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
Posts
No comments:
Post a Comment