8
$\begingroup$

Is there a possibility ( in theory ) to build a antimatter propulsion system, if so how can we control the collision of matter-anitmatter, will humans be able to control this force just like electricity ?

$\endgroup$
9
  • $\begingroup$ The US Air Force studied the problem during the cold war. Naive antimatter rockets are inefficient because it is hard to steer the exhaust (essentially impossible to steer the substantial neutrino component). $\endgroup$ Jun 28, 2013 at 13:03
  • 5
    $\begingroup$ @dmckee Let alone that storing antimatter for use will require huge magnetic fields, and thus large volumes. We have not yet solved the controlled fusion problem which is easier than antimatter delivery. $\endgroup$
    – anna v
    Jun 28, 2013 at 13:49
  • 4
    $\begingroup$ Alternatively to trying to steer the reaction products as exhaust, you could use them to heat a conventional propellant, as done by nuclear thermal rockets (NTRs). The problem there is that the exhaust velocity will be comparable to NTRs as anything higher will melt your spacecraft, so the reaction mass dominates the mass of the vehicle and the delta-v will be comparable as well. So the added (astronomical) cost of antimatter gains you little over regular nuclear thermal. $\endgroup$
    – Michael
    Jun 28, 2013 at 14:15
  • $\begingroup$ @dmckee: Why neutrinos? If you react hydrogen and antihydrogen, all you get is gammas...? $\endgroup$
    – user4552
    Jun 28, 2013 at 15:50
  • 2
    $\begingroup$ @BenCrowell You get all kinds of stuff, including lots of pions which means lots of neutrinos. This isn't $e^+ + e^-$, it is $(\text{bag of quarks and gluons}) + \overline{(\text{bag of quarks and gluons})}$. But I don't want to make that comment an answer because I don't know the details and can not provide a reference. $\endgroup$ Jun 28, 2013 at 16:31

1 Answer 1

19
$\begingroup$

The basic tragedy of space travel is expressed by the Tsiolkovsky rocket equation, which says that the amount of reaction mass you need grows exponentially with your $\Delta v/v_e$, where $v_e$ is the exhaust velocity. The advantage of antimatter propulsion is high energy density, but energy density doesn't have any direct, major effect on the amount of reaction mass needed. For a lot of purposes within the solar system, solar power provides plentiful energy for something like an ion drive (which has been successfully tested).

The real advantage of antimatter propulsion would be if you wanted to send a probe to another star and have it get there within a human lifetime. For this type of mission, you can't use solar power or beamed energy, and you need a relativistic $\Delta v$, which means that the energy density of chemical or fusion or fission reactions is not high enough. In this situation, the rocket equation is going to kill you unless the exhaust velocity is also highly relativistic. You can't achieve that simply by reacting the matter and antimatter in a chamber and letting reaction mass fly out through a nozzle -- the chamber would be destroyed. So the design you're probably going to end up with is a heat engine that generates electric power to run a particle accelerator, which shoots ions out as reaction mass at relativistic speeds.

There are many, many problems with carrying this out. Antimatter is ridiculously hard to produce in any significant quantity. It's hard to store safely. No way do you want ton quantities of antimatter anywhere near the planet you live on, because a disaster (or, say, a hijacking) would probably wipe out all life.

Assuming you get the engine running, the second law of thermodynamics says that the heat engine can't be 100% efficient, and the waste heat is likely to melt the entire vessel almost instantaneously.

The matter-antimatter reactions will produce a witch's brew of high-energy particles. Matter-antimatter annihilation (such as proton-antiproton annihilation, Amsler 1997) makes a lot of high-energy gamma rays, e.g., $\gtrsim 67$ MeV gammas from decay of $\pi^0$'s, and the energy of those gammas is hard to collect, because they're very penetrating. You also get muons and neutrinos from the charged pions that are produced. The neutrinos are lost energy. The energy from the charged particles (muons and undecayed charged pions) is the only energy in the process that's easy to collect in a controlled way. There will also be 511 keV gammas from electron-positron annihilation.

Amsler, http://arxiv.org/abs/hepex/9708025

$\endgroup$
7
  • $\begingroup$ +1 For the fatalistic meltdown of the almost working engine plan :) $\endgroup$ Jun 28, 2013 at 20:38
  • 1
    $\begingroup$ +1 on heat rejection; people often leave out this essential part of propulsion/craft engineering. $\endgroup$ Jun 28, 2013 at 20:38
  • 2
    $\begingroup$ I again want to note that 67 Mev is the lower limit of the gammas from pio, as on the average the pions from decay at rest of proton antiproton are five and the energy to be statistically distributed between them is 1880 or so. The pi0s will be in flight mostly and the average gamma energy over 150 MeV $\endgroup$
    – anna v
    Aug 14, 2013 at 5:24
  • $\begingroup$ @annav: Thanks for the comment. I've edited my answer accordingly. $\endgroup$
    – user4552
    Aug 14, 2013 at 13:39
  • $\begingroup$ My understanding of the antimatter engine was to combust in a magnetic bottle; the pions are kicked out the back, resulting in a relativistic ve. $\endgroup$
    – Joshua
    Feb 4, 2016 at 22:18

Not the answer you're looking for? Browse other questions tagged or ask your own question.