Tuesday, August 24, 2010

Antimatter - near future of the space flights and weapons

Antimatter is a most potent and efficient fuel known so far. While tons of chemical fuel are needed to propel a human mission to Mars, just tens of milligrams of antimatter can do the same thing. Only 1kg of antimatter can release the useful energy in equivalent of 20 megatons of TNT, which is nearly half of energy released by a hundreds of kilograms of fissile material in the Tsar Bomba (50 megatons of TNT), the most powerful nuclear bomb ever detonated. About 5 tones of antimatter would theoretically be enough to fuel all of the world’s energy consumption for a single year.

Concepts for using the benefits of antimatter already exists and they are technically achievable. However, 2 difficult problems must be overcome before antimatter can be put to use as a fuel source. The first is the cheaper creation of antimatter in sufficient quantities and the second is the storage of antimatter. As soon as these problems become solved, antimatter is going to be used in space missions and military as a fuel for engines and weapons.

Nature of antimatter
A Galactic Cloud of Antimatter, first shot of annihilation in space taken by NASA

Antimatter, nicknamed as “twin of matter” and “mirror matter”, consists of atoms with the same mass and inner structure as the matter atoms, but with opposite atomic properties known as spin and charge. Instead of electron and proton, the antiatom has antielectron (also called "positron") and an antiproton.

Mixing matter and antimatter can lead to the annihilation, which releases a huge amount of energy. 50-60% of that energy “escapes” in form of non-interactive neutrinos (very small, electrically neutral particle, which have almost no interaction with matter and thus escapes into outer space), and the rest releases as the kinetic energy in form of high-energy photons (gamma rays), and probably other particle–antiparticle pairs. That kinetic energy instantly heats up the environment and turns it into an ultra hot plasma, which then emits thermal radiation in the full EM spectrum.

Intensity of gamma rays depends of antimatter particles used in annihilation – antiproton, and especially antihydrogen would emit extremely high energy gamma rays which can damage and contaminate everything close enough, but positrons would emit only low energy gamma rays, 400 times less energy than antiproton would be, and there are no radioactive fallout after positron’s usage.

The energy per unit mass (9×1016 J/kg) is about 3 orders of magnitude greater than nuclear energy that can be liberated today using nuclear fission (8×1013 J/kg), and about 2 orders of magnitude greater than the best possible from nuclear fusion (6.3×1014 J/kg for the proton-proton chain). The reaction of 1kg of antimatter with 1kg of matter would produce 1.8×1017 J of energy (by the mass-energy equivalence formula E = mc²), or the rough equivalent of 43 megatons of TNT. Half of that energy would be “lost” through neutrinos, so the 1kg of antimatter would produce approximately 20 megatons of the useful energy. For comparison, Tsar Bomba, the largest nuclear weapon ever detonated, reacted an estimated yield of 50 megatons, but it used a hundreds of kilograms of fissile material (Uranium/Plutonium). Little Boy, the nuclear bomb dropped in Hiroshima 1945, completely devastated this city with “just” 13-18 kilotons of TNT.

Creation and storage of antimatter

In outer space, antimatter is created in collisions of high speed particles – cosmic rays. On Earth, it has to be created in particle accelerators (huge machines that smash atoms together). Creating the antihydrogen in enough quantities would be too long and too expensive process with today’s technology, and its use as fuel would free the high-energy gamma rays which are too contaminable and destructive. For this reasons, science is focusing on production of the smallest and the cheapest particles - positrons (anti-electrons), which emits low-energy gamma rays. Antihydrogen was produced experimentally at CERN in 1995, in quantity of few hundred thousand atoms.

Dr. Gerald Smith of Positronics Research, LLC, in Santa Fe, New Mexico, said that a rough estimate to produce the 10 milligrams of positrons needed for a human Mars mission is about 250 million dollars, using technology that is currently under development. Based on the experience with nuclear technology, it seems reasonable to expect positron production cost to go down with more research.

