The design and development of nuclear flight-propulsion systems requires the solution of very real problems associated with complex nuclear physics, sophisticated hardware operating at very high temperatures, and the lethal radiation produced by the fission process. Similar problems, although not as difficult, were solved first for nuclear weapons and then in the production of a large, relatively low-temperature submarine and
Of considerably greater interest from a long-term viewpoint would be fusion propulsion. Fusion is the nuclear process involving the combining of light nuclei to make heavier nuclei and, as in fission, convert a small amount of mass into a huge amount of energy. It is the primary process by which energy is produced in most stars and in so-called hydrogen bombs. Every civilization—even on distant stars—would become aware of the fusion process as it reached a minimal level of scientific maturity. There are many different reactions and processes which can be used in both fission and fusion devices. One of the most attractive for a space-propulsion system would be to cause the reaction of just those particles which, when made to fuse, produce only charged rather than neutral particles. These very high-energy particles then could be directed out the back of the rocket, using appropriate electric and magnetic fields. Neutral particles come off in all directions and cannot be directed or controlled, only slowed down and their heat absorbed . . . a very inefficient process. Using the right reactions in the right way, a space fusion-propulsion system could be designed to exhaust light ions having more than ten million times as much energy per particle as they can receive in a chemical rocket. A second advantage of considerable interest is that the fuel or propellant for a fusion rocket would be isotopes of hydrogen and helium, which are not only the lightest elements but are also by far the most abundant in the universe. Thus one could be certain of finding the raw materials for a fusion fuel stockpile in any star system to which one traveled.
There have been a number of studies published showing that staged fission and fusion deep-space propulsion systems are capable of round trips to nearby stars in a shorter time than an average life span. Chemical rockets would be used to launch starships into orbit or to the moon for relaunching from there because of the greatly reduced energy requirements on the moon. Clever design would be employed such as was used by the lunar landing program. Full advantage would be taken of every “free loading” possibility just as the Apollo vehicle takes advantage of the earth’s high rotation to the east near the equator and of the gravitational field of the moon and of staged rockets which fire in programmed succession on the way and by counting on the earth’s atmosphere to slow it down rather than carrying and firing retrorockets to slow it down on the way back. The final weight and cost depend almost entirely on the design assumptions rather than (as academic calculations so often assume) being independent of those design features. An early study of the required launch weight of a chemical rocket capable of sending a man to the moon and back concluded that the launch weight would have to be a million million tons. The launching was accomplished less than thirty years later with a chemical rocket weighing three hundred million times less.
Stars and planets along the way also would be used both for their fuel and solar energy and for gravitational assistance, just as the Pioneer spacecraft, which was without propulsion systems after leaving the vicinity of the earth, used the gravitational field of Jupiter to hurl itself past Saturn and eventually out of the solar system.
Earthlings are capable of building both fission and fusion deep-space propulsion systems if they are willing to spend the tens of billions of dollars required. However, these are not the only possibilities for interstellar travel. Other possibilities include:
An important aspect of the design of any interstellar propulsion system involves
An important point to bear in mind in any discussion of interstellar travel is that it would be done in a systematic fashion. Observations would be made, unmanned craft would be sent, followed by orbiters, the installation of refueling stations, manned craft, colonizers, travelers, and all the rest. It took only twelve years from the time the first small satellite was launched before we accomplished a manned landing on the moon.
onsidering that there are stars in our local neighborhood that are billions of years older than the sun, it would not be surprising if interstellar travel has been commonplace for billions of years. Several published papers have concluded that our Milky Way galaxy already has been colonized. Furthermore, it must be noted that travel between star systems is more likely to occur the closer the next system is. Zeta 1 and Zeta 2 Reticuli are both sunlike stars that are less than three light-weeks apart. Observers on a planet around one of them could easily observe planets around the other. One would certainly expect interstellar travel to develop earlier there than in our isolated corner of the neighborhood, where the nearest star to us is one hundred times farther away than the Zeta Reticulans are from each other.
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