A year ago, hours after seven astronauts lost their lives returning from space aboard the shuttle Columbia, President Bush went on national television and pledged that "the cause in which they died would continue."
"Mankind is led into the darkness beyond our world by the inspiration of discovery and the longing to understand," he told the nation Feb. 1 last year. "Our journey into space will go on."
Last month, as he set new targets for NASA of the moon and Mars, the president invoked the nation's collective history of migration and exploration.
"Mankind is drawn to the heavens for the same reason we were once drawn into unknown lands and across the open sea," he said.
But what has really driven the human impulse to explore, and does it still apply? What has become of societies that stayed home, or gave up exploration? And if we go, should we send robots, or must we go ourselves?
Such questions are driving the renewed national debate on the future of manned space flight, as scientists grapple with a year of both disaster and success.
"There is not a space exploration gene," said Charles D. Laughlin, professor emeritus of anthropology and religion at Carleton University in Ottawa. But modern man has inherited a body and brain of a creature that thrived because he moved out of Africa some 250,000 years ago and colonized Europe and Asia.
"So far as we know, they were not facing any calamity in Africa," he said.
They just moved out. And with their tools and clothing, their hands and large, clever brains, they were able to adapt to an astonishing variety of environments, from the rain forests to the tundra.
"We explored outward and filled the niches that no creature had theretofore been able to fill," Laughlin said. "We inherited that body and brain. ... We want to live anywhere it is possible to live. And now we're trying to live in space."
Robert Kurzban, an evolutionary psychologist at the University of Pennsylvania, said humans are peculiarly wired to seek out new information, wherever they can find it.
"If you have unique information it can give you an advantage," he said, "perhaps in finding out about good locations where game can be hunted, or food can be gathered, or ambushes can be held."
People especially like to pursue new information when they're confident they can get it at a reasonable cost - like solving a puzzle.
"In space exploration," he said, "we're getting to that point. We know there's information we can get, and that's where the motivational system that humans have kicks in."
Many societies have had periods of exploration and expansion, said Stephen J. Pyne, an Arizona State University professor who has studied the history of exploration. He noted especially the Arabs, the Chinese and the Polynesians.
Later, Portugal, then Spain, and later England, France and other Western European nations began sailing the world in pursuit of trade, wealth and empire, and to save souls.
Eventually, naturalists and scientists began going along, always on the lookout for information, and resources, that would prove an advantage for their societies at home.
Discovery, Pyne said, became "an index of national prestige and power." But it did not drive exploration. Nor did it make the process benign. Exploration could and did bring human and environmental calamity.
The devastation of American Indians by the weapons and diseases brought by Europeans is well known. But even the expansion of the Polynesians across the Pacific caused ecological disaster, wiping out 90 percent of the indigenous birds, said University of Hawaii anthropologist Ben Finney.
Disaster has always threatened individual adventures. The Darien expedition - a scheme by Scotland to take control of the Isthmus of Panama in 1698 - ended in failure. It cost 2,000 lives, 15 ships, half the wealth of Scotland and the country's independence.
When the great wave of European exploration and expansion ended at the close of the 19th century, Pyne said, there was a lull. It wasn't until the Cold War ignited the competitive energies of the United States and the Soviet Union that the next discovery period began.
That era was driven by military and political ambitions and fears more than science and exploration. And it carried humans into space and, in less than a decade, to the moon.
But with the Cold War over, advocates argue that wealth and power again justify the costs of the exploration of space.
"The United States is one of the most powerful economic forces the world has ever seen, and it follows directly from our investments in science and technology," said Neil deGrasse Tyson, an astrophysicist and director of the Hayden Planetarium in New York.
And that scientific prowess, he said, was developed among a generation of Americans inspired by the drama and excitement of the "space race" - particularly manned flight.
"We can send all the robots we want into space, but no one will ever give a ticker-tape parade for a robot," Tyson said. As an educator, he wants to be able to tell schoolchildren that they could be the ones to follow the robots to do the science on Mars.
Without humans in the game, said Carleton University's Laughlin, "it becomes merely another complex technological and scientific activity that most people on the planet cannot understand, cannot identify with, and balk at paying for."
And those humans in space need to have a mission worthy of the risks they take, Laughlin said. With the shuttle and space station locked in low Earth orbit, "we've been driving around the block for 30 years."
Not everyone agrees.
"I don't know what the human presence brings" to a mission to the moon or Mars, Pyne said. "You have to live in special suits, in special habitats. You can't touch, taste, feel or hear directly. You can see, but everything else is mediated by your need to survive.
"I'm a keen advocate for more exploration," he said. "I'd like to see us explore the solar system, but the idea that you have to do it with manned spacecraft as a prelude to colonization, is a huge diversion. ... We have so much we can do with the robots."
Putting people in space is expensive. But doing nothing - staying home - could carry high costs, too, some say.
Nations that have stopped their exploration and expansion have become vulnerable to others, said Laughlin.
"They start swirling around in a circle and they stop developing," he said, citing medieval China and Arabic cultures as examples. "The same damn thing could happen to us ... if our resources dry up or our political will disappears."
Worse, proponents of space colonization say a failure to move humans off the planet will leave the species vulnerable to extinction through environmental catastrophe, such as an asteroid strike.
"If the human race is to survive, it must positively become a multiplanet species, because something is going to make it impossible for us to survive here on Earth," said Roger D. Launius, chairman of the space history division at the National Air and Space Museum.
"That is the ultimate threat, and the reason why we've got to do it," he said. "It is also the one people don't take very seriously."