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.