CHAPTER 7D

SPACE EXPLORATION IN THE LONG RUN

How far can the human horizon in space expand? Space enthusiasts should not assume that this is merely an incremental matter. Let me consider first the possibility of colonizing the solar system, and then of expanding into the galaxy.

At Home in the Solar System

Colonizing the solar system requires that we solve the serious problem of prolonged exposure to radiation. Better shields, faster spaceships, and piles of dirt on top of our outposts should help a great deal. Better shields are within our technological means, and piling dirt on top of, say, inflatable dwellings should not present any major obstacles. Faster spaceships have been on the drawing board for a long time: fission rockets, fusion rockets, ion propulsion rockets, laser-propelled rockets, and even solar sails. Fission rockets and ion propulsion rockets would use tried-and-true technologies. Ion propulsion, for example, works on the principle of particle accelerators: you take a charged particle and you accelerate it by means of electromagnetic coils; the particle picks up a very large exhaust velocity and, by Newton’s Third Law, it propels the rocket forward. The solar sail would use the solar wind to move around the inner solar system with great ease and at great speeds. It would consist of very large sails made of very thin sheets of metal that would move under the pressure of the solar wind. The Planetary Society, an organization of space enthusiasts, has arranged with the Russian space program for a deployment in space of such a device in the near future. All these three systems could in principle reduce a trip to Mars from about a year, on the average, to perhaps as little as three months or less.

We also need to solve the physiological problems created by microgravity. Perhaps the best solution is simply to create what is sometimes called "artificial gravity", which is not more than an application of the equivalence between acceleration and gravitational attraction. If a ship accelerates at a rate of approximately 10m/sec2, which is the value of 1 g, a passenger would feel as if he were on the surface of the Earth (a favorite illustration is of an elevator that ascends to the heavens unbeknownst to an unsuspecting passenger; as the elevator accelerates, its floor comes up to push against the passenger's feet, which to him feels as if he were being drawn towards the floor).

Another way to achieve the same result is to construct very large ships or habitats that rotate. At the appropriate rate of rotation someone on the ground floor -- next to the outer shell -- would experience a centrifugal acceleration equivalent to the Earth's gravity. The reason this technique is not used on present spaceships is that, since they are relatively small, they would have to rotate extremely fast, which would subject the astronauts to coriolis forces (if you are an astronaut, your head would feel a certain acceleration and your stomach another). Those effects, in addition to the extremely fast rotation may play havoc with the control of the ship. Large structures on the scale of Gerard O'Neill's space colonies would not have such problems, but using a space colony as a spaceship would require seemingly prohibitive amounts of energy, at least in the near future. The most reasonable alternative would be to connect the spaceship to a cargo module by a very long tether, and to have the two rotate around their common center of gravity. If the distance between the space ship and the axis of rotation is 200 meters, for example, we would have the equivalent acceleration that a gigantic spaceship or a small space colony might enjoy.

The first colonies on the Moon, on Mars, and on the moons of the outer planets would be dedicated mainly to the scientific exploration of those worlds, but also to determining the feasibility of even bigger settlements. As our presence grows so will the opportunities for space science (although I am sure there will be drawbacks on occasion), but I presume that another transformation of great consequence will take place: if the solar system becomes our home, then all the arguments about the serendipity of space exploration will gain additional force. Surely the change in our views will not be about alien worlds far away --it will affect our understanding of our new habitat. Once we are up there in force, the panorama of scientific, technological, industrial, and social opportunities may seem vastly different from what we may imagine today.

As those colonies grow, they will transform their extraterrestrial environments. The point may come when we engage in the science, or art, of "terraforming," which at its most grandiose envisions transforming entire planets and giant moons into habitable worlds. If a world does not have enough water, for example, we bring to it one of the thousands of small asteroids from the Kuiper belt composed mainly of water. Crashing the asteroid into that world would also help to increase the density and raise the temperature of the atmosphere. In another world, Venus for instance, that receives too much sunlight, we could place a cloud of dust around it, and then manipulate its atmosphere to reduce its greenhouse effect considerably.

Decades, probably centuries, will pass before we understand planet dynamics well enough to attempt to transform any world into a second Earth. In any event, we need remember that the initial justification for setting up human habitats in places like Mars is the possibility for the profound transformation of our planetary science. In the case of Mars, the most compelling reason is the possibility of transforming our understanding of human life by comparing it with Martian life, or at least with the fossils of Martian life. But all the other “Earth” sciences are likely to profit as well – geophysics, climatology, etc. Thus it would be highly irresponsible to begin to terraform Mars before we learn from it what it has to teach us. Indeed, we will need to learn many lessons from Mars, and Venus, and Titan, before we can begin to feel confident that our terraforming theories will be worth undertaking on them or on any other world.

Speculations of this sort are not only entertaining, they bring up the possibility that the limits to human expansion into the cosmos may be very far from our home planet, perhaps very far from the solar system. To that possibility I now turn.

The solar system as a laboratory

Chapter 4O

The solar system as a laboratory

We often extrapolate with profit the bits and pieces we learn about the Earth to our understanding of other planets. Ideas about aspects of the Earth do serve as the basis for hypotheses about what we may find elsewhere. But this ability only strengthens our dependence on planetary science. For since those ideas about the Earth are also ideas about how a particular planet behaves, their worth is often appreciated only by seeing how they apply to other planets. Thus the solar system provides us both with an appropriate context in which to interpret our observations of the Earth and with valuable opportunities to test our ideas about the Earth.

