WASHINGTON — In the 1960s, with the United States lagging behind the Soviet Union's space program, the crisis was as obvious as the bleating of Sputnik overhead. Today it isn't.

"Some people say, 'What about China?' I don't think that's the same thing at all," said Stephen Garber, a NASA historian. "I'm not sure what the motivator would be. There's not the sort of fundamental rationale that everybody could easily understand in the general public."

Environmental decay would give humans a good reason to get off the planet, but that's a problem that is best addressed here at home. An asteroid hitting the planet is a serious threat — it's already happened several times in Earth's history — but a vague one; it could happen tomorrow or 100,000 years from now.

And nobody has figured out how to make a lot of money off space exploration. Space tourism is affordable for only a handful of people, and the idea of mining precious minerals and gases from asteroids or other planets faces huge technological hurdles.

Dale Ketcham, the director of the University of Central Florida's Spaceport Research and Technology Institute, said it would take nothing less than a meeting with intelligent alien life.

"That would make for some very interesting appropriations debates," Ketcham said.

Franklin Chang Diaz, a former astronaut who is now working on a new spaceflight engine, confessed that he was surprised about how things have actually turned out, 40 years later. When he started at NASA in 1980, he thought they would have returned to the moon by now and perhaps be heading to Mars.

"There's a lot more discussion, debate, and I think that's good, as long as it's not paralyzing," Chang Diaz said.

He remained optimistic, if only for the reason that humans as a species have inexorably achieved things that past generations thought impossible.

"Humans will eventually move out into space. There's no way to stop that outward movement. If the United States does not lead, someone else will lead," he said. "It's not a matter of what country, but actually when."