Physics of the Future: How Science Will Shape Human Destiny and Our Daily Lives by the Year 2100

was the Nuclear Test Ban Treaty of 1963, which prohibited aboveground testing of nuclear weapons. Without tests, physicists could not refine the design of the Orion, and the idea died.
    RAMJET FUSION
    Yet another proposal for a nuclear rocket was made by Robert W. Bussard in 1960; he envisioned a fusion engine similar to an ordinary jet engine. Aramjet engine scoops air in the front and then mixes it with fuel internally. By igniting the mixture of air and fuel, a chemical explosion occurs that creates thrust. He envisioned applying the same basic principle to a fusion engine. Instead of scooping air, the ramjet fusion engine would scoop hydrogen gas, which is found everywhere in interstellar space. The hydrogen gas would be squeezed and heated by electric and magnetic fields until the hydrogen fused into helium, releasing enormous amounts of energy in the process. This would create an explosion, which then creates thrust. Since there is an inexhaustible supply of hydrogen in deep space, the ramjet fusion engine can conceivably run forever.
    Designs for the ramjet fusion rocket look like an ice cream cone. The scoop traps hydrogen gas, which is sent into the engine, where it is heated and fused with other hydrogen atoms. Bussard calculated that if a 1,000-ton ramjet engine can maintain the acceleration of 32 feet per second squared (or the gravity felt on the earth), then it will approach 77 percent of the speed of light in just one year. Since the ramjet engine can run forever, it could theoretically leave our galaxy and reach the Andromeda galaxy, 2,000,000 light-years from earth, in just 23 years as measured by the astronauts in the rocket ship. (As stated by Einstein’s theory of relativity, time slows down in a speeding rocket, so millions of years may have passed on earth, but the astronauts will have aged only 23 years.)
    There are several problems facing the ramjet engine. First, since mainly protons exist in interstellar space, the fusion engine must burn pure hydrogen fuel, which does not produce that much energy. (There are many ways in which to fuse hydrogen. The method preferred on earth is to fuse deuterium and tritium, which has a large yield of energy. But in outer space, hydrogen is found as a single proton, and hence ramjet engines can only fuse protons with protons, which does not yield as much energy as deuterium-tritium fusion.) However, Bussard showed that if one modifies the fuel mixture by adding some carbon, the carbon acts as a catalyst to create enormous amounts of power, sufficient to drive a starship.
    Second, the scoop would have to be huge—on the order of 160 kilometers—in order to collect enough hydrogen, so it would have to be assembled in space.
    There is another problem that is still unresolved. In 1985, engineers Robert Zubrin and Dana Andrews showed that the drag felt by the ramjetengine would be large enough to prevent it from accelerating to near light speed. The drag is caused by the resistance that the starship encounters when it moves in a field of hydrogen atoms. However, their calculation rests heavily on certain assumptions that may not apply to ramjet designs of the future.
    A ramjet fusion engine, because it scoops hydrogen from interstellar space, can theoretically run forever.
    At present, until we have a better grasp of the fusion process (and also drag effects from ions in space), the jury is still out on ramjet fusion engines. But if these engineering problems can be solved, then the ramjet fusion rocket will definitely be on the short list.
    ANTIMATTER ROCKETS
    Another distinct possibility is to use the greatest energy source in the universe, antimatter, to power your spaceship. Antimatter is the opposite of matter, with the opposite charge; for example, an electron has negative charge, but an antimatter electron (the positron) has positive charge. It will also annihilate upon contact with ordinary matter. In fact, a teaspoon of antimatter has enough energy to destroy the entire New York metropolitan area.
    Antimatter is so powerful that Dan Brown had the villains in his novel
Angels and Demons
build a bomb to blow up the Vatican using antimatter stolen from CERN, outside Geneva, Switzerland. Unlike a hydrogen bomb, which is only 1 percent efficient, an antimatter bomb would be 100 percent efficient, converting matter into energy via Einstein’s equation
E
=
mc
2 .
    In principle, antimatter makes the ideal rocket fuel for a starship. Gerald Smith of