I am sending this message to Earth in an attempt to warn them about what I feel sure is going to happen, and what must happen. It is sad to think of what lies ahead, so no one wants to talk about it, but someone should, as the people of Earth ought to be prepared.

It is the latter half of the twenty-first century and there are a dozen Settlements in orbit about the Earth. Each is, in its way, an independent little world. The smallest has ten thousand inhabitants, the largest almost twenty-five thousand. I'm sure that all Earthmen know this, but you people are so entangled in your own giant world, that you rarely think of us except as some little inconsequential objects out in space. Well, think of us.

Each Settlement imitates Earth's environment as closely as it can, spinning to produce a pseudo- gravity, allowing sunlight to enter at some times, and not at others, in order to produce a normal day and night. Each is large enough to give the impression of space within, to have farms as well as factories, to have an atmosphere that can give rise to clouds. There are towns, and schools, and athletic fields.

We have some things that Earth has not. The pseudogravitational field varies in intensity relative to position within each Settlement. There are areas of low gravity, even zero gravity, where we can outfit ourselves with wings and fly, where we can play three-dimensional tennis, where we can have unusual gymnastic experiences.

We also have a true space culture, for we are used to space. Our chief work, aside from keeping our Settlements running efficiently, is to build structures in space for ourselves and for Earth. We work in space, and to be in a spaceship or a spacesuit is second nature to us. Working at zero gravity is something we have done from childhood.

There are also some things Earth has that we do not. We don't have Earth's weather extremes. In our carefully controlled Settlements, it never gets too hot or too cold. There are no storms and no unarranged precipitation.

Nor do we have Earth's dangerous terrain. We have no mountains, no cliffs, no swamps, no deserts, no stormy oceans. And we have no dangerous plants, animals, or parasites. If anything, there are some among us who complain that we are too secure, that there is no adventure-but then our people can always go out into space, and make long trips to Mars and to the asteroids, which you Earthpeople are psychologically unfit to do. In fact, there are plans by some Settlers to set up colonies on Mars and mining bases in the asteroid belt, but it may never come to that, for reasons I will describe.

The Settlements did not spring on humanity unawares. Even a century ago, Gerard O'Neill of Princeton and his students were making initial plans for such new homes for humanity, and science fiction writers had anticipated it even before that.

Oddly enough though, the difficulties that most foresaw turned out to be not those that plagued the Settlements. The expense of building them, the problems of providing an Earth-like environment, the gathering of energy, the matter of protection against cosmic rays were all solved. It was not done easily, but it was done.

The Sun itself supplies all the energy we need, and enough more to export some to Earth. We can grow food easily-more than we need, in fact, so that we can export some to Earth. We have small animals-rabbits, chickens, and so on, that can supply us with meat. We get what material we need from space, not only from the Moon, but from meteoroids and comets that we can trap and exploit. Once we reach the asteroids (if we ever do) we will have a virtually unlimited supply of everything we need.

What bothers us and produces an insuperable problem is something that few people foresaw. It is the difficulty of keeping up a viable ecology. Each Settlement must support itself. It contains people, plants, and animals; it contains air, water, and soil. The living things must multiply and maintain their numbers, but not outpace the ability of the Settlement to support them.

The plants and animals? Well, we control them. We supervise their breeding and we consume any excess. Maintaining the human population at a reasonable level is more difficult. We cannot allow human births to outstrip human deaths, and we keep the number of deaths as low as possible, of course. This makes our culture a nonyouthful one compared to Earth's. There are few youngsters and a large percentage of those mature and postmature. This produces psychological strains, but there is the general feeling among Settlers that those strains are worth it, since with a carefully controlled population, there are no poor, no homeless, and no helpless.

Again, the water, air, and food must be carefully recycled, and much of our technology is devoted to the distillation of used water, and to the treatment of solid bodily wastes and their conversion to clean fertilizer. We cannot afford to have anything go wrong with our recycling technology, for there is little room for slack. And, of course, even when all goes well, the feeling that we eat and drink recycled materials is a bit unpalatable. All is recycled on Earth, too, but Earth is so large and the natural cycling system so unnoticeable, that Earthpeople tend to be unaware of the matter.

