One hundred million miles away, the mechanical innards of NASA’s Spirit rover have begun to hum in the brittle cold of the martian air. The rover is a synthetic geologist on wheels, small enough to fit in your kitchen, and the space agency is reveling in the fact that Spirit has managed to elude the silent death that has claimed so many of humankind’s envoys to the Red Planet.

The boost to NASA’s confidence, badly eroded by the loss of the shuttle Columbia, is surely a good thing. If Spirit and its sister rover, Opportunity, perform well, the Bush administration may support a major new space initiative, perhaps a return to the moon or a human expedition to Mars.

Those would also be good things, but such judgments, coming from a scientist, may seem obvious and self-serving. American taxpayers will rightfully ask why it’s important to shell out $800 million to send a pair of cybernetic skateboards to another world.

One answer is the interest and value of the science. For two centuries, Mars has beguiled us with its Earth-like appearance. Venus is closer, but Mars is charismatic; it is sufficiently similar to our own planet to warrant the hope that it once spawned life. And the possibility of discovering life beyond Earth is a siren song to anyone with curiosity, even if, as is surely the case for Mars, that life is no more sophisticated than bread yeast.

NASA’s approach to learning whether microbes ever populated the Red Planet is to look for signs of ancient lakes, rivers or oceans. Spirit will explore a flat-bottomed crater that may once have held a body of water half the size of Lake Erie. Its mission is to find evidence for this erstwhile lake by examining the rocks littering the crater floor.

If Spirit discovers that water once ebbed and flowed on Mars, the next questions are: For how long? Long enough to germinate life? NASA will send a string of robot explorers to address this question, and to ultimately seek out microscopic Martians. The carrot that hangs before us is deliciously seductive: If another world — the next world out from the sun — is proved to have supported life, that would imply that the cosmos is drenched with living things. We could conclude that planets with life are as common as phone poles.

That’s the science, and it’s exciting. But science is no more than curiosity imbued with logic. Surely, in a world awash in political upheaval, epidemics and poverty, curiosity is a dispensable luxury.

It’s not. Curiosity is hard-wired into our behavior because it has survival value. For 300 millenniums, it has driven us to exploration and understanding. The former has encouraged the discovery of new resources, and the latter allows us a comfortable life in a pitiless world.

Curiosity is the silent motor of progress, without which we are condemned to a steadily worsening existence as we burn through our resources.

Humans display many behaviors that separate us from the beasts. Art, music, poetry … the list is easily formulated. Curiosity, neither incidental nor trivial, is on that list. In simpler times, it drove our ancestors to wander across the mountains and, on occasion, to find a valley that was better than where they started. Today, scientific curiosity turns up answers to questions that previous generations could barely ask.

The Spirit rover is a small actor in a long play with a large cast. It is aptly named, for it represents not only the best of our enterprises, but also an essential quality of our being. Spirit is mechanical in construction only. It is quintessentially human.

Seth Shostak is senior astronomer at the SETI Institute (Search for Extraterrestrial Intelligence) in Mountain View, Calif. He wrote this column for the San Jose Mercury News.