THE DREAM OF CENTURIES will soon be fulfilled when intelligent beings walk the deserts of the fourth planet from the sun. They will be the first Martians, and they will be us.

Manned exploration of Mars, long the fondest hope of space visionaries and calculating engineers, will probably be attempted within the next 20 years by Americans, Russians or both. This will be the first step in the inevitable colonization of the most hospitable world in the solar system for us apart from Earth.

The question is, how long will the colonists be content to live there in sterile cocoons? When will the yearning to remake Mars in Earth's image become irresistible? Will people someday bask lightly clothed on the beaches of a new Earth -- a radically transformed Mars?

"Terraforming" planets to be habitable by Earthlings has long been a staple of science-fiction stories. The word was first used by Jack Williamson in two science fiction novels in the 1940s, but the concept is as old as Utopia. Within the last two decades, the idea has acquired a new respectability as planetary scientists, biologists and engineers have begun to study terraforming -- or "planetary engineering" -- in detail.

While some of these studies at first may have been treated as mere experiments to exercise scientific cunning, their conclusions have increasingly pointed to the real possibility of altering the most opportune world -- Mars -- starting sometime in the next two centuries. There are many uncertainties, not the least of which are the ethical questions surrounding this undertaking.

An irate reader may be forgiven his reaction: "Don't we have enough problems with Earth's own environment without charging off to inflict our ignorance on other worlds?" To which space scientists answer that only by understanding the dynamics of other ecospheres can we hope to fathom the workings of spaceship Earth. (This assumes there is no indigenous life on Mars. Scientists would consider disrupting an existing Martian ecology unethical.)

In imagining the process by which a planetary atmosphere could be transformed to our needs, we come to grips with natural and culturally induced forces that may alter our own biosphere for better or for worse. We may decide never to remake another world, yet we will have learned much about the intricate network of cycles which have sustained life on this planet for billions of years in the face of potentially catastrophic insults.

If we do remake a world, which will it be? Astronomers for a long time thought that cloud-shrouded Venus might be a habitable twin of Earth, perhaps replete with tropical rain forests and graced with life- sustaining oceans. In the early 1960s the veil was pulled off the second planet from the sun by the U.S. Mariner-2 fly-by mission. Erstwhile Aphrodite was revealed to be Dante's Inferno.

Temperatures on the Venusian surface were over 460 degrees Celsius -- more than enough to melt lead. Moreover, any living intruder unwise enough to descend to the surface would be crushed by an atmosphere -- predominantly carbon dioxide -- with 100 times Earth's sea-level pressure. This on a planet roughly the same size as Earth and only 30 percent closer to the sun.

Inklings of Venus's hellish nature came even before this first successful mission to a planet. In a 1961 paper in Science, a young Carl Sagan assumed Venus had a hot greenhouse of an atmosphere, and suggested concrete steps to terraform her. Venus's upper atmosphere could be seeded, he suggested, with a particular strain of microscopic algae that would split the carbon dioxide into oxygen and residues of carbon that would fall to the surface. Rains would come and over hundreds of years the atmosphere would moderate in temperature and presssure, paving the way for life on the surface.

That suggestion misjudged the mind-boggling difficulty of transforming Venus. Not only were the pressures and temperatures found to to be greater than anticipated, but Venus was discovered to have a "day" equivalent to 243 Earth days. For a planet with a year of 225 Earth days, this meant that one face of the planet was oriented toward the sun for a substantial period. In this extreme condition, atmospheric circulation could be nothing like that of Earth.

Airless Mercury revolves in its orbit closer to the sun than Venus and is an even less likely candidate for terraforming by foreseeable technology. It, too, has a day that is a substantial fraction of its year -- 59 and 88 Earth days respectively.

Earth's moon may in the far future glisten with oceans and cloud cover, but these will have been imported from other bodies in the solar system -- perhaps comets. The 2,160-mile-diameter moon, with its surface gravity only one-sixth Earth's, will forever be a weakling in holding an atmosphere -- speeding molecules would rather leap into space than hover over Selene.

The so-called "gas giant" planets of the outer solar system -- Jupiter, Saturn, Uranus and Neptune -- are nothing like the terrestrial dust-mote planets nearer the sun. The giants have "surfaces" that would resemble nothing familiar -- slushy swamps or oceans of liquified gases would best describe them. Even with familiar land- like surfaces, the high atmospheric pressures would crush humans.

The warming light of the sun grows ever more dim as we journey outward. For each factor of two increase in distance there is a four-fold light reduction. Distant and tiny Pluto may be the gateway to the stars, but it is a dark and inhospitable outpost -- not a likely world to plant an oasis.

There are many substantial rocky moons surrounding the gas giant planets and some of these might yet be turned into new Earths. But far more appealing for the near term is the only reasonably warm world that is eminently accessible with current space technology -- Mars.

