In our own halting, herky-jerky fashion, human beings are becoming inhabitants not just of a planet but of a solar system.

In our own halting, herky-jerky fashion, human beings are becoming inhabitants not just of a planet but of a solar system.

We have scouts out there, robotic spacecraft, including a couple winging their way into the interstellar void. We have big plans for sending astronauts once again beyond low-Earth orbit. For an exploring species, one driven by curiosity and material desires, this solar system is a target-rich environment. There are places to go, things to see.

There is, for example, Titan, the great moon of Saturn, with its weather-carved landscape, its methane rain, its valleys and rivers and lakes. Too cold for liquid water and for life as we know it, it is still a chemist's paradise, rioting with the kind of carbon-based molecules that suggest that this is a universe primed for life's efflorescence.

There's Jupiter's moon Europa, covered with what look to be icebergs shifting above a deep subsurface ocean.

There's Enceladus, another Saturnine moon, a weird-looking place with part of its face smooth, part ragged, and the surface erupting in geysers of ice, like a cold Yellowstone.

Of course there's Mars, which, though chilly, rusty, nearly airless and blasted by radiation, is by comparison with Venus or Mercury a veritable Club Med. We've found water ice on Mars, and there may be some liquid stuff deep underground. The planet has just about as much land surface as Earth. A Martian day lasts about 24 hours. On the warmest days, the air temperature is about the same as at a Green Bay Packers playoff game at Lambeau Field.

Never getting much ink, but still intriguing, are the asteroids, some of them hundreds of miles across, a few of them crossing the path of Earth. They may be like giant tanker ships of exploitable resources, waiting for someone to bring them in to port.

Much closer to hand, and perhaps unfairly unfashionable these days, is our own moon. It could potentially supply raw materials to fuel fleets of spacecraft. The far side has the nice feature of being radio-quiet, protected from our wireless chatter and other electromagnetic noise. It's the perfect place to put a telescope to scan the deep universe.

This solar system is not entirely ideal — it's chockablock with hostile environments and not a single benign location apart from our own to plop down and hold a cookout. But it is a scientific wonderland and, perhaps, full of practical opportunities that we simply haven't envisioned. If nothing else, it beckons us to great adventures.

There are many people who make the plausible argument that we can't afford to throw money into space when we have so many practical problems on terra firma. Finding water on, say, Europa may seem esoteric to earthlings who don't have clean drinking water at home.

But we already are, in our own half-interested way, a space-faring civilization. The basic principles of spaceflight are known to us. The big, overriding question is: Where do we go from here?

That question was simpler half a century ago, on the first day of October, 1958, when the National Aeronautics and Space Administration began operations. At that point we just needed to get into space, up beyond the atmosphere, into that high frontier.

The how was generally understood, too: We would put people on top of rockets. Rockets had been around only for a few decades and had their first dramatic impact when the Nazis used them to bombard England during World War II. Huge new rockets were a side effect of the Cold War. What wasn't known for sure in 1958 was whether a person could survive in a weightless environment. Also, our rockets initially had a nasty habit of exploding on the launchpad.

The why was obvious: We were in a white-knuckle contest with the Soviet Union, which had a head start into space, bigger rockets and maybe (we wondered) all manner of malign purposes for orbital platforms. Maybe they'd drop bombs on us from up there.

Half a century on, NASA faces a more complicated political, budgetary and technological landscape, and its ambitions are not limited to human flight. Bigger space telescopes are coming, the search for extraterrestrial life and distant solar systems is in full swing, and robotic space probes regularly produce dramatic findings. But the public's eye tends to focus on the agency's grand plans to fly astronauts beyond Earth orbit — and no one knows if they will come to fruition.

NASA is in the process of finishing the international space station, retiring the space shuttle and building a new architecture for putting humans in space, with the Constellation program. The agency is following a road map called the Vision For Space Exploration, adopted in 2005 with President Bush's imprimatur. That plan calls for a return of astronauts to the moon and the establishment of a permanent base there as a staging ground for missions that might someday include putting humans on Mars.

But without a space race or any national groundswell of opinion in favor of ambitious human spaceflight, the Vision has to proceed in an incremental, bureaucratic manner, keeping within a flat NASA budget. That means that most of the money to build the new system will become available only when the shuttle is retired in 2010. That also means the United States will not be able to launch astronauts into space for about five years. The plan calls for us to hitch rides from the Russians. But the U.S.-Russia relationship has been deteriorating.

This makes the next few years a slippery time for NASA. At any point Congress or the White House could decide that the nation's priorities do not include sending people back to the moon. Joseph Alexander, of the Space Studies Board of the National Academy of Sciences, says he worries that NASA is being "set up to fail."

"The program is in danger of completely running aground at this point," Alexander said. "Within the constraints that this administration has put on NASA's budget, you can't get there from here."

NASA Administrator Michael Griffin knows all the challenges the agency faces, but there is no more persuasive advocate for a civilian, government-run spaceflight program.

Why go at all? Partly it's prestige, Griffin said. It's definitely a strategic move. History tends to be written by countries that explore. Griffin emphasizes that we won't know in advance how space will be useful, or even whether it will be useful at all. It could be like Mount Everest — or it could be more like one of those North Sea oil platforms. Or maybe even like North America, a resource-rich place colonized by people from around the world.

He panned back for the cosmic view of why we go.

"Fundamentally, it's about long-term human survival," he said. "If we believe that human life is worth preserving, then we have to face the fact that the history of life on Earth is the history of extinction events. Diversification of our portfolio is a good thing in the long run."