Then, too, there is always the fear that a sizable meteor may strike and damage the outer shell of a Settlement. A bit of matter no larger than a piece of gravel might do damage, and one a foot across would surely destroy any Settlement. Fortunately, the chances for such a misadventure are small and we will eventually learn to detect and divert such objects before they reach us. Still, these dangers weigh upon us, and help mitigate the feeling of over-security that some of us complain about.

With an effort, however, with close attention and unremitting care, we can maintain our ecology, were it not for the matter of trade and travel.

Each Settlement produces something that other Settlements would like to have, in the matter of food, of art, of ingenious devices. What's more, we must trade with Earth as well, and many Settlers want to visit Earth and see some of the things we don't have in the Settlements. Earthpeople can't realize how exciting it is for us to see a vast blue horizon, or to look out upon a true ocean, or to see an ice-capped mountain.

Therefore, there is a constant coming and going among the Settlements and Earth. But each Settlement has its own ecological balance; and, of course, Earth has, even these days, an ecology that is enormously and impossibly rich by Settlement standards.

We have our insects that are acclimated and under control, but what if strange insects are casually and unintentionally introduced from another Settlement or from Earth?

A strange insect, a strange worm, even a strange rodent might totally upset our ecology, inflict damage on our native plants and animals. On numerous occasions, in fact, a Settlement has had to take extraordinary measures to eliminate an unwanted life-form. For months every effort had to be taken to track down every last insect of some species that, in its own Settlement, is harmless, or that, on Earth, can keep its depredations local.

Even worse, what if pathogenic parasites-bacteria, viruses, protozoa-are introduced? What if they produce diseases against which another Settlement and, of course, Earth itself, have developed a certain immunity, but one against which the Settlement that suffers the invasion is helpless. For a while, the entire effort of the Settlement must go into the preparation or importation of sera designed to confer immunity, or to fight the disease once it is established. Deaths, of course, occur invariably.

Naturally, there is always an outcry when this happens and a demand for more controls. As a result, no one from another Settlement, and no one returning to his own Settlement from a trip elsewhere, can be allowed to enter without a complete search of his baggage, a complete analysis of his bodily fluids, and a certain period of quarantine to see if some undetected disease is developing.

What's more, rightly or wrongly, the inhabitants of the Settlements persist in viewing Earthpeople themselves as particularly dangerous. It is on Earth where the most undesirable life-forms and parasites are to be found; it is Earthpeople who are most likely to be infested, and there are parties on all the Settlements who support the notion-sometimes quite vehemently-of breaking all contacts between the Settlements and Earth.

That is the danger of which I want to warn Earthpeople. Distrust-and even hatred-of Earthpeople is constantly growing among the Settlers.

As long as Earth is only a few tens or hundreds of thousands of miles away, it is useless to talk of breaking off all contact. The lure and attraction of Earth is too great. Therefore, there is now talk-it is only a whisper, so far, but it will grow louder, I assure you-of leaving the Solar system altogether.

Each Settlement can be outfitted with a propulsive mechanism, making use of microfusion motors. Solar energy will suffice us while we are still among the planets and we will pick up small comets as a source of hydrogen fuel, in the process of leaving all the planets behind, when the Sun becomes too distant to be of use to us.

Each Settlement will say good-bye to Earth, then, and launch itself as an independent world into the unimaginable wastes between the stars. And who knows, someday a million years hence a Settlement may find an Earth-like world, empty and waiting, that it can populate.

But that is what I must warn Earth of. The Settlements will someday leave, and if you build others, they will eventually leave, too, and you will be left alone. And yet, in a way, your descendants will be expanding into, and populating, the entire Galaxy. You may find that a consoling thought as you watch them disappear.