Extraterrestrial Civilizations - Isaac Asimov [106]
The nearest planet to Earth in the direction away from the Sun is, of course, Mars. It is, at its closest, some 58 million kilometers (36 million miles) away, closer than any other planet but Venus. Traveling Mars-ward means steady progress in the direction of decreasing intensities of Solar radiation. Furthermore, Mars is a cold world that can be explored for indefinite periods even with the Sun in the sky (provided there is some protection against the Solar ultraviolet other than Mars’s thin and ineffective atmosphere).
Nevertheless, the round trip to Mars would certainly take more than a year of travel time. Even though that will be broken, for a shorter or longer time, by a landing on a planet that next to Earth itself is the most comfortable in the Solar system, the task would surely stretch human endurance to the limit.
And beyond Mars? To reach the larger asteroids, or the satellites of the giant planets, would mean crossing the much greater spatial gaps of the outer Solar system, and the voyages would take years and even decades one way. Manned voyages of such lengths do not seem practical at the moment.
Beyond the Moon, then, we are left with only Mars as a sizable target and that only as a borderline possibility.
SPACE SETTLEMENTS
In a practical sense, then, our initial triumphs in space do not seem to count for much. It looks as though we will be confined to the Earth-Moon system for the foreseeable future.
That may be true, however, only because I have been assuming so far that Earth itself is the base to be used for space exploration. Is there an alternative?
If we are to be confined to the Earth-Moon system, it would seem that the Moon is the only possible alternative. Suppose we establish an elaborate base on the Moon, one where it is possible to build space vessels and gather fuel. The Moon has a much smaller escape velocity than Earth, so it would take considerably less energy for a launching from the Moon than from the Earth. There would be more energy left for acceleration and deceleration, so the time lapse for a given trip would be smaller. It would not be sufficiently smaller, however, to make the trips practical.
But wait. Because we, and all the life forms we know, live on the surface of a world, we have a natural tendency to find anything else unnatural. In 1974, the American physicist Gerard Kitchen O’Neill (1927–) suggested the alternative of artificial settlements for human beings in space. It was not an altogether new concept and had been used in science fiction on occasion, but it had never before been put forward in such careful detail.
O’Neill even suggested two places as bases for humanity; places that were not on the Moon, but were just as far as the Moon is from Earth.
Imagine the Moon at zenith, exactly overhead. Trace a line against the sky due eastward from the Moon down to the horizon. Two-thirds of the way along that line, one-third of the way up from the horizon, at a distance equal to that of the Moon, is one of those places. Trace another line westward from the Moon down to the horizon. Two-thirds of the way along that line, one-third of the way up from the horizon, at a distance equal to that of the Moon, is another of those places.
Put an object in either place, and it will form an equilateral triangle with the Moon and Earth. It is 384,400 kilometers (238,900 miles) from the Moon to the Earth. It is that same distance from either point to the Moon, or from either point to the Earth.
What is so special about those places? Back in 1772, the Italian-French astronomer Joseph-Louis Lagrange (1736–1813) showed that in those places any small object would remain essentially stationary with respect to the Moon. As the Moon moved about the Earth, any small object in either of those places would also move about the Earth in such a way as to keep step with the Moon. The competing gravities of Earth and Moon would keep it where it was.
If the small object were not exactly in the place, it would wobble