I would guess antimatter rockets are what the majority of people think about when talking of rockets for the future. This is hardly surprising as it is such an attractive word for the writers of science fiction.
It is, however, not only interesting in the realm of science fiction. Make no mistake; antimatter is real. Small amounts, in the order of nanograms, are produced at special facilities every year. It is also the most expensive substance of Earth, in 1999 the estimated cost for 1 gram of antimatter was about $62.5 trillion.
It may surprise you to learn that the reality of antimatter is hardly a modern discovery, its existence was predicted back in 1929 and by 1953 Eugen Sanger (a German rocket scientist) had proposed its use for spacecraft propulsion.
The reason it is so attractive for propulsion is the energy
density that it possesses. Consider that the ideal
energy density
for chemical reactions is 1 x 107 (10^7) J/kg, for
nuclear fission it is 8 x 1013 (10^13) J/kg and for
nuclear fusion it is 3 x 1014 (10^14) J/kg, but for
the matter-antimatter annihilation it is 9 x 1016
(10^16) J/kg. This is 1010 (10 billion) times that of
conventional chemical propellants!
This represents the highest energy release per unit mass of any
known reaction in physics. The reason for this is that the
annihilation is the complete conversion of matter into energy
governed by Einstein's famous equation
E=mc2, rather
than just the part conversion that occurs in fission and
fusion.
Let us not get carried away here though, this is not some
incredible new technology that will power us throughout the
galaxy. At the most basic level the antimatter rocket is still a
Newtonian rocket, governed by the three laws of motion and it
still conforms to Einstein's theory of special relativity, in
other words it can not exceed the speed of light.
Still if we are enable to develop such a propulsion system in
the future it will surely render any other Newtonian rocket
obsolete overnight, the system has the highest predicted
efficiency,
specific impulse and probably the highest
thrust to
weight ratio. There does seem to be a serious amount of
disagreement over this last point, the general feeling seems to
be that the
thrust to weight will at least comparable to today's
very powerful chemical rockets.
What this means is that only 100 milligrams (1/10 gram) of
antimatter would be needed to match the total propulsive energy
of the Space Shuttle (all those huge tanks of fuel!).
This fact has led to the interesting observation that future
advanced spacecraft, such as the antimatter rocket, will not be
designed around their propellant tank like conventional craft.
Instead the craft will be designed around the reactors (for
nuclear craft) or around the systems and chambers to cause
annihilation (for antimatter craft). Radiation shielding will
also become a key component of spacecraft design.
Antimatter Storage at Penn State |
| Courtesy Penn State University Antimatter |
Antimatter consists of exactly the same elementary particles as matter, but the electric charge, along with all other quantum numbers, is reversed. For example matter has electrons (negatively charged) and antimatter has positrons (positively charged).
Antimatter does not exist in nature - or at least certainly no
where near us, which is just as well. If it did it would
immediately annihilate with matter and explode with more force
than we have ever experienced.
This means we have to manufacture it and then very
carefully store it; it is only produced at certain high-energy
laboratories around the world (probably most famously at CERN in
Geneva).
The actual manufacturing is achieved in a particle accelerator
creating extremely high energy collisions, which results in the
kinetic energy being converted to matter (subatomic particles),
some of which is antimatter.
Storage is possible because it may be controlled in magnetic
fields, thereby avoiding the obvious problem of trying to store
it in structural containers.
Production efficiency is extremely poor and collection, storage and handling of antimatter is exceeding complex.
Antimatter Propulsion SystemsOnce we have produced and stored the antimatter we can use it in propulsion by releasing it into a chamber and allowing it to annihilate with normal matter which produces its tremendous energy in the form of energetic sub-atomic particles.
There are actually two choices for propulsion.
Should we choose the proton-antiproton or the electron-positron
annihilation?
Well electron-positron annihilation produces high energy gamma
rays which are impossible to control, hence useless for
propulsion, and on top of this are potentially very dangerous.
Whereas the proton-antiproton annihilation produces charged
particles (mostly pions moving at velocities close to that of
light) that can be directed with magnetic fields, maximising
propellant mass.
The fact that there is this mass left over after the
annihilation means that the full conversion of mass to energy has
not occurred as it does in the electron-positron annihilation,
therefore slightly less energy has been produced.
This energy, however, still far exceeds any other method and the
resulting particles allow this energy to be harnessed by
directing it with magnetic forces. In other words the perfect
reaction does not produce perfect propulsive result.
Another important advantage for antimatter rockets over nuclear rockets is that heavy reactors are not required, the reaction is spontaneous.
There are four main designs for an antimatter rocket, they are listed here in increasing specific impulse:
Solid Core - Annihilation occurs inside a solid-core heat exchanger, the reaction superheats hydrogen propellant that is expelled through a nozzle. High efficiency and high thrust, but due to the materials the specific impulse is only 1000secs at best.
Gas Core - Annihilation occurs in the hydrogen propellant. The charged pions are controlled in magnetic fields and superheat the hydrogen, there is some loss in the form of gamma rays that can not be controlled. specific impulse of 2500secs.
 |
| Courtesy MSFC |
Plasma Core - Annihilation of larger amounts of antimatter in hydrogen to produce a hot plasma. Plasma contained in magnetic fields, again some loss in form of gamma radiation, the plasma is expelled to produce thrust. There are no material constraints here so higher specific impulse is possible (anywhere from 5,000 to 100,000secs).
Beam Core - Direct one to one annihilation, magnetic fields focus the energetic charged pions that are used directly as the exhausted propellant mass. These pions travel close to speed of light so the specific impulse could be greater than 10,000,000secs.
The spacecraft will have to be designed to be very long as the annihilation products travel close to the speed of light.
Journey TimesEstimates for travel times to Mars for an advanced antimatter rocket using the beam core approach are anywhere from 24 hours to 2 weeks, it is probable that it will be somewhere inbetween. Compare this to the space shuttle using its conventional chemical propulsion when a trip to Mars would take between 1 and 2 years.
ProblemsProduction
We would need at least several milligrams of antimatter to fuel a beam core antimatter engine in local operations and several kilograms for interstellar travel to Alpha Centuri. Given that currently 1-10 nanograms of antiprotons are produced a year at Fermilab (Chicago) and CERN (Geneva), a beamed core engine is not feasible in the near future.
Storage
The Penning trap has been developed, it is a portable
antiproton trap which is capable of storing 1010
(10^10) antiprotons for one week using the superposition of
electric and magnetic fields. The next stage is an improvement to
1012 (10^12) antiproton storage.
For complete antimatter propulsion it is thought that
1020 (10^20) anti-protons will need to be stored.