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Generation Spaceships
They will have to endure the cramped conditions, the lack of privacy and high workload of a mission. There will be a number of simulated scenarios to undergo including the launch, walking on the Martian surface and an emergency or two. All contact with the outside world is going to be the same as would be experienced if the volunteers were out in space: it will be subject to a lengthening delay. And to make matters worse, all they will have to eat are the types of rations used on the International Space Station. The goal of these experiments is to discover just how the astronauts on the actual mission to Mars might react to their lengthy period of isolation, shortages of supplies and stress, and how the dynamics of a group might develop over such a lengthy period. All of this is for a trip to a near neighbour, just one planet out from Earth. Science recently has been discovering more and more extrasolar planets, systems where we might find a suitable environment to support human life. Mankind has a history of pushing frontiers, of striving to go that little bit further. So in future decades mankind may well be looking to venture outside our solar system even if we do not have superluminal (faster than light) travel. The question, though, is: just what will that involve? Even travelling a distance as short as that to Mars is taxing our levels of technology and commitment. At its furthest point from Earth, Mars is a mere 21 light minutes, or 400 million kilometres away. The nearest star to us, other than the Sun, Proxima Centauri is 4.3 light years away, or about 40 trillion kilometres. The nearest star known to have a planetary system, Gliese 581, is nearly five times that distance at 20.5 light years. The fastest man-made objects so far have been the Voyager spacecraft, which reached speeds of some 59,000 kilometres per hour as they left the solar system. Even if we averaged this speed then reaching Proxima Centauri would take 80,000 years. Methods of Propulsion These craft used conventional means of propulsion. Traditional engines, be they internal combustion engines or rockets, require the vehicle to carry a fuel supply. On an interstellar trip this would be prohibitive in terms of weight, as well as in terms of the immense amounts of fuel we would be sending off into the heavens. So other methods of propulsion are required. There are several suggestions for how to achieve this. One possible method of propelling an interstellar craft is Nuclear Pulse Propulsion (also called External Pulsed Plasma Propulsion in a recent NASA mission proposal). This method involves a craft being accelerated by the Plasma Waves generated by a series of nuclear explosions directly behind the vehicle. This method, of course, exposes the craft to massive amounts of radiation and so would require the crew compartments to be heavily shielded, which would add to the weight. In the 1960s an American physicist called Robert W. Bussard proposed a Ramjet, powered by collecting Hydrogen from the Interstellar Environment by means of a scoop (called a Ramscoop) on the front of the spacecraft and funnelling it into a nuclear fusion reactor. There are a number of problems to overcome with this option, not withstanding the fact that we have yet to design a nuclear fusion reactor. As the density of the interstellar environment is very low, the Ramscoop needs to be very large: on the order of tens of kilometres in diameter. Solid Ramscoops cannot be used, so an Electromagnetic or Electrostatic Field would be needed to funnel the required particles to the reactor. For this type of Ramscoop to be able to collect fuel the particles ahead of the ship would need to be Ionised, possibly by means of a laser. Since Bussard outlined his Ramjet, other scientists have analysed the concept and have cast doubt on the feasibility of such a device. Since the 1960s it has been discovered that the region surrounding our Solar System has a considerably lower particle density than previously thought and so would not provide as much fuel. Also Ramjet design attempts have so far not been able, in theoretical models, to provide sufficient thrust to overcome the drag effect of collecting the Hydrogen Ions. The Ramjet's saving grace may lie in combining it with another propulsion method. Another such method is a solar sail. Solar sails are effectively large mirrors. When photons from the Sun (or any other star) reflect off such a mirror they give the craft a very small push. This push is minute and lessens as the distance from the source object increases, just as the brightness of a light bulb diminishes as you walk away from it. But it could provide a reliable, if slow, acceleration. Sunlight, however, is not the only method of providing the push such a sail would need. Any form of radiation would work, so a series of microwave emitters constructed in space could be used as launch platforms for such craft. The Human Factor These current concepts of interstellar propulsion all envisage a maximum cruising velocity of no more than 10 per cent of light speed. This is an estimated maximum though, not an absolute guarantee, so for the rest of this article an average speed of 5 per cent of light speed will be assumed. Our trip to Proxima Centauri would therefore take a little over eighty years, and to reach Gliese 581 we would need four centuries. We also need to consider the accelerations and decelerations of the craft. Human beings are "squishy": accelerate us too quickly and we suffer. Astronauts are trained to withstand accelerations of 8 or 9 times Earth gravity for short periods - but these accelerations are very short-lived. Even at a constant acceleration of 10 G, accelerating to the speeds of Voyager would take many minutes. Any passengers would be long dead before this speed could be reached. This should not be considered much of a restriction. Accelerating at a rate of 1 G would still result in reaching our 5 per cent of light speed in just a matter of days, a small fraction of the total length of such a voyage. So what would we need to consider for such a project to be feasible? Even before we think of the technical