

And this is just the exhaust mass from your rocket system... remember those exhaust products have a lot of kinetic energy (making a bigger boom) and only a proportion of the mass fed into the reaction chamber will come out as charged particles so there's a lot more gamma radiation flying around as well. Frisbee's work on beam core rockets includes a breakdown of annihilation products... the key ones there are the 200MeV early gammas. The NIST attenuation data doesn't go up that high, but 100MeV gamma rays can reach out to ~2.65km of sea-level air before 99.5% of the luminosity has been absorbed, making them a substantially greater radiation hazard than the longer-wavelength exhaust gamma rays. These may, in fact, be country-sterilising levels of bad, but again: I'm not going to work out the precise details because the situation is unambiguously bad.

This is the price you pay for a high-thrust, high-exhaust velocity rocket. Torch drives are incredibly dangerous, even at modest thrust levels, and a rocket capable of taking off from a planet will not have a modest thrust!

And for those trying to wiggle out of the problems I've just outlined:

  • Don't think you can escape by doing a horizontal takeoff... an Airbus A321 has a comparable mass and a max thrust of ~150kN, and that thrust would still require over a gram and a half of pions per second from a beam-core rocket!
  • In a low gravity environment, the radiation level is still terrifyingly high. In a milligee gravitational field you're only spitting out 10mg/s of pions to just lift off, but that's still a mass-only luminosity of ~980GW of hard gamma rays. You really don't want anything near that, and certainly no people.
  • It probably isn't even a good idea to fire up this rocket at milligee levels even in low orbit above an Earthlike planet. The gamma ray luminosity could cause high altitude EMP and it might be bright enough to cause eyesight damage on the surface below (though I'm not 100% certain of that, I'm pretty certain it would be a bad thing to look at).
  • Even a tiny 1N pion rocket, enough to accelerate 100kg at a milligee, is a gigawatt gamma source. An unshielded human 10km away in a vacuum (say, someone wearing a normal spacesuit at one end of Phobos watching you leave from the other end) is exposed to ~0.8W/m2 of gamma rays from the exhaust. If they had a cross-sectional area of 0.6m2 and a mass of 60kg, that's a dose of 8 milligrays per second, equivalent to 8mSv (equivalent to a chest CT) a second. An unshielded human just 1km away in a vacuum receives enough gamma rays to develop radiation sickness in just one second, and a fatal dose in 10 seconds.

You can see that there's really no safe way to operate a pion rocket anywhere near anything that isn't very well protected indeed (and details of that protection belong in a different question/answer).

Your problems don't end there, of course... pion rockets have many other problems including a worse delta-V than you might expect due to reaction mass decay (see the adjusted relativistic rocket equation in Frisbee's paper) and the fact that reaction cross-sections for particle-antiparticle annihilation are actually kinda low (Relativistic rocket: Dream and reality).

Some of these are surmountable. For takeoff from an inhabited world with reasonable gravity like Earth or Mars, one might use a solid-core antimatter rocket. There were some details of this in a previous edit of this answer (still present in the edit history) but I've removed them for brevity. Feel free to ask a separate question on the best ways to safely depart Earthlike worlds with high-power engines, though!


I am pretty sure that if you were using matter-antimatter annihilation as an energy source, you most likely are NOT using a reaction mass drive. I would presume it would be some sort of space/time warping, or pinching along the lines of an Alcubierre drive. Or maybe some sort of gravity channeling drive. Something that requires very high energies, but not to propel a reaction mass, to power some other physical mechanism. It would be akin to trying to figure out how to use a fusion reactor energy to drive a ship.

How it would produce useable energy, as AlexP has suggested, is speculative at best. Would it be entirely in some containment vessel? Akin to how we generate nuclear power today? With the radiation very carefully contained and shielded? In deep space, of course, there are no worries about radiation poisoning of the environment, until well-traveled space lanes are established.

Using the power core in-system would definitely require shielding of some sort, but just like nuclear fission reactors today, there would be no need to discharge radiation onto the environment. The drives would have some sort of intermediary energy transfer system the way we use water/steam to transfer energy from nuclear power reactors.

I would expect in-system drives to use some form of stored energy, or would use this energy to perhaps power a plasma reaction drive, but that would be an entirely different environmental concern.

  • $\begingroup$ check hard science tag requirements, do not jump in it with fiction $\endgroup$
    – MolbOrg
    Mar 7, 2021 at 2:50
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    $\begingroup$ @MolbOrg As AlexP says, there is no hard science about something that there is no hard science on. The entire question is speculative at best, and as such is pure science fiction. $\endgroup$ Mar 7, 2021 at 14:15
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    $\begingroup$ "is speculative at best" - agree, it has some fundamentals, but it not enough for hs in this case. if hs answer can't be provided - then no answer has to be provided to the q under that tag. seems quite simple. $\endgroup$
    – MolbOrg
    Mar 7, 2021 at 14:33
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    $\begingroup$ @MolbOrg Rule of thumb for hard science - when the number of assumptions required exceed the number of facts available, it should not have a hard science tag? $\endgroup$ Mar 7, 2021 at 15:00
  • $\begingroup$ Maybe, I would say it is a good rule for science based , but something still laking for good hs, but it can be as a bare minimum. HS probably more on a side - we have all the data and we calculate derieve a conlusion if something works or not, and we refer our data. On the same set of data we can ask different questions, as an extreame - all what is known to humans and any question about reality - why sky is blue as an example. Answers to hs q's needs to be science - yes on a simple level, crude, but as best we can do here on WB. $\endgroup$
    – MolbOrg
    Mar 8, 2021 at 8:52

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