Louis D. Friedman is a former manager of Advanced Planetary Studies at JPL and has been Executive Director, The Planetary Society since 1980. Charlene Anderson is Associate Director of the society.

Its a big solar system out there; there are many worlds holding wonders that can be revealed by terrestrial explorers, be they robots or humans.

Of all the worlds in our solar neighborhood, there is only one where we can imagine our species settling, as pioneers have settled terrestrial frontiers throughout history. There is only one world with the atmosphere and water that could be made to support our type of life. There is only one world within reach of our wandering footsteps and that world is Mars.

To send human beings to the Red Planet has been a primary goal of the Planetary Society almost since its beginning in 1980. In 1986, we circulated and published the Mars Declaration, signed by leaders in many fields around the world, calling for the spacefaring nations to join their capabilities and send a human mission to our neighboring world. With that declaration, The Planetary Society established itself as a credible voice calling for humanity to take a decisive step to the next frontier.

At the Planetary Society, we work toward the day when technologies will have lowered the cost of human space flight, making possible a practical political initiative among the international space agencies to send humans to Mars. At the same time, we are committed to the robotic exploration of Mars, and to an international Mars Sample Return campaign beginning in 2003. Following that, robots will set up outposts on Mars and begin preparing for human landings.

When imagining the future of our species among the planets, its important to remember that the pacing question behind all this exploration is whether life existed in the past, flourishes now, or could be established on other worlds. The driving force behind space exploration is our attempt to understand life in the universe and the human relationship to the origin and evolution of life.

Mars dominates our thinking about extraterrestrial life because it is the only planet we know of with accessible oxygen and water that could support our type of life, and with a wet and warm past when extraterrestrial life might have arisen.

But that is not to say it is the only possible abode for life. In fact, some scientists believe that, if we want to search for extraterrestrial life that flourishes today, the best place to look is Jupiters moon Europa.

Since the Voyagers flew past in 1979, scientists have wondered if liquid water resides in underground oceans protected by Europas icy crust. Europas orbit around Jupiter, locked in to the resonance of the other large Galilean moons and the massive planet, causes it to be repeatedly flexed by tidal forces. On Io, these tides drive continuous volcanic eruptions. On Europa, they might provide the energy for life.

All this talk of oceans on Europa, and the imaginings of life, will remain speculation until we determine to explore that ice world with a will. NASA is planning to send a spacecraft, equipped with various remote-sensing instruments, to orbit Europa and determine if there is layer of liquid water beneath the ice. But that will not be enough to tell us if anything is actually living down there.

Europa may be the target of the first interplanetary submarine. A few brave thinkers are already pondering how to probe through the kilometers-thick ice shell and break through to the Europan ocean. Once there, well need some sort of mobile robot to seek out potential abodes for live. A remote-controlled submarine could be just the thing. And then imagine the robotic surrogate of our life-form could be the first visitor to drop in on the native Europans ... even if they are only microbes. What a landmark in exploration!

Europa is not the only other planet relevant to the question of life. Saturns moon Titan has a thick atmosphere, rich in organic compounds. It may resemble the early Earth, but locked in a deep freeze. The Cassini spacecraft, carrying the Huygens probe to Titan, may reveal pre-biotic organic chemistry that could tell us much about how to search for extraterrestrial life.

One startling realization thats come from our explorations of the outer planets satellites is that the so-called "habitable zone" around our Sun is not restricted to Earths vicinity. Weve found conditions that might support life on a moon of Jupiter, and some of the chemical stuff of life on a moon of Saturn. This raises the possibility habitable zones may lie at varying distances from stars, increasing the possibilities for life-friendly planets around other Suns.

This realization has energized the search for extrasolar planets. We are just beginning to discover planets around other stars. While it is too early to know what will find, we can be sure well see a great range of conditions on extrasolar planets, and the search for habitable planets will be an important subject in the next few decades. Finding a habitable planet, or an Earth-like planet around another star, will be another major milestone in exploration.

There are so many worlds to explore. There are so many tools at our disposal. What we need is the vision, the will and the means to follow our dreams to the planets.

To make that exploration happen is the role of The Planetary Society. Our mission is to explore new worlds and to seek other life. We actively promote the robotic exploration of other worlds, and the human exploration of Mars. By 2008, we want to see samples of Mars returned to Earth and the beginning of a Martian outpost placed on the Red Planet.

We are actively working to see an orbiter circling Europa and, eventually, probes beneath the ice. Orbiting beyond Earth, we hope to see space platforms for searching for extrasolar planets. We look for new developments to advance the search for extraterrestrial intelligence.

Humanity, teamed with its technology, is taking its first tentative steps into the solar system. There are many worlds to explore, and many adventures waiting those with the will to undertake them.