Antimatter propulsion

Antimatter is real. Particle physicists have been creating antimatter for at least half a century as a byproduct of high-energy particle accelerator research. But what is it? First theorized by Paul Dirac in 1928, antimatter—particles with the same mass as their normal matter cousins but with opposite charge—is regularly produced in many facilities used for particle physics research. The antimatter counterpart of the electron is the positron. The proton's antiparticle is the antiproton, having the same mass as a proton but an opposite charge. When a particle and its antiparticle meet, they do something very interesting and potentially very useful: they annihilate each other and convert all their combined mass into energy. It is the most efficient mass-to-energy conversion process known—far more efficient than nuclear fission or fusion.

Created when atoms of normal matter smash into each other at relativistic speeds, antimatter particles quickly encounter normal matter atoms and annihilate them. These events have been studied and recorded for decades. Even under hard vacuum conditions, atoms of antimatter will eventually cross paths with atoms of normal matter and annihilate them. It is thus very difficult to store antimatter for any significant length of time. Conventional vacuum chambers cannot be used to store these elusive atoms because they too are made of normal matter and readily annihilate when the antimatter atoms come into contact with their walls. It should be noted that nature, too, produces antimatter. When high-energy cosmic rays (which are nothing more than atoms moving at very high speeds with an origin in deep space) enter the Earth's atmosphere, they collide with atoms in the atmosphere. Some of the enormous energy of the cosmic rays is converted into matter—antimatter pairs, which soon interact and annihilate.

Since antimatter atoms have essentially the same physical properties as normal matter, they follow the same laws of physics. Charged particles in the presence of a magnetic field experience a force acting upon them that is perpendicular to both the lines of magnetic force and their direction of motion. Similarly, charged particles in the presence of an electric field will also experience a force, this time aligned along the electric field line. Any charged particle, matter or antimatter, will experience these forces— though, since their charges are of opposite sign, the force will push the antimatter particle in a direction opposite to the normal matter particle. No matter, the bottom line is that electric and magnetic fields can be used to "trap" antimatter particles in vacuum, thus mostly preventing them from coming into contact with the vacuum chamber walls. The antimatter ions spiral along the magnetic field lines, reflecting back from both the north and south poles of the magnet in much the same way that ions are trapped along the Earth's magnetic field lines in the ionosphere. These magnetic traps are called Penning traps and have been successfully used to trap charged particles for extended periods of time.

How, then, might these energetic atoms be used for space travel? And how is this relevant to "living off the land in space''? To answer the first question, matter—antimatter collisions, under controlled conditions, can be used to create enormous amounts of energy. It is, in effect, a very efficient battery. Unfortunately, it is also a very expensive battery. Global production of antimatter hovers near a nanogram (that's 0.000000001 gram) per year. For a viable propulsion system, one would need several grams of antimatter. And, by the way, it would have to be stored with nearly 100% efficiency lest it should inadvertently become a very powerful bomb, which would be the result if a significant fraction of the antimatter were to get free and annihilate all at once. If you recall that this reaction is orders of magnitude more energy efficient than nuclear fission or fusion, you can then imagine the disaster that would ensue as a result of such an event. For comparison, the total energy released by the space shuttle engines (and its 1.7 million kilograms of fuel) during launch is the equivalent of 0.1 gram of antimatter! Unfortunately, antimatter is also one of the most expensive items on earth to produce. In 1999 Dr George Schmidt of NASA calculated the cost to produce 1 gram of antimatter at 62.5 trillion (yes, trillion) dollars. How, then, might this be relevant to "living off the land in space''?

Given its highly volatile nature, the large-scale production ofantimatter would be a very dangerous venture. Ideally, it should be produced in a remote location so that any industrial accident would not kill a large nearby population and devastate a continent. It should be produced in a location where the energy required to manufacture it is plentiful and cheap—one doesn't get all this energy for nothing. It still takes a lot more energy to create antimatter than can be extracted from it. The beauty of the antimatter is its high-energy density and storability for use on relatively small spacecraft. One place where these conditions are simultaneously met is the Moon. As sunlight is plentiful, large solar array farms could be assembled to provide the power required to run the accelerators and make the antimatter. And the Moon certainly satisfies the "remoteness" criteria should there be an accident, endangering only the astronauts/workers and not an unwilling population. In this case, nature provides us with the real estate and the power—now all we humans need to do is discover how to affordably mass-produce the stuff.

