Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!henry From: henry@utzoo.UUCP (Henry Spencer) Newsgroups: net.space Subject: Re: Matter/Antimatter drive Message-ID: <5650@utzoo.UUCP> Date: Sat, 1-Jun-85 19:52:56 EDT Article-I.D.: utzoo.5650 Posted: Sat Jun 1 19:52:56 1985 Date-Received: Sat, 1-Jun-85 19:52:56 EDT References: <1993@mordor.UUCP> Organization: U of Toronto Zoology Lines: 78 > Could someone who understands particle physics please explain what the > idea is behind a matter/antimatter drive. Specifically, what does it > take to make the antimatter... Existing particle accelerators can make antimatter, albeit at hideously low efficiencies. High-energy physicists are, I believe, doing some work with antiproton beams; the technique is to isolate antiprotons from the debris produced when a proton beam hits a target, and then accumulate them in a storage ring until you've got enough to be useful. Decelerating them to lower velocities is not hard. "Cooling" them -- removing the random component of their velocities -- is harder but the physicists already have adequate answers for this. Bringing them to a full stop has never been done, but presents no serious problems. Combining them with positrons to make anti-hydrogen is easy. Handling the result is tricky, but there are enough different ideas about how to do it that the problem should be solvable. The major hassle remains inefficient production, largely because the existing accelerators were not designed as antimatter factories. Dr. Robert Forward (senior scientist at Hughes, and advanced-propulsion consultant to the USAF) says that there appears to be no major obstacle to getting the efficiency up quite a bit from where it is now, if one assumes a large dedicated facility. > ...and how would one turn the energy from a > matter/anitmatter explosion into thrust? Positron plus electron equals gamma rays, ugh. Fortunately, proton plus antiproton isn't that simple. First you get both neutral and charged pions. The neutral pions are impossible to do anything with, both because they lack charge and because their life is very short. The charged pions are a different story; much of the energy of the proton-antiproton reaction comes out as their kinetic energy. Their lifetime is sufficient that they travel several meters. Since they are charged, they can be bullied about with magnetic fields. So one can build a magnetic nozzle that will get them going rearward. When they decay, the major product is muons. These too are charged, and their lifetime equals a kilometer or so of motion. So if the charged pions are too much of a problem, you can use a magnetic nozzle on the muons instead. You lose some energy in the pion decay, but it's still workable. Either way you get an exhaust of charged particles at very close to the speed of light, plus a spray of gamma rays and other ugly things that one would rather live without... In practice, there is a problem with this. The exhaust velocity is pretty high, but the thrust will probably be low because the gamma rays and other uncharged trash will limit the annihilation rate -- too much radiation and the coils that produce the magnetic nozzle will absorb enough to melt. For many purposes, it is probably better to use the matter-antimatter reaction to heat something else, probably hydrogen. The exhaust velocity will be lower, hence you get less ship velocity for a given amount of fuel, but the thrust will be much higher and hence the engine will be more useful. There's a whole range of tradeoffs. For interplanetary work, lower exhaust velocities will be plenty and the higher thrust will speed things up considerably. For interstellar rockets, you want the highest possible exhaust velocity, within the restriction that the acceleration time shouldn't be too ridiculously long. And if you want a really sexy interstellar drive, consider using antimatter to heat the gas gathered in by a Bussard ramscoop... > On the last episode of PBS's Space Flight last night, some futurist > said that in order to make a resonable flight to another star, we > would need the technology for a matter/anitmatter drive, There are other ways, but antimatter may well be the most promising. > which I think > he indicated would be available within the next 100 to 200 years. Forward says that existing technology is probably good enough to make antimatter cost-competitive for in-space propulsion. (Remember that the current alternative is lifting large quantities of liquid hydrogen and oxygen from the ground, which is expensive.) If he's right -- and he's a professional in this area -- it's probably going to happen a lot sooner than "100 to 200 years". -- Henry Spencer @ U of Toronto Zoology {allegra,ihnp4,linus,decvax}!utzoo!henry