requirements of the ship there is the simple matter of the crew. These voyages are far longer than a single human life span. So we either need to induce a form of hibernation among the crew and have them sleep through the voyage, or accept that the initial crew of the mission will be dead long before the ship reaches its destination, as will their children, grand children and several more generations. Scientists have recently been moving suspended animation from the realms of science fiction and into reality. In 2005 a team at the Fred Hutchinson Cancer Research Center in Seattle, Washington led by Mark Roth proved that Hydrogen Sulphide could be used to induce a hibernation-like response in mice, a species which does not hibernate in nature. Further experimentation is ongoing in larger species, including dogs and pigs, although it will probably be quite some time before this is a viable option for humans. In the meantime we are left with long term multi-generation missions. This type of long length mission will require a considerably larger crew than any space mission mankind has yet undertaken, as too small a group would not allow for sufficient genetic diversity to maintain a viable colony. The minimum accepted number is about 180 people, with a relatively even split between men and women. Although it has been proposed that to save weight and resources in the initial phases of the project a smaller all female crew could be chosen with frozen sperm replacing the need for men in the first generation. Closed Ecology Sustaining a population even as small as this over such a long term so far removed from Earth provides more problems to overcome. Human beings have rather basic needs—we require oxygen, water and food. We produce waste materials that poison our environment. In everyday life on Earth these are not things that concern most of us overly. The Earth is a vast ecosystem that is self-balancing (when we are not overtaxing its regenerative abilities with our industrial lifestyles). Microbes reprocess our waste, providing raw materials for the plants, which convert the energy of the Sun into a form we can eat providing the oxygen we breathe in the process. On short space missions astronauts and cosmonauts take all they need for their missions with them, their food, their water and the air they breathe. But as with fuel, the weight of sufficient supplies is prohibitive so the vessels must be self-sustaining, being capable of generating their own supplies. Essentially what is needed is a miniature Earth-like ecosystem. Many such systems have been set up on Earth. The Soviet Union built such environments in the 1960s and 1970s. The Americans built Biosphere 2, a three-acre enclosed ecological system built in Arizona in the late 1980s. It included its own ocean, rain forest, desert, and temperate zone. Eight people entered Biosphere 2 on September 26th 1991 and remained sealed inside for two years, despite problems including lower than desired agricultural produce. Problems in a subsequent experiment led to the project being ridiculed in the popular press and losing favour with the scientific community. The European Space Agency has since announced MELiSSA (Micro-Ecological Life Support System Alternative). This project is intended to develop the kinds of technology we will need for an Interstellar Generation Starship to sustain its inhabitants over many decades. Generations But beyond simple biology there is also the matter of the crew society. Anthropologists and psychologists believe that the traditional military structure of all current space missions and organisations will not be suitable for such multi-generation endeavours. Over such lengthy periods a more tribal or familial structure will be necessary, with the basic bonds between parents and offspring, brothers and sisters providing the glue to keep the society stable, and avoiding the anarchy portrayed in many science fiction tales featuring generation ships. If you have ever played Chinese Whispers you will be all too aware of just how passing a message between people can change its meaning with each person interpreting the message slightly differently. Over the course of generations this could be extreme. Instruction manuals might almost become religious texts and crew positions almost holy. Equally, the knowledge of how things work could easily become lost, with the people maintaining the systems becoming little more than mindless automata endlessly repeating instructions passed down to them by previous generations. To a large extent, then, it will be necessary to automate as many of these processes as is possible. Equally it is important to consider boredom. On Earth the range of activities in which one can participate is vast. In a closed environment this range is reduced. Thankfully , though, as technology advances, miniaturisation will allows these craft to include far more entertainment media without the need to substantially increase weight. Destination If we accept that all the hurdles in building such a ship have been overcome and a target world has been selected, there is still the matter of what happens when the ship arrives. It is unlikely that any planet out there will be exactly like Earth. It could be that it is close enough that hardy settlers would be able to adapt to their new home, but more likely the planet will need to be adjusted to suit Earth-type life forms. So, if we achieve the scientific advances necessary to reach a new Earth, converting this world to be suitable for human life—Terraforming—will then be the final hurdle we will need to overcome for the desire of a human colony world out there amongst the stars be achieved.
Story Copyright © 2007 by I. E. Lester. All rights reserved.
About the author
Having worked through all the fiction of Asimov (as well as Heinlein, Clarke, Moorcock, and many others) he moved onto Asimov's non-fiction, encouraging a love of science.
He studied Mathematics and Astrophysics whilst at University and works as a software designer. When not reading sf or factual science, he can often be found watching cricket or rugby, or wandering medieval streets in France or Italy.
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