Antimatter particles have the same mass as normal matter particles, but opposite electrical charges. Matter and antimatter mutually annihilate each other on contact and are converted to pure, 100% energy. This energy usually takes the form of a combination of gamma rays, neutrinos, antineutrinos, and pions. This total energy conversion makes forms of antimatter very attractive as a spacecraft fuel. One gram of antimatter, annihilating with one gram of normal matter, can generate as much energy as 23 Space Shuttle external fuel tanks. Antimatter rockets are thought to be able to provide specific impulses of up to 10 million seconds.
Antimatter responds as readily to magnetic and electrical fields and bottles as normal matter, so containing and directing antimatter for use in a spaceship engine does not represent as huge a problem as many assume. No, the big problem with using antimatter as a form of propulsion is with obtaining and storing it.
Antimatter is very rare and short-lived in nature, so it must be manufactured artificially. Today, it can only be produced in the amount of nanograms (billionths of a gram) per year at about $62.5 trillion dollars per gram, making it the most expensive substance on Earth. This could be unfortunate, as dozens of kilograms would have to be made to make interplanetary flights possible, and tons of antimatter would have to be produced for interstellar missions. Production methods for creating and storing antimatter would have to be increased a billionfold while its cost would have to decrease on a similar scale before antimatter propulsion could be considered practical. The relatively high tech levels of advanced antimatter rockets covered in this article represent not only the engineering hurdles in building the rocket but also the difficulty in obtaining and storing tremendous amounts of antimatter.
Proton-antiproton collisions (as opposed to electron-positron or hydrogen-antihydrogen collisions) are preferred for propulsion, as the reaction produces a large percentage of charged particles (pions) that can be contained and directed for thrust with electromagnetic fields.
Antimatter reactions produce large amounts of radiation as their by product, including gamma rays and pions, making heavy shielding an absolute necessity on almost all missions using the technology.
|I.C.A.N. MICRO FUSION/FISSION PROPULSION|
Tech Level: 15
|An ICAN antimatter rocket as envisioned by PSU researchers.|
|Detail of an ICAN rocket's engine section.|
ICAN is significant in that it needs only a very modest amount of antimatter (approximately 140 nanograms for a nearby interplanetary mission) in order to work, an amount that can be produced within about a year or so at significantly equipped facilities such as Fermilab.
The ICAN scheme is being studied by the Pennsylvania State University and is being considered for a manned Mars missions. The most recent engine configuration, called ICAN-II, could theoretically make a trip to the red planet and back again in only 120 days.
|AIMSTAR ANTIMATTER ROCKET|
Tech Level: 16
|An AIMStar-powered rocket approaches an unnamed gas giant moon.
The AIM in AIMSTAR stands for Antimatter Initiated Microfusion. Like the ICAN scheme, the AIMStar is being developed by the Pennsylvania State University, specifically for an interstellar "precursor" mission that would carry a probe well beyond the heliopause to a distance of 10,000 AUs from the sun. Also like the ICAN scheme, the AIMStar engine tries to make use of existing or near-term antimatter technology, specifically penning traps, and apply it to space propulsion.
A penning trap is basically a powerful magnetic bottle with specific electrical fields used to hold anti-protons. Pellets of fission/fusion fuel (similar to the ICAN propellant pellets, above, but smaller) are "shot" through the trap, basically compressed onto the outer layer of the antiparticle mass in the trap as it passes through. The energy of the antimatter annihilations initiates a fission reaction, which in turn sparks a fusion burn in the compressed deuterium-tritium mix. This superheated plasma is then expelled for thrust.
After each such "burn" the antiprotons in the penning trap are allowed to reset back to their original configuration, minus about 0.5% of their original mass, which was used up in the burn cycle annihilations. After every 50 burns, new antiprotons are injected into the magnetic bottle to reload the trap. The AIMStar engine would fire at about 200 burns per second.
Fuels being considered for the AIMStar are a deuterium-tritium (DT) mix and a deuterium-helium-3 (DHe3) mix. The DT fuel provides much more energy and higher thrust, but the tritium for the DT mix is much harder to obtain than helium-3 and the reaction produces far more radiation than the DHe3 fuel.
The AIMStar engine would require about 28 micrograms of antimatter for the proposed 10,000 AU mission, and has an upper specific impulse of about 61,000 seconds.
Solid Core antimatter rockets would function very similarly to NERVA solid-core nuclear rockets. Antiprotons annihilate protons, heating a tungsten or graphite heat-exchanger. The tungsten and/or the graphite would help to absorb the gamma rays and pions produced by the reaction. Hydrogen fuel is pumped through narrow channels between the heat exchangers, heating the hydrogen to a plasma state, which is then expelled for thrust. Because of the material limitations of the system, solid-core antimatter rockets would be capable of specific impulses of "only" 1000 or so seconds.
This scheme injects antiprotons directly into the hydrogen fuel stream. Magnetic fields are used to contain only the energetic charged pions which spiral into the hydrogen gas to heat it. The resultant plasma is then expelled through a conventional rocket nozzle. Gas core antimatter rockets are less efficient than solid core models, but because they are less constrained by the melting points of their material components, they can achieve specific impulses of up to 2500 seconds.
This scheme is similar to the Gas Dynamic Mirror Fusion Propulsion engine. The gas core system uses a relatively small amount of antimatter to heat the hydrogen; the plasma core injects a much larger amount of antimatter into the hydrogen fuel, using powerful magnetic fields to contain the high energy pions that result from the annihilation reactions to heat the resultant plasma to a superheated state. This plasma is then exhausted for thrust. This engine is not limited by the material melting points of its compnents, and thus can achieve specific impulses in excess of 100,000 seconds at significant thrust levels. It does, however, require full kilograms of antimatter to go any significant distance.
The beam-core thruster employs a diverging magnetic field just upstream of the annihilation point between the antimatter and low-density hydrogen. The magnetic field then directly focuses the energetic charged pions as the exhausted propellant. Since the charged pions are traveling close to the speed of light, the specific impulse of the device could possibly range as high as 10 million seconds, but at very low thrust levels.
The beam core scheme has a matter/antimatter annihilation ratio of nearly 1:1 and would need metric tons of reaction mass for deep space missions. However, it has the fuel efficiency to be made into a true interstellar rocket, able to obtain up to 40% lightspeed. It could reach the nearby stars with an antimatter fuel requirement of a "mere" ten metric tons.
|Diagrams of advanced antimatter rockets.|
Indistinguishable From Magic by Robert L. Forward
On The Web:
JPL’s pages on antimatter propulsion:http://www.islandone.org/APC/Antimatter/07.html#ICAN http://www.islandone.org/APC/Antimatter/02.html
Pennsylvania State University’s web site dedicated to antimatter propulsion:http://www.engr.psu.edu/antimatter/
Page with antimatter rocket travel times:http://www.futurespace.de/library/database/antimatter.html
A technical PDF presentation of PSU’s AIMStar scheme:http://www.engr.psu.edu/antimatter/Papers/AIMStar_99.pdf
A more layman-friendly article on AIMStar and ICAN:http://popularmechanics.com/science/space/2002/6/antimatter_spaceships/print.phtml
A very informative site discussing the nature of antimatter and the feasibility of antimatter drives:
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