Time was when the planet Mars looked like another Earth. The astronomer Schiaparelli, peering through his Milanese telescope at the celebrated conjunction of 1877, saw what he thought were “channels,” announced them as “canali” and had them understood as “canals” by half the literate population of Earth. Including nearly all the astronomers, who promptly turned their telescopes in the same direction and discovered more.

Canals? Then they must have been dug for a purpose. What purpose? To hold water — there was no other explanation that saved the facts.

The logic of the syllogism was compelling, and by the turn of the century there was hardly a doubter in the world. It was accepted as lore that Mars held an older, wiser culture than our own. If only we could somehow speak to them, what marvels we would learn! Percival Lowell mused over a sketching pad and came up with a first attempt. Draw great Euclidean shapes on the Sahara Desert, he said. Line them with brushwood, or dig them as trenches and fill them with oil. Then on some moonless night when Mars is high in the African heavens, set them afire. Those alien Martian eyes that he took to be fixed firmly to their alien Martian telescopes would see. They would recognize the squares and triangles. They would understand that communication was intended, and out of their older wisdom they would find a way to respond.

Not everybody believed quite as much quite so firmly as Lowell. Some said that Mars was too small and too cold ever to harbor a hugely intelligent race. Dig canals? Oh, yes, that was a simple enough peasant skill, and a race that was dying of thirst could well manage to scratch out ditches, even enormous ditches visible across interplanetary space, to keep itself alive. But beyond that, the environment was simply too harsh. A race living there would be like the Eskimos, forever trapped on the threshold of civilization because the world outside their ice huts was too hostile to grant them leisure to learn abstractions. No doubt when our telescopes were able to resolve the individual Martian face we would see only a brutish mask, stolid and stunned, brother to the ox; able to move soil and to plant crops, yes, but not to aspire to a life of the mind.

But, wise or brutal, Martians were there — or so thought the best opinion of the times.

Then better telescopes were built, and better ways were found to understand what they disclosed. To the lens and the mirror was added the spectroscope and the camera. In the eyes and understanding of astronomers Mars swam a little closer every day. At every step, as the image of the planet itself grew more sharp and clear, the vision of its putative inhabitants became more cloudy and less real. There was too little air. There was too little water. It was too cold. The canals broke up, under better resolution, into irregular blotches of surface markings. The cities that should have marked their junctions were not there.

By the time of the first Mariner fly-bys the Martian race, which had never lived except in the imagination of human beings, was irrevocably dead.

It still seemed that life of a sort could exist, perhaps lowly plants, even a rude sort of amphibian. But nothing like a man. On the surface of Mars an air-breathing, water-based creature like a human being could not survive for a quarter of an hour.

It would be the lack of air that would kill him most quickly. His death would not be from simple strangulation. He would not live long enough for that to happen. In the 10-millibar pressure of the surface of Mars his blood would boil away and he would die in agony of something like the bends. If he somehow survived that, then he would die of lack of air to breathe. If he survived both of those — given air in a backpack, and a face mask fed with a mixture of gases that did not contain nitrogen, at some intermediate pressure level between Earth- and Mars-normal — he would still die. He would die from exposure to unshielded solar radiation. He would die from the extremes of Martian temperature — at its best, a warmish spring day; at its coldest, worse than Antarctic polar night. He would die from thirst. And if he could somehow survive all of those, he would die more slowly, but surely, from hunger, since there was nowhere on the surface of Mars one morsel that a human being could eat.

But there is another kind of argument that contradicts the conclusions drawn from objective facts. Man is not bound by objective facts. If they inconvenience him, he changes them, or makes an end run around them.

Man cannot survive on Mars. However, man cannot survive in the Antarctic, either. But he does.

Man survives in places where he ought to die, by bringing a kinder environment with him. He carries what he needs. His first invention along those lines was clothing. His second, storable food, like dried meat and parched grain. His third, fire. His most recent, the whole series of devices and systems that gave him access to the sea bottoms and to space.

The first alien planet men walked on was the moon. It was even more hostile than Mars, in that the vital supplies of which Mars had very little — air, water and food — did not, on the moon, exist at all. Yet as early as the 1960s men visited the moon, carrying with them air and water and everything else they needed in life-support systems mounted on their spacesuits or built into their landing modules. From there it was no trick to build the systems bigger. It was not easy because of the magnitudes involved. But it was straightforward scaling up, to the point of semi-permanent and not far from self-sustaining closed-cycle colonies. The first problem of support was purely logistic. For each man you needed tons of supplies; for each pound of cargo blasted into space you spent a million dollars’ worth of fuel and hardware. But it could be done.

Mars is orders of magnitude more remote. The moon circles the Earth only a quarter of a million miles out. At its very closest, a few times in a century, Mars is more than a hundred times as far.

Mars is not only distant from the Earth, it is farther than Earth from the sun. Whereas the moon receives as much energy per square inch as the Earth does, Mars, by the law of inverse squares, gets only half as much.

From some point on Earth, a rocket can be sent to the moon at any hour of any day. But Mars and Earth do not circle each other; both circle the sun, and as they do so at different speeds they are sometimes not very close and sometimes very far. It is only when they are at minimal travel distances that a rocket can efficiently be sent from one to another, and those times occur only once in every period of two years, for one month and some weeks.

Even the factors in Mars’s makeup which make it more like the Earth work against maintaining a colony there. It is bigger than the moon, and thus its gravity is more like Earth’s. But because it is bigger and pulls harder a rocket needs more fuel to land on it, and more fuel to take off again.

What it all comes down to is that a colony on the moon can be supported from Earth. A colony on Mars cannot.

At least a colony of human beings cannot.

But what if one reshapes a human being?

Suppose one takes the standard human frame and alters some of the optional equipment. There’s nothing to breathe on Mars. So take the lungs out of the human frame, replace them with micro-miniaturized oxygen regeneration cat-cracking systems. One needs power for that, but power flows down from the distant sun.

The blood in the standard human frame would boil; all right, eliminate the blood, at least from the extremities and the surface areas — build arms and legs that are served by motors instead of muscles — and reserve the blood supply only for the warm, protected brain. A normal human body needs food, but if the major musculature is replaced by machines, the food requirement drops. It is only the brain that must be fed every minute of every day, and fortunately, in terms of energy requirements the brain is the least demanding of human accessories. A slice of toast a day will keep it fed.