IN JANUARY 2004, President Bush announced an ambitious plan "to gain a new foothold on the moon and to prepare for new journeys to worlds beyond our own." It seemed, we noted then, an odd moment to embark on a dispensable project of great expense -- given a yawning budget deficit, pressing health care and educational needs, and a long-term struggle against terrorism. Now, as the country faces another great expense in the aftermath of Hurricane Katrina, NASA has unveiled its proposal for making Mr. Bush's vision a reality. Both the moment for embarking on this endeavor and the justifications for it seem odder than ever.

The agency's new administrator, Michael D. Griffin, says that the $104 billion project, which would put American astronauts on the lunar surface by 2018, can be done without adding to the agency's budget (beyond keeping pace with inflation) and without taking away from the agency's valuable programs outside the area of human space flight. We hope he's right, but given NASA's history of cost overruns and inevitable cost pressures, both assertions seem optimistic. If they don't prove true, after all, Mr. Griffin won't be around to answer for them -- and yet another NASA chief will have to deal with yet another program that's gone way over budget.

More fundamentally, we believe that the needs of NASA -- and the country -- can, at this point, be better served by continuing and expanding robotic exploration. In a visit to The Post the other day, Mr. Griffin emphasized the inherent limits of robots; NASA, he said, had concluded that a human could achieve in a single day what it would take a robot 90 days to do.

But sending a human into space costs far more than dispatching a robot. The inherent risks of human spaceflight can be minimized but not eliminated. And humans, at least in the near future, will not be able to remain on the moon, Mars or elsewhere nearly as long as robots; the rovers Spirit and Opportunity are still cruising the Martian surface after landing in January 2004. All in all, 90 days to one seems like a pretty good trade-off.

Indeed, Mr. Griffin was refreshingly candid in acknowledging that favoring humans over robots wouldn't make sense if science were the only consideration. He argued, instead, that "extending the range of human habitat out from Earth into the solar system" -- and, indeed, beyond -- is critical for human survival. "In the long run, a single-planet species will not survive," Mr. Griffin said, adding, "There will be another mass-extinction event. If we humans want to survive for hundreds of thousands or millions of years, we must ultimately populate other planets."

To Mr. Griffin, this existential threat, however far-off, justifies spending the small sliver of the federal budget -- seven-tenths of 1 percent -- devoted to space travel. We'd be more willing to finance this sliver if we didn't suspect it would ultimately grow into a hefty slice -- or risk being wasted altogether. At some point, when the technological path is clearer and the capacity to finance it greater, a mission to Mars or farther could be an important extension of the human drive to explore new worlds.

For now, though, the country ought to focus its limited resources on paying for things such as making sure poor children get health care now -- not figuring out how to get our descendants to Alpha Centauri hundreds or thousands of years into an uncertain future.