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It may be the biggest environmental issue facing us in the century ahead -- and scientists even now are debating it as they plan inter-planetary expeditions: should we or should we not remodel the planets to make them more like Earth? PNS associate editor Walter Truett Anderson attended a conference in Vienna in late August at the International Space University where terraforming was a topic of hot debate.

VIENNA -- If you've listened closely to the latest round of space-travel talk following the discovery of the now-famous Martian rock, you may have heard occasional invocations of the word "terraforming." That's a word totally absent from most people's vocabularies but regularly bandied about by space scientists, sci-fi writers and other interplanetary-travel enthusiasts. Depending on your point of view, it stands for either the most awesomely inspiring or overbearingly arrogant idea ever conceived by the human mind.

Terraforming means making other planets more like Earth.

It means turning the bitterly cold, nearly airless and probably lifeless Mars into a place where human beings can walk about without space suits. It means warming up the surface of a planet where average temperatures now hover around 60 below -- Celsius. It means, in its more expansive versions, covering the Red Planet with rivers and lakes and oceans, meadows and farms and forests, making it habitable for the adapted relatives of Earthly planets and animals.

Is such a thing possible? A surprising number of scientists have studied the idea in great detail, and have worked out exhaustive plans and models for how such a massive ecological transformation might be achieved.

The subject was discussed recently here in Vienna, at the annual summer session of the International Space University -- an institution whose faculty includes leading space scientists from around the world, and whose students include many young people who hope someday to travel through the solar system. The discussions touched on the various strategies that might be employed in terraforming and also the pros and cons -- because there are those who believe that, for scientific or ethical reasons, it might be best to leave Mars the way it is, in all its barren grandeur.

Most of the serious terraforming scenarios combine the grandiose and the gradual. They talk about nothing less than the transformation of an entire planet's life systems, but envision it as a series of steps that might be taken over a long period of time -- at least centuries, and perhaps millennia.

In this vein one of the leading terraformers -- Christopher McKay of NASA Ames Research Center in California -- has written about creating an atmosphere something like what we had on Earth about a thousand million years ago, before oxygen levels began to rise. It would be a thick mix of carbon dioxide and other gases, suitable for plants and microorganisms, warm enough to keep people from turning blue but not suitable for them to breathe. A more advanced situation would be a humanly-breathable atmosphere something like what we now have on Earth.

How would any of this be done? Definitely not by shipping loads of air and water to Mars -- a notion that all but a few enthusiasts dismiss as impractical. Instead, the terraformers envision releasing the carbon dioxide in Mars' soil and polar icecaps to create a "greenhouse effect" that would ultimately lead to a warmer, oxygen-rich atmosphere. Alternatively, they propose putting satellites with huge mirrors into orbit above the polar icecap to reflect sunlight on it and hasten melting, or finding asteroids of ice in space and steering them to Mars to be melted down and added to the water supply.

In discussing these ideas, terraformers cite such sci-fi classics as Kim Stanley Robin's trilogy Red Mars, Green Mars and Blue Mars, which chronicles how an expedition eventually remodels the entire plant, paving the way for a new human civilization.

Robinson's books reflect most of the current scientific and technological thinking about planet-remodeling -- as well as the pros and cons. In his story the Greens want to modify the planet while the Reds prefer to leave it alone. While the Reds win many of the arguments, the Greens proceed to change Mars -- even as human beings move on to terraform Venus, various asteroids, and the moons of the outer planets.

All this is pretty fanciful stuff. Yet as new interplanetary expeditions are being proposed, terraforming may be the biggest environmental debate facing us in the century ahead.