Positrons are currently impossible to store in large quantities. By their very nature, positrons are positively charged and therefore exert a Coulombic force of repulsion against one another. This Coulombic force is extremely powerful and only the smallest amounts of positrons can be stored adequately with current technology. Beside that, positrons (and any particles of antimatter) would annihilate anything what touches, so it has to be stored in the inside of electro-magnetic fields. There are currently specially constructed magnetic storage devices called Penning traps, but these devices are not suited for high density storage of antimatter that would be required for space propulsion. Dr. Gerald Smith claims that this problem will be overcame.
Penning trap - portable version

NASA’s antimatter spaceship plan
Image right: This is an artist's concept of an advanced positron rocket engine, called an ablative engine. This engine produces thrust when material in the nozzle is vaporized (ablated). In the image, the engine emits blue-white exhaust as thin layers of material are vaporized by positrons in tiny capsules surrounded by lead. The capsules are shot into the nozzle compartment many times per second. Once in the nozzle compartment, the positrons are allowed to interact with the capsule, releasing gamma rays. The lead absorbs the gamma rays and radiates lower-energy X-rays, which vaporize the nozzle material. This complication is necessary because X-rays are more efficiently absorbed by the nozzle material than gamma rays would be. Credit: Positronics Research, LLC

Beside production cost and storage problem, a big problem represents high-energy gamma rays which would be emitted in annihilation of antiproton and especially antihydrogen in antimatter propulsion. These rays are very energetic and can damage the engine and contaminate the whole ship.
The NASA Institute for Advanced Concepts (NIAC) is funding a team of researchers working on a design for an antimatter powered spaceship that avoids this nasty side effect by producing gamma rays with much lower energy. Previous designs were based on antiprotons, but new ones will use positrons, which makes gamma rays with about 400 times less energy. They also investigate how these low energy gamma rays can be absorbed or converted into harmless X-rays.

The current Reference Mission calls for a nuclear reactor to propel the spaceship to Mars. This reactor is very complex and many things can go wrong during the mission. Nuclear reactors are also  very radioactive and after their fuel is used up.

Positron reactor will be much simpler and will not produce any leftover radiation after spending the fuel. Another significant advantage is the speed – with nuclear reactor, spaceship would arrive on Mars in 180 day, but with positron reactor the same journey should take just 45 days.

Antimatter weapons

Huge energetic potential of antimatter can dramatically improve capabilities of the army. When the problem of positron storage became solved, it will be possible to make the bomb small enough to hold in one’s hand but powerful enough to destroy one whole district. With antimatter engines, aircrafts will gain a huge improvement in range, ceiling and speed. Submarines will be simpler and faster comparing to today’s nuclear ones. Another possibility is antimatter-powered "electromagnetic pulse" weapons that could fry an enemy's electric power grid and communications networks, leaving him literally in the dark and unable to operate his society and armed forces.
250 grams of antimatter, about the weight of five eggs, could be used to make a hand-grenade sized bomb as powerful as a ten megaton nuclear weapon. Photo from Edit International
Of course, antimatter will also be used for cataclysmic bombs - either pure antimatter bombs or antimatter-triggered (catalyzed) nuclear weapons.

Pure antimatter bomb will be the positron bomb because it would be the cheapest and the less contaminable explosive. As mentioned, there is no radioactive fallout after spending the positrons because they can emit only low energy gamma rays (400 times less energy then antiprotons would emit). And keep in mind that the 2-3kg of antimatter can release approximately the same energy as the hundreds of kilograms of nuclear material in Tsar Bomba (50 megatons of TNT) which leaves behind a heavy radioactive contamination.

Antimatter catalyzed weapons requires much less quantity of antimatter, which means that this will be probably the first official use of antimatter. They also can result in less long-term contamination than conventional nuclear weapons, and their use might therefore be more politically acceptable. Igniting fusion fuel requires at least a few kilojoules of energy, which corresponds to around 10−13 gram of antimatter, or 1011 anti-hydrogen atoms. Fuel compressed by high explosives could be ignited using around 1018 protons to produce a weapon with a one kiloton yield. These quantities are clearly more feasible than those required for "pure" antimatter weapons, but the technical barriers to producing and storing even small amounts of antimatter remain formidable for now.

1 comment:

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