But theres another reaction to these calamities thats just as predictable: the post-mortem gnashing of teeth by pundits as they rhetorically ask themselves and us whether we should even be scouting the nearby fringes of space. Many raise the man-versus-machine argument: why do we continue to risk people when robots can do the job? Those who are willing to tread thinner ice pose a more radical question: why do we need a space program at all?

Why indeed. Unlike many, Ive had to confront this query explicitly. In 1997, the year we watched Sojourner successfully reconnoiter a rocky patch of martian real estate, I visited Australia to give a series of school lectures. One bright-faced young student asked me this: "Why does America spend billions of dollars putting motorized skateboards on Mars while people are starving in the streets?"

Theres a facile answer to this: we do it for knowledge. We are engaged in a voyage of exploration and learning, and to undertake such journeys is a fundamental and noble drive.

Thats true, and probably true at a basal level. In humans as well as ants, an urge to pioneer the unknown if it exists in at least a few individuals monumentally improves the survival chances for all others. Not everyone need be a scientist, but if no one is, then society will stagnate or worse.

The politically correct justifications for exploration are knowledge and spin-off technology. The first has a lofty appeal, and the latter can be understood by nearly anyone. Were not talking Tang here, but telecommunications, terrestrial monitoring (weather satellites, for example), GPS, and much of the revolution in electronics.

Majestic as this answer is, it often fails to resonate with the public. The citizenrys vision of the space program isnt so much what weve learned about metabolism and microgravity, but consists instead of a mental pastiche of video clips in which astronauts in shorts, clearly having a good time, juggle their weightless food. And the question such imagery begs is this: Is the manned space program, despite all the scientific rationale, fundamentally just an expensive adventure for a chosen, lucky few?

Well of course not. But it is disingenuous I think, and even counterproductive, to minimize the lure and more importantly, the value of simply pushing the envelope of human experience; of savoring the adventure that allows you to bounce around in zero-G. To downplay the emotional in favor of the intellectual is easy and immensely acceptable; but its a self-deceiving argument. Knowledge may be the goal, but adventure is usually the motivation. We go to the mountain largely because "its there."

History tells us this is so, as does a simple examination of our heroes. Its fashionable in 2003 to recall the remarkable trek of Lewis and Clark, whom Thomas Jefferson charged with charting Americas newly purchased territories west of the Mississippi. The two explorers made copious and valuable observations of flora, fauna, geography, and native cultures, all of which proved useful in opening the continent. But today what we most remember and admire about their expedition is not the volume of data, but the triumph of their journey; an odyssey of survival in dangerous and unknown lands.

The same is true of another frontier of exploration: Antarctica at the beginning of the twentieth century. Nearly everyone knows the bittersweet story of Robert Scott, who died on the ice while walking back from the pole, a sled burdened with rock samples still at his side. But at least as inspiring are the tales of Ernest Shackleton, who was required to sail 600 miles in an open boat across the worst seas in the world to save his polar expedition. Shackleton did rather little science, but the energy that drove him to test the unknown and survive are now widely admired.

Probing the edge of the known is neither easy nor safe. This is tautologically true, and we cannot expect otherwise. All great exploration efforts are vulnerable to disaster. Few among the famous sea-going navigators of the late 18^th century survived to retirement, and entire ships were routinely lost.

So when exploration comes to grief, let us at least moderate the temptation to damn the enterprise just because of its clear component of bravado and adventure. Gene Rodenberry pictured the best of our descendants as those who would "boldly go where no one had gone before." He simply said what all of us truly know: there is enormous value in the "going."