In contrast to the low-gravitational environment that exists on Ceres, the gravitational acceleration in the outer atmosphere of Jupiter is 2.53 times greater than that experienced on the Earth. British engineer Paul Birch has suggested, therefore, that a vast honeycombed shell might be built around Jupiter at a stand-off distance of 42,000 km from the upper cloud deck. Such a suprajupiter structure, as Birch calls it, would have a surface gravity the same as that on Earth, and a surface area some 318 times larger than that of Earth. Here indeed, albeit in the form of a proxy surface, is a terraformed Jupiter, a vast world with a core full of energy and resources. Indeed, there is so much wealth in terms of surface area and energy within a suprajupiter system that Birch estimates it could support a population of some 200 billion people.
Smaller versions of Birch's suprajupiter have been proposed by engineer Kenneth Roy (The Ultimax Group Inc., Oak Ridge, Tennessee) and co-workers, who suggested at the Space Technology and Applications International Forum at Albuquerque in 2004 that shell worlds might be constructed around large asteroids and planetary moons. Such structures are similar in concept to the World-house idea advocated by Richard Taylor (and as discussed in Chapter 6), where the idea is to build a spherical roof around the parent body and establish a breathable atmosphere underneath it. In some sense, this is still spacecraft living, but it is at least living large. In keeping with the notion espoused by Gerard O'Neill that, ''At least some of the settlers in space will model their cities and villages on the prettier areas of old Earth,'' the shell worlds, according to Roy, will have surfaces designed to look and feel-like our home world. Indeed, the idea appears to be that one might only tell a manufactured world from the Earth by the lower gravity that will be experienced on the smaller, less-massive shell worlds.
The issue of what spaceship colonies, shell worlds, and supra-mundane planets might look like to their initial inhabitants and future citizens is a topic that has drawn some impassioned debate over the years, with both extremes of the possibilities being argued. Some believe that the new worlds should be doppelgangers (even clones) of the Earth, while others promote the idea of ''new world, new outlook,'' with functionality, or perhaps more to the point, safety being the only design constraint. While the interior design and color scheme of shell worlds, space colonies, and supramun-dane planets is a problem for others to consider, there are some properties of these potential habitats that are truly constrained by the requirement that the environments must support human life. The atmosphere cannot be just any old collection of gases; surface gravity cannot be just any value; the day/night cycle cannot be just any combination of hours.
In what has become a classic reference source (albeit a little dated now) on the conditions for planetary habitability is the report prepared by Stephen Dole (of the Rand Corporation) in 1964 for the US Air Force. Entitled Habitable Planets for Man, Dole comments in his introduction that the ''central purpose of this book is to spell out the necessary requirements of planets on which human beings as a biological species (Homo sapiens) can live.'' Among the key issues that Dole considers for habitability are:
• atmospheric pressure and composition
• surface gravity
• temperature variations (diurnal and annual).
If an atmosphere is to be breathable by humans then it must contain oxygen and it must provide a minimum surface pressure. For an atmosphere providing 1 bar (105 Pa) surface pressure, the same as the Earth's, the percentage volume of oxygen must be greater than about 10%, or else hypoxia will result, and it must be less than about 70%, or oxygen toxicity will ensue. In addition, to avoid catastrophic fires from being ignited the volume percentage of oxygen must not exceed 25% of the total. For a surface pressure of 0.5 bar (5 x 104 Pa) the volume percentage of oxygen must exceed 20% to avoid hypoxia, but there is no upper limit with respect to toxicity. If one can control the flammability constraint, the lowest-possible pressure for a breathable pure oxygen atmosphere is about 0.14 bar (1.4 x 104 Pa).
Nitrogen is the dominant gas in the Earth's atmosphere, accounting for 78.08% of the volume (oxygen accounts for 20.95% of the volume; see Table 5.2), and provided its partial pressure is less than about 3 bar (3 x 105 Pa), then it won't be narcotic. Important as well, the nitrogen also acts a fire inhibitor, and it is hence a vital component to a nurturing breathable atmosphere.
The surface pressure (PS) conditions for a breathable atmosphere places very specific conditions upon the mass of the atmosphere (Matm) and surface gravity (g), since Ps = g (Matm / 4 p R2), where R is the radius. Since the oxygen and nitrogen will need to be either mined or manufactured, the smaller the amount of material required to produce an atmosphere the better. Most of the surface pressure will have to come from the atmospheric mass, since there is an upper limit to the surface gravity under which humans can work comfortably. Indeed, Dole argues that the upper limit is of order 1.25-1.5 times the Earth's surface gravity.
It has already been made clear that the presence of liquid water is vital to human survival. On a terraformed world, therefore, a mean temperature that is above the freezing point of water is clearly required. In artificial worlds, liquid water need not necessarily be present on the surface (it must, of course, be available), but the typical temperature still needs to be above zero in order for humans to feel comfortable in their surroundings. Dole notes, for example, that the majority of people on Earth live in those regions where the annual temperature variation falls between about 5°C and 27°C. Although there are also geographical and climate conditions that apply to this majority distribution of the populace, the temperature range seems a reasonable one to aim for on any new world. The human body can certainly withstand a greater range of temperatures than the annual variation just given, but exposure to temperatures lower than about -10°C for more than 1 day will result in hypothermia. Temperatures warmer than about 35°C for more than 1 day will further result in hyperthermia. For temperatures outside of these extremes, protective clothing of one sort or another will need to be worn, and this will severely limit everyday new-world activity and life.
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