[WARNING] Approximate physics incoming

A few months ago I was looking for a way to get to Gliese 581 within 100 years. So I've been reading a lot from Wikipedia and other sites about different propulsion devices. Then I found an wonderful article from NASA about the antimatter propulsion and more precisely the positron reactor.

So, my question is, assuming I could produce and store enough positrons, could I use a reactor like this to travel to a stellar system to another? I wrote down it would make my spaceship go to a speed of "very" approximately 63,000,000 km/s using more or less 100 kg of antimatter, is it true?

enter image description here

  • 2
    $\begingroup$ Is it this the NASA article?: nasa.gov/exploration/home/antimatter_spaceship.html If so you should add it into your question. $\endgroup$
    – kingledion
    Dec 22, 2016 at 20:20
  • $\begingroup$ Yeah sorry, I should have done it, I was afraid to put every single link, I will do it next time if I make a reference. Thanks to have pointed it out. $\endgroup$
    – user31296
    Dec 22, 2016 at 22:50
  • $\begingroup$ Said as if 100kg of Antimatter isn't much at all :P. I'd flatly refuse to enter a shuttle with 100 grams of the stuff on board, let alone a prototype fuel-tank full of it; so please, excuse me for sitting out on this expedition... $\endgroup$ Dec 22, 2016 at 22:54
  • $\begingroup$ When I wrote it I wanted to make a joke but I'm kinda new so I try to be serious. ^^ $\endgroup$
    – user31296
    Dec 22, 2016 at 23:08
  • $\begingroup$ 0.2c isn't so much, you may try to ask another question how you can accelerate your DeathStar to 0.2c and stop it at the taget - without FTL and as close to the scientific truth as possible. There are interesting options, and Thermonuclear engines one of them in combination with mass driver launch and solar sails and such. Payload will be 13.5% for the speed, maybe not so exciting as using antimatter but it isn't that bad, better then today payloads. $\endgroup$
    – MolbOrg
    Dec 23, 2016 at 4:11

3 Answers 3


Expanding on kingledion's answer, the interaction between positrons and electrons produces gamma radiation at a very specific energy as well: 512 kEV. Gamma radiation at that sort of energy will penetrate materials fairly easily, and most reaction materials like hydrogen or water will be virtually transparent to the inferno of gamma radiation, meaning most of the energy will simply pass through the reactor and end up irradiating the structural members of the vessel. The stuff you want to heat up isn't going to become as hot as you want (a superheated plasma for maximum ISP), while many of the things you would prefer to remain cool are at blazing temperatures.

You will actually have more success using anti protons, either interacting with normal matter protons in a beamed core engine, or beamed into the reaction mass flow to heat a larger quantity of water or hydrogen.

Antiprotons are made up of antiquarks, and the reaction of antiprotons creates a stew of charged particles. Most of these particles can be directed with magnetic or electrostatic fields so the energy they have is directed away from your ship, and hopefully out with the blazing plasma of your exhaust stream. Far less energy is going to be deposited on your ship's structural members, and the overall efficiency is going to be much greater (i.e you 100kg of antiprotons will provide far more usable energy and thrust than the same 100kg of positrons).

enter image description here

The difference between positron and anti protons

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Simplified diagram of an antiproton rocket

So while an positron rocket seems to be going about things the hard way, using antiprotons to energize the rocket, either in a beam core engine or as a means of heating reactant seems to be the better way to go.


There are more technical hurdles than you are mentioning

I would group the technical hurdles into two general groups:

  • How to point the energy in the right direction? Electron-positron annihilation produces gamma photons. These go off in directions based on the orientation of the electron and positron as they approached interaction. In order to generate thrust, you either need the gamma to go out the back end of your vessel to impart momentum, or you need to super-heat a plasma with said gammas, and that plasma needs to go out the back of your engine.

  • How to protect the engine materials from irradiation? If you are letting gammas go blasting around willy-nilly, they are going to run into teh structural components of your engine. Gammas have a lot of energy and can do a lot of damage to the materials over time. Given a ~100 year trip through space, your engine could be damaged beyond repair before you get where you are going.

Then the third hurdle is covered in your assumption, how to produce and store sufficient positrons; which is actually the toughest challenge of all. The linked article is not very good on details, and doesn't really address any of the three technical hurdles. The article mentions ablating something to provide the thrust, but doesn't really say what or how it would be directed.

I would say, based on the information in that article, that this design is not feasible until more engineering is done.

  • $\begingroup$ I was expecting that kind of answer, I'm trying to be as close to the scientific truth as possible so thank you, I will make more research. $\endgroup$
    – user31296
    Dec 22, 2016 at 23:12

For the production and storage of positrons, the SLAC Linear Collider at CERN used a crystal tungsten target and a low energy, 5 GeV electron beam as its positron source.

And this paper describes using a tungsten heat shield for collecting the gamma radiation and using compton scattering to transfer the momentum from the reaction to the craft. It implies that micro-propulsion is possible however the engine is limited by the melting point of tungsten, and suggests a liquid tungsten heat shield might alleviate this limitation.

Why store the positrons at all if you could just produce them in situ and use them up immediately? And tungsten also blocks gamma radiation. So just design your engine nozzle so that it redirects the radiation away from the craft. The more you block in one direction the faster you go, so that's kind of the goal anyway.

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    $\begingroup$ Question: what do you use to contain and support a molten tungsten gamma shield? $\endgroup$
    – Zeiss Ikon
    Mar 8, 2021 at 19:00

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