Breakthrough Propulsion Physics

Barring some revolutionary change in the known laws of physics, nature seems to have placed a nearly insurmountable barrier between us and the rest of the universe—the immense distances that must be traversed for any explorers, robotic or human. Crossing many light years in a human lifetime—or even in the lifetime of a human civilization let alone the potential lifetime of the spacecraft making the journey—appears to many to be nothing more than a dream. Even using antimatter-driven spacecraft, or the solar-sail-propelled version described in Chapter 13, it will take centuries, if not millennia, to reach nearby stars. Unless nature has provided an as-yet unknown "escape clause,'' we may never journey beyond the closest stellar systems.

Nevertheless, new theories and phenomena are being reported in the scientific literature that at least give spacefarers pause for thought—and hope. Perhaps science-fictional concepts like wormholes, space-warping drives, and vacuum energy are both real and usable? If so, will our "live off the land'' philosophy extend to journeys beyond Sol? The answer is a resounding, "yes!" If, and only if such exotic theories are validated will we see significant human exploration beyond the home solar system—we simply cannot otherwise take enough energy or propellant with us on such a journey. Aside from limited and infrequent interstellar journeys using sails, there seems to be no possible method of establishing the galactic empire of science fiction books and films.

From 1996 through 2002, NASA funded the Breakthrough Propulsion Physics (BPP) Project at the NASA Glenn Research Center. Though the funding was modest, the project attempted to rigorously investigate many of these exotic physics ideas and recent theoretical or experimental claims regarding phenomena that might lead to faster-than-light travel. Using the peer-review process and involving credible members of the scientific community, the BPP Project investigated quantum tunneling, vacuum energy extraction, Woodward Transient Inertia, and others. The Project also assessed similar theories and work sponsored elsewhere. No obvious breakthroughs were found. Does that mean that nature has not provided humanity with a way to reach the stars? No—it does mean, however, that we don't know if she has or not.

The following concepts may have a Technological Readiness Level near Zero in AD 2006. But who knows what they might lead to in the far future?

Replacing the Rocket: Antigravity

No one can deny that the launch of a large rocket is an awe-inspiring and dramatic sight. It is also dangerous and inefficient. Space travel would be safer and less expensive if a gentler device could be developed to replace the reaction engine, something perhaps akin to the magic carpets of ancient fables.

One conceptual rocket replacement—an approach that has captured the hearts of both science fiction authors and Hollywood special-effects experts—is antigravity. Simply strap on your spacecraft, plug in your convenient antigravity device, and float slowly up into the cosmos.

As discussed by Gregory Matloff in Deep-Space Probes, there is a theoretical basis for antigravity. In the early instants of the universe, gravity was linked with the other forces of nature, including electromagnetism. Might an electromagnetic machine be possible that could convert Earth's gravitational attraction into repulsion?

Some theoretical work suggests that the answer to this question is "yes." A number of experiments, some using test masses suspended over rapidly spinning superconductors, have been performed.

To date, these experiments have not proven that the theory is valid. And any experimentally suggested antigravity effects are very small, perhaps non-existent or just very close to the limits imposed by experimental precision. But antigravity remains a tantalizing concept and research will certainly continue.

Replacing the Rocket: Thrust Machines

Imagine a machine that could directly convert electrical or mechanical energy into spacecraft kinetic energy at high efficiency. Such a device could act in a similar manner to an antigravity device, to accelerate a spacecraft, aircraft, flying car, or flying city skyward with little or no reactive thrust. Humans, like birds, would become creatures truly at home in three dimensions.

From time to time, such devices have been proposed and tested. To date, all have proven to be impossible to implement or have been shown to directly violate the basic laws of mechanics or thermodynamics.

The classic space drive of this type, the "Dean Drive'' of the 1960s, is discussed by Eugene Mallove and Gregory Matloff in The Starflight Handbook. Awarded a US patent, this device purports to convert rotary motion into linear motion. But controlled laboratory tests on similar devices demonstrated that they will not function as advertised.

A more recent device, developed by British engineer Roger Shawyer, is still under evaluation. This device purports to function by converting direct-current electrical power in a microwave tube directly into thrust. To date, no one has independently demonstrated the device.

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