Antimatter Propulsion
Space travel
in the future will extend to destinations previously impossible. For such long distance travel to be
successful, the densest fuel possible will be necessary. Compacted antimatter provides the perfect
solution. When the concept of antimatter propulsion is mentioned, one
immediately thinks of Star Trek.
With that automatically assumed, it is usually dismissed as nothing more
than the fantasy of a science fiction writer.
A real antimatter engine will not be as fantastic as the one from Star
Trek (creating a warp field and traveling at speeds exceeding that of
light); it will work more like a Newtonian rocket, through action and
reaction. Antimatter, however
far-fetched it seems, truly exists and is being researched by the scientific
elite of not only the United States but of countries around the world.
Antimatter
is, by definition, a form of matter in which the electrical charge or other
property of each constituent particle is the reverse of that in the usual
matter of our universe. The basis of
antimatter propulsion is antimatter/matter annihilation. To aid in the understanding of this concept
we a simple parallel: “Imagine energy as a sheet of metal going through a
coin-making machine. The coins punched
out would be matter, the space left in the sheet would be antimatter.” With this straightforward analogy, one can
easily see that when antimatter and matter are brought together they produce
energy. This concept, however crude,
explains antimatter/matter annihilation.
Broken down
to the subatomic level, matter consists of protons, neutrons, and
electrons. Predictably, antimatter
consists of equal but opposite subatomic components: anti-protons,
anti-neutrons, and positrons. In 1928
Paul Dirac combined the quantum theory and special relativity in an equation to
describe the behavior of electrons.
Dirac stated that his equation, in the same manner as the equation x2,
has two solutions: one negative and one positive. With that reasoning, he said that there must exist a particle
equal to the electron but with the opposite charge, an “anti-electron.” In 1932 Carl Anderson, studying cosmic ray
showers, observed the trail left by the first-known anti-particle, which he
described as “something positively charged, with the same mass as an electron.”
The Bevatron,
a particle accelerator capable of colliding protons with enough energy to
create anti-protons, was built by Ernest Lawrence at Berkeley, California in
1954. A team of physicists including
Emilio Segré built a detector
to see the anti-protons produced, and in 1955 the anti-proton was
discovered. A few years later, in 1959,
the anti-neutron was discovered using the same apparatus. With anti-protons and anti-neutrons,
scientists next attempted to make an anti-nucleus, and they succeeded in 1965
by making an anti-deuteron (one anti-proton and one anti-neutron). The slow progress reached its crescendo in
1995, when the first atom of antihydrogen was synthesized by German and Italian
physicists in Switzerland.
One aspect of
antimatter propulsion focuses solely on the annihilation anti-protons. When this happens, three to seven particles
called pions are produced. Usually
there are two neutral pions and three charged pions; the neutral pions almost
immediately convert into high-energy gamma rays, but the charged pions travel
an average of 21 meters at 94% of the speed of light (282,000,000 m/s). The charged pions could be collected in a
thrust chamber made of magnetic fields, and once there provide a means of
propulsion. The energy could be aimed,
by way of a magnetic nozzle, to directly provide thrust for a spacecraft. This is the aforementioned Newtonian rocket
application of antimatter propulsion.
To demonstrate the power of conceptual antimatter propulsion: the Space
Shuttle Main Engine provides 455 seconds of impulse (a measure of
efficiency). Nuclear fission could
attain 10,000 seconds, nuclear fusion supply 60,000 to 100,000 seconds, and
matter/antimatter annihilation could deliver an amazing 100,000 to 1,000,000
seconds.
Despite the
incredible power of antimatter propulsion, a large quantity of anti-protons
would be needed to reach celestial destinations. Unfortunately the cost for synthesizing anti-protons is an overwhelmingly
exorbitant amount: one gram of antimatter is $62.5 trillion. According to Nasa/Marshall employee Harold
Gerrish, improvements on current equipment could lower the price to $5000 per
microgram. Fermilab, outside Chicago,
Illinois, currently produces fifteen nanograms a year. In addition to being so expensive, many of
the anti-protons produced are wasted.
Antihydrogen
could be used to power antimatter engines in the same way that antiprotons
could be used. When a free anti-proton
picks up a positron in an atomic orbit, antihydrogen is formed. To date, antihydrogen has only existed at
relativistic speeds, and it could not be contained. Antihydrogen, although yet unattainable in useful conditions, is
a giant step in the right direction for antimatter research and
development. It is not yet known how
antihydrogen will differ from anti-protons in antimatter propulsion and
antimatter/fusion hybrid propulsion.
With even a small source of antihydrogen a cornucopia of antimatter
research doorways will be opened.
Even though
proton/anti-proton annihilation propulsion is not yet financially feasible,
anti-protons could still be put to good use in the realm of propulsion. This alternate possibility would utilize anti-protons
to activate inertial confinement fusion.
This occurs when anti-protons penetrate the nuclei of heavy atoms,
annihilating the protons and causing the heavy nuclei to fission, the fission
fragments would heat the fusion fuel, and initiate the fusion reaction. This fusion/antimatter hybrid would give an
impulse of 13,500 to 67,000 seconds, which is still 30-147 better than the
Shuttle Main Engine. A
hybrid-powered craft would need just
micrograms of antimatter to reach the Oort cloud (just beyond Pluto), which
would cost about $60 million. Reaching
actual stars would necessitate metric tons.
Another
hybrid propulsion system exchanges a nuclear reactor with a tungsten heat
exchanger core. The products of the
antimatter reaction (gamma rays and pions) would be stopped in the tungsten and
their energy used to heat hydrogen gas passing through the exchanger. This engine would use one microgram per
second of antiproton fuel while providing an impulse of 1100 seconds.
It
is my belief that antimatter is the future of the space program. Once a cost and energy efficient way of
producing it is learned, applications will have know no bounds. The synthesizing of heavy anti-elements and
annihilating them with their normal element counterparts will yield a
tremendous amount of energy and it only a matter of time before such feats are
possible. Without antimatter, space
propulsion would be limited to current rocket technology, nuclear fusion, and
nuclear fission.
The
AMS (Alpha Magnetic Spectrometer) was flown on space shuttle Discovery on
flight STS-91 in June 1998, and is scheduled to be used on the International
Space Station. It will search for dark
matter and the origin of cosmic rays.
Cosmic rays contain antimatter, as discovered by Carl Anderson in
1932. As technology advances and begins
using antimatter propulsion engines, it could become possible to refuel
spacecraft while they are in space.
Antihelium exists in cosmic rays and though it seems unlikely, one day
space craft may be able to use existing antimatter in the universe as fuel
rather than producing it on earth at such a high price.
As
stated above, antimatter propulsion is not yet financially feasible. I predict that it will take some major
scientific breakthroughs before its full potential is realized, and many years
before it is replaced by a more efficient propulsion system.