People have been walking on the surface of Mars for more than a century, in tales of science fiction and fantasy. Now, however, the possibility is real enough that many people think the question is not whether humans will go to Mars, but when they will go, how they will get there and who will go first.
No one yet knows the best way to get to Mars. While a consensus is growing that it will be some kind of multinational effort, experts said, the feat is within the grasp of one prosperous nation like the United States, if getting to Mars has a very high priority.
The cost of such a venture has been estimated at $60 billion or more. But Dr. Robert Zubrin, founder of the Mars Society, an advocacy group, estimates it could be done for about the price of the Apollo Moon project, $30 billion to $40 billion in today's dollars.
Dr. Zubrin, author of the book ''The Case for Mars,'' a blueprint for humans' settling the planet, calls for an increasingly popular approach of sending unpiloted vehicles in stages to provide equipment and a way for the people who would land later to get home.
This approach would avoid the difficulty of developing one giant ship with equipment and supplies for a round trip. The first landing would be a spaceship that would be a return vehicle. It would land intact, so it could take off again, but it would have used up its fuel.
Supply and mining and processing vessels would follow. The humans would arrive last. They would dig for subsurface water and other materials they could convert into fuel for the trip home.
Mars is the obvious first choice for travel to other planets. Of all the other planets in the solar system, Mars -- even with its present cold, hostile environment -- is the most like Earth. About half Earth's diameter with one-third the gravity and only 1 percent of the atmosphere, spacecraft have found that Mars nonetheless harbors the important asset of water beneath its surface and in its frozen poles.
Every two years its orbit draws it closer to Earth, and its presence in the night sky is a constant reminder of what might be. This summer, it came within 34.6 million miles of Earth, its nearest approach in almost 60,000 years.
Only Venus is closer, and with an average temperature of about 850 degrees, it is not welcoming. In contrast, Mars is pleasantly cool, from about 200 degrees below zero right up to freezing.
From the presence of water and relatively temperate climate comes another powerful attraction. ''The fascination with Mars centers around the issue of life,'' Dr. Zubrin said. ''Mars is key to answering that critical question, Is there life somewhere other than Earth?''
Dr. Louis D. Friedman, executive director of the Planetary Society, said, ''The fundamental question people want answered is 'Where am I from and where am I going?' '' He added, ''Mars can tell us about the origin of life and the destiny of life.'' Finding evidence of past or present life, even if only primitive microbes, would mean that life as humans know it did not occur only on Earth, he said. And, he said, Mars is the only planet we know of that humans have a chance of colonizing to prove they will not be forever Earth-bound.
''Mars becomes the experiment,'' Dr. Friedman said. ''If we can't make it on Mars, then Earth is our limit and we are going to have to re-examine our relationship to our home planet.''