A new twist on the nuclear thermal rocket: forget the uranium, use antimatter:
When antimatter meets matter, both annihilate in a flash of energy. This complete conversion to energy is what makes antimatter so powerful. Even the nuclear reactions that power atomic bombs come in a distant second, with only about three percent of their mass converted to energy.
Previous antimatter-powered spaceship designs employed antiprotons, which produce high-energy gamma rays when they annihilate. The new design will use positrons, which make gamma rays with about 400 times less energy…
Positrons are directed from the storage unit to the attenuating matrix, where they interact with the material and release heat. Liquid hydrogen (H2) circulates through the attenuating matrix and picks up the heat. The hydrogen then flows to the nozzle exit (bell-shaped area in yellow and blue), where it expands into space, producing thrust.
Not quite as sexy as the science fiction versions (which, it has to be admitted, are typically a bit sketchy on the details), but whatever works.
Such an engine would be safer for the astronauts and for the environment for several reasons: it would reduce the travel time to Mars, increasing safety for the crew by reducing their exposure to cosmic rays; the reactor would not be radioactive after its fuel is used; and there should be no risk for the public even if the reactor exploded during its launch because “gamma rays would be gone in an instant.”
Uh-huh…as would a good chunk of the spaceport. I don’t want to sound like the Atomic Fearmonger here, but I suspect that being “gone in an instant” would be small comfort to the living things that shower of gamma rays passed through along the way.
(And speaking of Bruce, who I haven’t lampooned much lately, how can someone read the accounts of the recent protests in France against reforms to unsustainable employment regulations aimed at reducing youth unemployment and get the story so completely wrong? I guess the facts don’t matter when you see the world through Marx-colored glasses.)
I don’t want to sound like the Atomic Fearmonger here, but I suspect that being “gone in an instant” would be small comfort to the living things that shower of gamma rays passed through along the way.
The point, however, is that, for a sufficiently large rocket, the gamma rays won’t go past the distance at which living things would have been roasted anyway by the explosion of a purely chemical launcher. This is because the distance over which the annihilation gammas are absorbed in air is independent of the size of the rocket.
And it is reassuring that the gammas will not create any activation products, as the annihilation radiation from antiproton reactions would.
If crew are on this launcher they’ll need to be shielded, but that’s probably true anyway due to imperfect confinement and trapping of gammas in the positron storage system and engine, and forward scattering of gammas from the engine in the air around the launcher.
What caught my attention there was his seemingly dismissive attitude about the gamma radiation — but you’re right, anyone near enough to feel the gammas is going to have bigger problems to worry about a fraction of a second later.
Any bets on how soon we hear the anti-nuclear crowd fretting about “antimatter pollution” contaminating the globe?
I think one piece I read suggested that the gamma ray effect would not go past a kilometer. If true, that seems fine with me, since one would not want to be within a kilometer of any large rocket as it takes off.
Of course our favorite nut case will certainly very soon see evil plots to build an “antimatter bomb” or something like that.
If the rocket benefits from anti-electron use because the resulting low-energy gamma rays don’t induce radioactivity in nearby materials (i.e.: the engine and spacecraft), would an anti-electron bomb be similarly “clean”? If so, one could have all the destructive capacity of a nuclear weapon without the radioactive side-effects.
))) would an anti-electron bomb be similarly “clean”?
Yes. One could imagine an integrated positron-powered cruise missile that would simply turn off the confinement system when it reached its target.
The device would probably not have the energy density of a nuclear weapon (the positron storage device would have only a small fraction of its mass as positrons), but it might be more powerful than a conventional warhead. Also, it would produce its energy as gammas, which could be useful against personnel or electronics.
This is not to claim this would or would not be practical, mind you.