Through history Mars has been a magnet for the imagination of scientists, dreamers and poets. It is a superficially dry planet with a thin atmosphere and is only half the diameter of Earth. Yet it is Earth-like in a number of ways already. The surface of Mars matches almost exactly the area of Earth's continents. The Martian day is only a fraction of an hour longer than an Earth day and the present tilt of its rotation axis to the planet's orbit plane -- 25 degrees -- approaches Earth's 23.5 degrees, which gives it similar seasonal rhythms. Mars orbits the Sun in 687 Earth days but in an elliptical path more eccentric than Earth's, producing greater seasonal temperature variation than on Earth. Both planets have polar ice caps, though in the case of Mars much of the southern cap is solid carbon dioxide or "dry ice" while the northern cap is mainly water ice.

The surface gravity on Mars is a comfortable one- third Earth gravity, enough to hold a substantial atmosphere, if one were created. Unfortunately, the surface atmosphere pressure on Mars now is a tiny 0.6 percent of Earth's surface pressure and is composed of 96 percent carbon dioxide, 2.6 percent nitrogen, less than 2 percent argon, a minuscule 0.03 percent water vapor, and almost no oxygen. Earth's atmosphere is almost 80 percent nitrogen with the balance as oxygen and only minor amounts of other gases.

Life on Earth is protected from killing solar ultraviolet radiation by a high-altitude layer of ozone gas (molecules with three atoms of oxygen). Mars lacks this shield. Martian surface temperatures range from minus-143 Celsius to occasional balmy equatorial highs of 27 degrees Celsius (80 Fahrenheit). The average temperature is 60 degrees Celsius (108 Fahrenheit) lower than Earth -- a very cold world for sure.

The greatest difference between Mars and Earth is the extreme sparsity of life on Mars. The two Viking spacecraft that tested Martin soil for life in 1976 produced inconclusive results at best, although most scientists would agree Viking showed Mars to be, at least currently, a fairly "dead" world. Life may hide under some yet unturned stone, or perhaps life arose on Mars, evolved to an unknown height, and then perished. One of the strongest reasons for detailed exploration of Mars is to search for the fossil evidence of such extinct life.

With the absence of definitive evidence for life, science must ask why Earth and Mars differ in this fundamental respect. Gullies and erosional features seen all over the planet by orbiting spacecraft are compelling evidence for abundant ancient waters. Why isn't Mars today more like Earth?

According to most proposals, terraforming Mars would begin with efforts to increase atmospheric pressure and temperature. The polar caps would be key here, since if they could be melted, the carbon dioxide and water vapor thus evolved would initiate a self-generating greenhouse heating effect by trapping the energy from the sun. Depending on how much water could be obtained from the poles -- a current gap in our knowledge -- temperatures could rise substantially. Water vapor is a much more efficient producer of the greenhouse effect than carbon dioxide.

Some proposals would darken the caps with finely dispersed Martian soil to increase solar energy absorption. Other suggestions involve gigantic gossamer-thin orbital mirrors fousing sunlight on the ice. Calculations indicate that enough gas could be generated by polar melting to create a surface pressure equivalent to Earth's -- although predominantly carbon dioxide -- in only 100 years. Increasing atmospheric pressure on Mars would permit transport of heat from lower latitudes to polar regions ("advection"), causing further ice melting.

The great hope for Mars is the permafrost that is assumed by many scientists to exist below the surface over much of the planet. Vast underground reserves of frozen water are almost certainly present, given all the erosion that almost has to have been caused by flowing water at some point in Mars' history. Hopefully, biologically critical reserves of frozen nitrogen exist also.

The warming trend set in motion by polar ice melting could lead in many thousand years to large basins of free water. Moderate temperatures and tolerable pressures could allow people to move about the surface, protectively clothed against the intense ultraviolet radiation from the sun and wearing oxygen masks, but otherwise free to roam the land.

Any serious attempt to terraform Mars probably would have to employ life forms on a planet-wide scale, first to generate an oxygen atmosphere and then to insure its stability. This was precisely the conclusion of the landmark scientific study of Martian terraforming, led in 1975 by Robert MacElroy and Melvin Averner with the support of NASA and Stanford University.

To get oxygen into the Martian atmosphere will require the services of trillions of beings. Much as Earth was transformed over millions of years by oxygen- producing bacteria, so could the Martian carbon dioxide be broken down in the presence of water to oxygen and carbohydrates. All the studies point toward the use of algae or lichens to complete the Martian transformation.

Existing bacteria and lichens might actually be able to live on Mars even now, yet the efficiency of transforming carbon dioxide would not be great. The 1975 NASA study concluded, "Mechanisms of genetic engineering currently available or under development could be used to construct organisms far better adapted to grow on Mars than any present terrestrial organism."

