michaelm@3comvax.UUCP (05/16/86)
THE WORLD, THE FLESH, AND THE DEVIL Freeman J. Dyson Institute for Advanced Study Princeton, New Jersey IV. Big Trees I have spoken about the two first steps of biological engineering. The first will transform our industry and the second will transform our earth-bound ecology. It is now time to speak of the third step, which is the colonization of space. I believe in fact that biological engineering is the essential tool which will make Bernal's dream of the expansion of mankind in space a practical possibility. First I have to clear away a few popular misconcpetions about space as a habitat. It is generally considered that planets are important. Except for Earth, they are not. Mars is waterless, and the others are for various reasons basically inhospitable to man. It is generally considered that beyond the sun's family of planets there is absolute emptiness extending for light years until you come to another star. In fact it is likely that space around the solar system is populated by huge numbers of comets, small worlds a few miles in diameter, rich in water and the other chemicals essential to life. We see one of these comets only when it happens to suffer a random perturbation of its orbit which sends it plunging close to the sun. It seems that roughly one comet per year is captured into the region near the sun, where it eventually evaporates and disintegrates. If we assume that the supply of distant comets is sufficient to sustain this process over the thousands of millions of years that the solar system has existed, then the total population of comets loosely attached to the sun must be numbered in the thousands of millions. The combined surface area of these comets is then a thousand or ten thousand times that of Earth. I conclude from these facts that comets, not planets, are the major potential habitat of life in space. If it were true that other stars have as many comets as the sun, it then would follow that comets pervade our entire Galaxy. We have no evidence either supporting or contradicting this hypothesis. If true, it implies that our Galaxy is a much friendlier place for interstellar travelers than it is popularly supposed to be. The average distance between habitable oases in the desert of space is not measured in light years, but is of the order of a light day or less. I propose to you then an optimistic view of the Galaxy an an abode of life. Countless millions of comets are out there, amply supplied with water, carbon, and nitrogen, the basic constituents of living cells. We see when they fall close to the sun that they contain all the common elements necessary to our existence. They lack only two essential requirements for human settlement, namely warmth and air. And now biological engineering will come to our rescue. We shall learn how to grow trees on comets. To make a tree grow in airless space by the light of a distant sun is basically a problem of redesigning the skin of its leaves. In every organism the skin is the crucial part which must be most delicately tailored to the demands of the environment. The skin of a leaf in space must satisfy four requirements. It must be opaque to far- ultraviolet radiation to protect the vital tissues from radiation damage. It must be impervious to water. It must transmit visible light to the organs of photosynthesis. It must have extremely low emissivity for far-infrared radiation, so that it can limit loss of heat and keep itself from freezing. A tree whose leaves possess such a skin should be able to take root and flourish upon any comet as near to the sun as the orbits of Jupiter and Saturn. Farther out than Saturn the sunlight is too feeble to keep a simple leaf warm, but trees can grow at far greater distances if they provide themselves with compound leaves. A compound leaf would consist of a photosynthetic part which is able to keep itself warm, together with a convex mirror part which itself remains cold but focuses concentrated sunlight upon the photosynthetic part. It should be possible to program the genetic instructions of a tree to produce such leaves and orient them correctly toward the sun. Many existing plants possess structures more complicated than this. Once leaves can be made to function in space, the remaining parts of a tree -- trunk, branches, and roots -- do not present any great problems. The branches must not freeze, and therefore the bark must be a superior heat insulator. The roots will penetrate and gradually melt the frozen interior of the comet, and the tree will build its substance from the materials that the roots find there. The oxygen which the leaves manufacture must not be exhaled into space; instead it will be transported down to the roots and released into the regions where men will live and take their ease among the tree trunks. One question still remains. How high can a tree on a comet grow? The answer is surprising. On any celestial body whose diameter is of the order of ten miles or less, the force of gravity is so weak that a tree can grow infinitely high. Ordinary wood is strong enough to lift its own weight to an arbitrary distance from the center of gravity. This means that from a comet of ten-mile diameter, trees can grow out for hundreds of miles, collecting the energy of sunlight from an area thousands of times as large as the area of the comet itself. Seen from far away, the comet will look like a small potato sprouting an immense growth of stems and foliage. When man comes to live on the comets, he will find himself returning to the arboreal existence of his ancestors. We shall bring to the comets not only trees but a great variety of other flora and fauna to create for ourselves an environment as beautiful as ever existed on Earth. Perhaps we shall teach our plants to make seeds which will sail out across the ocean of space to propagate life upon comets still unvisited by man. Perhaps we shall start a wave of life which will spread from comet to comet without end until we have achieved the greening of the Galaxy. That may be an end or a beginning, as Bernal said, but from here it is out of sight.