RENO, NEVADA -- Where should humans plant their feet next, beyond Earth orbit? Both the Moon and Mars loom large for those wanting to put our planet in a rocket ship's rearview mirror. But each destination stirs up debate as to cost, timing, and practicality.

No doubt, one person's vision of the future can be another individual's unwanted diversion.

That was the scene at the 40^th American Institute of Aeronautics and Astronautics (AIAA) Aerospace Sciences Meeting, held here January 14-17. At special sessions on space colonization, the lunar dust flew and the Martian mudslinging got heavy-handed.

For Paul Spudis, a geologist at the Lunar and Planetary Institute (LPI) in Houston, Texas, the Earth's Moon is the ideal setting for the first colony.

A human return to the Moon -- and planting a lunar base there -- is made all the more feasible by purported findings of lunar ice, Spudis emphasized.

"Going to the Moon's poles. That's where I would go to make humanity's first foothold off the planet. We now know the Moon does have water, both at the north and south poles. The current estimate is on the order of 10 billion metric tons. I happen to think that's an underestimate," Spudis added.

Because that water ice is in concentrated form, far less energy to process the material is needed. Small manufacturing plants, followed by even larger processing machinery, can convert that water ice into fuel and water to support human settlers on the Moon. Also, exporting lunar polar water is "a very obvious market," Spudis suggested. Fuel made from water ice could be supplied to commercial and governmental clients operating in cis-lunar space - the distance between the Earth and the Moon, he said.

"You can make propellant sell it, and hopefully, sell it at a profit," Spudis said.

Spudis said that the federal government does have a role in getting America back to the Moon and getting to Mars. But that role is not in colonizing those celestial bodies. "The only way we're ever going to really colonize the solar system is to produce commodities of some type that people want," he said.

And what are those products?

"I don't think we have enough imagination to imagine the markets of the next century, let alone the markets of the following century," Spudis concluded.

Go direct to Mars. Go past the rock pile that is the Moon.

"Mars is the most promising place to establish a new branch of human civilization," countered Robert Zubrin, president of the Mars Society. Historically, where best to locate a civilization has been dominated by the richness of the resources of the location involved, he said.

"It's true that the Moon is not completely bereft of resources. But compared to Mars, it's like Greenland compared to North America. Yes, there may be water at the Moon's poles. There's enough water on the Moon to cover the whole world with a layer of water a few microns thick," Zubrin said.

In contrast, Zubrin added, Mars is a water world. Current estimates are there's enough water on the Red Planet today to cover that globe some 2,000 feet (600 meters) thick -- a difference of five orders of magnitude, he said.

Sporting a 24-hour day similar to our Earth, Mars's day-night cycles are favorable for plant growth. While sunlight is more intense on the Moon than on Mars, lunar day-night cycles run in two-week time intervals. Then there's the Moon's lack of atmosphere, while the Martian atmosphere is thick enough to mask out harmful solar-flare energies, Zubrin said.

"What this means is, on Mars, we would be able to grow plants in greenhouses by natural sunlight," Zubrin suggested.

Among other Mars pluses, Zubrin underscored that where there has been the action of water, there is the formation of many mineral ores. Furthermore, geothermal reservoirs below Mars' surface, he said, could be available and yield a huge source of energy for future colonists.

"The Moon is so resource-poor that the establishment of a colony there is vastly more difficult than establishing one on Mars," Zubrin emphasized. "We're talking about a new branch of civilization. The fundamental resources to do it are not there on the Moon they are there on Mars," he said.

"We want to create a new society and that's all there is to it. I believe we should commence, essentially immediately, a program to send humans to Mars. I think we can have humans on Mars within 10 years. We already know more about Mars than they knew about the Moon in 1961," when then President Kennedy green-lighted America's quest to send humans to that world.

"We are certainly prepared to initiate a humans-to-Mars program now," Zubrin said. Initial sojourns back and forth to the Red Planet will set the stage for more extensive bases. "Once we can become self-sufficient on Mars, then I think we can start colonizing Mars," Zubrin concluded.

"Going to Mars is going to require a political decision on the part of some government," said Douglas O'Handley, a space engineer for Orbital Technologies Corporation in Madison, Wisconsin. "Frankly, I don't think we're there," he argued.

On the other hand, getting to the Moon on a commercial basis is conceivable in the near future, O'Handley said. "I think a lunar base can be established by 2010 as the next step. It's doubtful whether it's going to be the United States that does it, unfortunately."

O'Handley reported on the results of a recent study on a self-sustaining lunar base. That lunar colony would manufacture raw materials and build structures for use on the Moon, building up transportation infrastructure, as well as produce oxygen, nitrogen, carbon dioxide, water and food for life support and export elsewhere.

"The colony would be considered "self-sustaining" when it can achieve the goal of surviving without any supplies from Earth for a period of 52 months," O'Handley reported.

Energy needs of a colony -- no matter where it's located -- is always a daunting issue, O'Handley emphasized. "No one is going to sell any permanent colony anywhere in the solar system unless we have nuclear power. If we have a problem in the United States with nuclear power, we can buy it from the French and let the Russians launch it. Doing that, we've eliminated the whole U.S. problem," he said.

Sustaining a lunar operation solely on solar power is worrisome. Better to have nuclear power as a primary source, with solar energy as a backup, O'Handley noted. Learning how to live, work and sustain a colony on the Moon would give a leg up on any future plan to sustain a Mars base, he added.

Could the Moon harbor evidence for how life may be sprinkled elsewhere?

O'Handley said there is growing scientific interest in the prospect that lunar water ice -- deposited directly there by impacting comets -- could carry amino acids.

"Detecting amino acids in the lunar water ice is an exciting possibility. It would do more to put the nail in the coffin as to how unique life is or how common," O'Handley said.

While the land of Oz had its "lions, tigers, and bears," Mars has algae, mosses, and lichen. Those are early ingredients for the transformation of Mars said James Graham, a scientist in the Department of Botany and Center for Limnology at the University of Wisconsin-Madison.

Over the decades, Mars has been viewed as a potential global engineering project. Numbers of scientists have speculated on how best to reset the climate clock on Mars back to an earlier time. Doing so is called "terraforming" -- the ability to transform the dry and cold Mars we know today into one that is more Earth-like.

For example, spreading dark substances over Mars' polar caps, thereby inducing them to melt and thicken the atmosphere was one idea, Graham said. Another proposal was to orbit the planet with giant reflecting mirrors. These huge reflectors would focus additional sunlight onto the Martian surface and ostensibly raise the planet's temperature.

More recently, and arguably more reasonable research, suggests that the manufacture and release of greenhouse gases on Mars could be used to drive the globe's climate to a warmer stable state.