A process that might ordinarily take tens of thousands of years could be substantially reduced by genetically redesigned life. Clearly, even very efficient terraforming on Mars may require millenia, a project duration not compatible with current myopic political time frames. But the first stage -- pressure elevation and the warming trend -- could be amazingly rapid.

Other Mars terraformers are not content with the slow path to remake Mars. James Oberg, author of "New Earths," suggests that diverted asteroids could be crashed into the Martian surface to create depressions six to eight miles deep. Gravity would cause such atmosphere as exists to "puddle" in such a hole. The weight of all the atmosphere pressing down on the bottom of the depression would immediately produce Earth-like pressures -- a kind of localized terraforming. Oberg's "air holes" are matched in their daring by his "ice-teroids," -- icy chunks from the asteroid belt or stolen fragments of Saturnian moons rich with frozen water -- sent on collision course with Mars to enrich its air.

The costs required for many of these planetary engineering proposals are of course enormous, but who can say what humanity would not do to gain another world?

The energy needed for terraforming is conveniently provided free by the sun once low-energy human activities -- seeding the planet with life and dispersing dust on the polar caps -- have set the process in motion. It would require capturing only 1 percent of the solar energy reaching Mars to free the frozen carbon dioxide and give the planet Earth-like pressures within 100 years. Liberating oxygen from the carbon dioxide -- the job of the new Martian life -- would take longer, but again, the sun would provide the energy.

Meanwhile, back on Earth, the "Mars Underground" community of scientists and engineers struggles to make the first step to Mars a reality. Wernher von Braun in 1953 drew up the most detailed plans for a massive manned expedition to Mars in "The Mars Project." Many other Mars mission engineering studies were done in the 1960s. In 1981, a report edited by biologist Penelope Boston, "The Case for Mars," described the results of a conference held in Boulder, Colo., to plan for an early Mars base. A second conference last summer renewed the commitment to Mars and updated plans for a 15-person research expedition.

Since production of food, biologically important gases, and rocket fuels is possible on Mars, the philosophy would be to establish a permanent colony at the outset, to which crews would rotate on shifts lasting from two to five years. Initial Mars missions could be conducted for one-third to two-thirds the cost (in fixed dollars) of the Apollo project. Our rapidly advancing space technology would eliminate the need to reinvent many wheels.

The exploration of Mars may commence after or in parallel with a manned lunar base -- itself viewed as a logical leaping point from the manned space station due in 1992. The president's science adviser, George Keyworth, asked in October at a Washington symposium on space activities in the 21st century, "What steps should be taken in parallel with the lunar base? Do we go to Mars, and if so, why?"

In the background is the widely held belief that the Russians intend to go to Mars. Their cosmonauts have already exceeded the approximately six-month flight duration of a current-technology one-way trip to Mars. It would not be surprising to see them land on Mars' moons within 15 years to begin an extended scientific exploration of the planet. Lunar astronaut, Edwin Aldrin, was recently quoted, "I would stick my neck out and say a cosmonaut will walk on Mars before we walk on the moon again."

Carl Sagan in a recent Discover magazine essay sees in Mars an opportunity to divert energies of conflict. "Suppose the people of Earth are one day fortunate enough to discover new leaders in Washington and Moscow dedicated to a new beginning; and to seal that new beginning they embark on a dramatic joint enterprise -- something like the Apollo program, but with cooperation, not competition the goal." The "Mars Underground" is hoping that a president who speaks so eloquently of American pioneers exploring a once-New World might be instrumental in opening an even newer one.

It may not be for strictly pragmatic purposes that we go to Mars to change it forever. We may go for aesthetic or psychological reasons through the energies of dreamers and misfits -- a course not unknown in history. An outpost of human life on Mars would be the ultimate insurance policy for the continuity of our race. Planetary disasters -- asteroids crashing into the Earth, nuclear winters, and environmental troubles unforseen -- could perhaps then not still the last human beings.

In 1982, James Oberg wrote in "Mission to Mars": "The metamorphosis of old Mars into a living, terraformed planet would be more than a metaphor of the ultimate conquest of the god Mars' influence in all terrestrial civilizations, or of the victory of life over death which has been the spark behind so much human aspiration. The spread of Earth-born life beyond the world of its biological origin would be an event of galactic significance, both for what would still lie ahead of a newborn multiplanetary human civilization, and for what would be left behind." CAPTION: Picture, Astronauts in pressure suits collect geological samples on Mars; in jet-powered backpacks, they can hover and fly short distances. Behind are cylindrical housing and laboratories, shielded from radiation by layers of dirt, and greenhouses. Two solar-powered reconnaissance dirigibles hover above the colloing towers of buried nuclear power plants. A shuttlecraft, its parachutes cast off, prepares to land near berths for other craft that shuttle between the Martian surface and interplanetary transports. By Robert McCall Copyright (c) 1984 Discover Magazine