Furthermore, in order to understand the evolution of the Earth, we need to know what factors originate and shape planetary environments. But it is difficult to determine the range of those factors when we only see them in operation on the Earth. Although, as we saw earlier, we can carry out some experiments on the Earth's weather (and perhaps on its geophysics) the range of global controlled experiments is likely to be very restricted for a long time to come. That is a problem, for in science when we have an idea we want to see how it works, we want to look at it from different angles. Fortunately for us, the solar system can take the place of the laboratory. If we want to know how the mass of a planet influences its tectonics, we look at several planets with a variety of masses and examine their tectonics. We cannot vary the global conditions of our planet at will. But we can look at other worlds in which those variations occur naturally and see how other factors are correlated with them.

On Earth, the oceans provide a buffer for heat and supply most of the water vapor in the atmosphere. What other influences are there on planetary weather? The study of planets without oceans begins to answer that question. How much of a factor is the planet's rotation? Jupiter, with its gigantic, three-layered atmosphere, rotates every ten hours. In Venus, by contrast, the day is the equivalent of 243 Earth days (the atmosphere, however, rotates 60 times faster than the planet). Computer models of weather systems that control for this and other factors have been tested by the actual performance of the atmospheres of the other planets. This gives us to some degree the analog of manipulating our own atmosphere to test our ideas about the Earth's weather.

Atmospheric density is another important influence on a planet's weather. On Venus, which has a very dense atmosphere (about ninety times that of Earth), the temperature variations are minimal, 10 to 20 degrees Celsius, between poles and equator. This characteristic surely contributes to the absence of some of the weather patterns familiar on Earth. On the other hand on Mars the atmosphere is very thin, less than one-hundredth that of Earth. Apart from making practically impossible the presence of liquid water on the surface, the thin atmosphere does not provide a barrier to break the atmospheric waves. Whereas on the Earth atmospheric waves encounter resistance and form eddies, on Mars they can run their course, so to speak. The result is that on Mars the weather patterns are far more regular, repeatable, and periodic than on the Earth. Since the structures that organize the Earth's weather are not periodic, our weather is very difficult to predict. Nevertheless we can hope that the comparative study of planetary atmospheres will continue to identify the factors that contribute to the behavior of our own weather.

The point is not that comparative planetology guarantees better weather forecasting, though it might, but that in giving us a much wider experience with weather systems it may transform our views about weather. It is the transformation of those views that may provide a great impetus, some day, to much improved weather forecasting. And similar remarks may apply in the case of improvements in our knowledge of tectonics with respect to earthquake prediction. It may of course turn out that all the knowledge in the universe would not suffice to improve the prediction of weather and earthquakes past a certain level of precision. But the improvements may be great before we reach that level. A strategy that aims to give us the deepest understanding of these phenomena and the limits of their predictability is surely the best one to follow. That is what comparative planetology offers.

The transformation and fine-tuning of our views about the Earth is by no means the only benefit likely to accrue from the exploration of the solar system. We will also increase the precision with which we can observe and predict a variety of environmental states. This increase will come, partly because we will be better able to specify relevant factors and parameters of the global environment, as a result of our comparative study of the gravitation, magnetism, atmospheres, morphology, topography, geology and chemistry of the Earth and other planets; and partly because of the advances in the technology necessary to carry out that comparative study. At the present time, for example, lasers that bounce off satellites can be used to measure the movement of the tectonic plates and the vibrations of the ground around volcanoes. In this and many other new ways, the scientific exploration of the solar system will make it easier for us to ask more fruitful questions and secure more precise answers in our quest to understand and monitor the world to which we were born.[1]

Moreover, we have also seen that when trying to understand our own planet we often are unaware of many of the crucial factors that affect it. That is so because those factors often are the result of trends and forces that have developed sometimes for billions of years, or because they are hidden from our view or caused by interactions with the rest of the solar system. So we make up theories to guide our thinking. But those theories have little contact with the results that might shape them in fruitful directions if we limit ourselves to direct observations of the Earth. A generation ago we had observed up close only one planet: the Earth. Today we have examined over forty planetary bodies in the solar system. Those other bodies give us the opportunity to test the mettle of our ideas about our own world. The inevitable transformation of those ideas is therefore a transformation of our understanding of the Earth.

It is perhaps sensible to adopt a posture of skepticism toward the sensational predictions of catastrophe. The world has never suffered a scarcity of doomsayers. But it may well be that humans do have an increasingly greater impact upon the environment. And it also seems that the present rate of change is higher than what would have been brought about by other natural processes. The evidence is inconclusive but enough to make it at least prudent to look into these matters. The Earth may have seen greater changes in its past than we are liable to inflict upon it; but we should not rest assured that we will endure whatever we carelessly bring about.

We are rather in the position of a blind man in a china shop. If he moves he loses; if he does not move he loses too. Neither recklessness nor paralysis is to be recommended. To avoid them both he needs to know what the shop is like and how he can move about in it. To give him sight would be the greatest gift. The Earth is our china shop, and the satisfaction of our curiosity through space science can help us see where we are going. Wisdom requires that we accept that gift.

[1] One interesting illustration of this point can be found, once again, in the environmental problem of CO₂. Most of the dire scenarios include the melting of the polar caps. But determining how the caps would melt needs at least two kinds of investigations: (1) a way of measuring changes in the ice cap that can be correlated with increases in global temperature, and (2) a general theory about the formation and evolution of ice caps that will allow us to infer trends from such measurements. Fortunately space exploration has given us the means to take accurate measurements that would be practically impossible otherwise: polar satellites that make the precise comparisons needed for a fine determination of changes in the ice caps.