"We stay bound to Earth at our peril," Gott said.

Gott makes the case in his new book, Time Travel in Einstein's Universe, where he warns that the risk of developing technology to the point that you can colonize space also raises the possibility that the technology will be used for ill purposes, such as biological or nuclear warfare.

"So it was especially heartening to me to see that Stephen Hawking has embraced this idea as well," Gott said. His reasoning extends beyond Hawking's narrow focus on viruses:

While he stopped just short of saying that current bioterrorism efforts could fuel a stronger desire in society for space settlement, Gott said it could have an effect. "The current situation has perhaps made us more keenly aware of the dangers we face staying confined on the Earth where disasters, either natural or of our own making, may do us in."

Many scientists argue that there is no need to worry about the mortality of civilization right now. Eric Croddy is an expert on chemical and biological weapons at the Monterey Institute of International Studies. Croddy said the threat of a virus wiping out the entire human species is simply not real.

Even the most horrific virus outbreak in history, the 1918 Spanish Flu epidemic that killed between 20 million and 40 million people, including hundreds of thousands in the United States, eventually stopped.

Experts say new strains of the influenza virus emerge every few decades and catch the human immune system unprepared, but prevention measures and ever-evolving medical treatments overcome the outbreaks.

"I'd be much more concerned about an asteroid hitting the planet," Croddy said.

Croddy accused Hawking of speaking more from a religious, apocalyptic view than from anything based on the facts of science.

"What he said is more biblical than scientific," Croddy said. Besides, he added, "Earth's not such a bad place."

Most space-colonization enthusiasts share this planet with Croddy, as well as his view of it. But whether stated or not, the desire to ensure survival has always permeated their plans.

Asteroids, in fact, frequently top the list of reasons to flee. Ample evidence suggests that many species -- including dinosaurs -- have perished as the result of colossal impacts in the past.

Stephen Hawking has broken no new ground in suggesting fear as a motivating factor for intelligent beings to develop an exit strategy.

Fear, in fact, has a long history of pushing humans to new frontiers. Cold War worries, more than anything else, put Neil Armstrong on the Moon. Fear of British rule and religious oppression helped to create the United States. And fear mixed with opportunity drove early humans to leave Africa, settle new lands, then later to migrate away from advancing glaciers.

But fear is just one factor that could push earthlings to the next frontier. Pure profit potential and the lure of scientific discoveries may prove to be the more productive enticements.

"I feel we should not go into space out of fear, either fear of asteroids, nuclear war, worldwide epidemics, pollution or industrial collapse," said Bruce Mackenzie, a member of the board of directors of the National Space Society.

"Those are all valid reasons, but they are not good long-term motivations," said Mackenzie, who stressed that he speaks for himself and not the organization. "I prefer positive motivations, such as the almost unlimited resources offered by asteroids, moons and other planets."

Whether opportunity or fear will eventually push us off the pale blue dot that has been home to hominids for more than a million years, no one is going anywhere anytime soon. At least not on a permanent basis.

Even Tumlinson, the director of FINDS and arguably the most energetic and productive proponent of space settlement, expects the whole process to take a generation. Sure, the first tourist has already flown. Others may soon follow. Mars could conceivably be visited in a decade or two.

But Tumlinson's ultimate goal is to have people calling space their home -- forcing FedEx to add rockets to its fleet of planes and trucks. And he hopes to live to see it happen, in 35 to 40 years.

Similar goals were voiced with great confidence 40 years ago, of course. But Tumlinson thinks the mechanisms are now in place to make it a reality. NASA has done its job, punching open the near frontier, he says. Now it's time for the space agency to get out of the way. Private enterprise, say Tumlinson and many of the other true believers, is poised to take over the quest to the Moon and beyond.

"It's no longer pie in the sky," says Tumlinson.

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