Photo: Man wearing astronaut helmet and gloves

Astronaut gear, such as this helmet and gloves, must be airtight, resistant to external temperatures and air pressure, and comfortable enough to work in.

Photograph by Cary Wolinsky

Written by Sir Arthur Clarke

Republished from the pages of National Geographic magazine

One weekend in 1939, shortly before the outbreak of World War II, visitors to London's Science Museum might have come across a dozen young men peering through a contrivance of rotating mirrors at a spinning disk a few meters away. Hanging just above them was the 1903 Wright Flyer—but the experimenters had set their sights on something far more ambitious than flight in the atmosphere. They believed that one day it would be possible to travel to other worlds and were trying to design the navigational equipment needed for such voyages.

I am now the only survivor of that little group of British Interplanetary Society members. Though none of us could have guessed that the first moon landing was only three decades in the future, we were already concerned with the problems that might arise during the voyage. One of these—well known even to Jules Verne when he wrote From the Earth to the Moon in 1865—was that for most of the time the occupants of a spaceship would be weightless.

Because this condition cannot be reproduced on Earth for more than a few seconds, no one knew how the human body would react to it. Some horrifying scenarios had been predicted: One held that the heart would race uncontrollably in zero gravity, so that any foolhardy astronauts could expect a swift but merciful death.

However, there appeared to be a simple solution: Make the living quarters of the spaceship a slowly revolving drum, so that centrifugal force gave the occupants the sensation of weight, allowing them to walk on a cylindrical "floor." My late friend Stanley Kubrick showed this, on rather a lavish scale, in 2001: A Space Odyssey's orbiting Hilton hotel.

So we premature 1930s space cadets had designed a spinning spaceship, but how to observe the moon and stars if we were revolving several times a minute? Fortunately, astronomers had long ago solved this problem for the Earth, which revolves once a day, with an instrument known as a coelostat. This employs mirrors moving in such a way that the reflected sky appears stationary.

To demonstrate the British Interplanetary Society's considerably higher speed version, we used a spinning disk on which we had painted the letters "BIS." These were quite unreadable until one peered into the coelostat, where they appeared motionless. I am indeed happy to say that the society is still very active and is now the world's oldest organization devoted to space exploration.

As it turned out, our fears of weightlessness were much exaggerated, although there may be long-term effects about which little is yet known. Humans have now lived in space for longer than a year, and indeed some astronauts became so addicted to freedom from gravity that they were reluctant to return to Earth.

Weightlessness does have certain problems, however, and NASA's famous—or infamous—flying laboratory, the aptly named "Vomit Comet", has been used to study some of them. The plane's carefully controlled flight pattern can produce weightlessness for nearly half a minute. One of the great unsung moments in the conquest of space was when the Vomit Comet was used to test space-toilet design. The heroic volunteers had a mere 30-second window of opportunity…

It is fortunate that the only planets in the solar system with gravity greater than Earth's are the gas giants Jupiter and Neptune, which have no solid surfaces and so are unlikely targets for future explorers. On the Moon and Mars—the two most promising destinations—gravity is one-sixth and one-third the Earth's, so we would have a reassuring feeling of enhanced strength there. However, should permanent settlements be established, any children born on those worlds may never be able to risk visiting the home planet. In the centuries to come we may therefore see a form of gravitational segregation, as our species divides into various tribes, adapted to zero, fractional, and one gravity.

There is nowhere beyond the Earth, as far as we know, where we will find environments in which unprotected humans can survive. Sometime in this millennium we will confront this dilemma: Should we leave our planetary neighbors unaltered, or should we modify them and make them closer to the heart's desire? With technologies that have already been the subject of much study, "terraforming"—a word invented in the 1940s by science fiction writer Jack Williamson—would be feasible for some of the bodies in the solar system. Thus we might orbit giant solar mirrors to warm frozen Mars, or sunshades to cool torrid Venus. Environmental groups, rightly pointing out the mistakes we have made on this planet, will have much to protest in the exciting centuries to come.

Barring catastrophes—natural or manmade—the third millennium will be the real age of space. Sooner or later today's hopelessly inefficient rockets (with payloads measured in fractions of a percent!) will be superseded by technologies that will make space travel no more expensive than atmospheric flight. The real cost, in terms of energy, of putting a human into orbit will be only a few hundred dollars, not the present millions of dollars. One way of approaching this target would be to use a "space elevator," which lift people into space using cables lowered from satellites in geostationary orbit, as I described in my novel The Fountains of Paradise.

What we may discover during the forthcoming exploration of the solar system will shape the future of humanity. And beyond the planets lies the inconceivably vaster universe of stars and galaxies. Though we may well be the only children of this particular sun, can we be egocentric enough to believe that self-styled H. sapiens is the single intelligent life-form the cosmos has produced in billions of years in trillions of worlds?

The truth is indeed out there, and one day we will find it—or it will find us. Then we will learn whether we are closer to the angels or the apes.

What's in National Geographic magazine this month? Find new articles, maps, photos, and more online.

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