President George Bush is said to be on the verge of announcing plans to build a permanent science base for men on the moon that could serve as a steppingstone for sending astronauts to Mars.
Senior administration officials said Mr Bush will announce his plans in a speech next week. The president wants to aggressively reinvigorate the space programme, still reeling from the Columbiatragedy nearly one year ago, said the officials.
But a Nobel-winning physicist who investigated the shuttle accident is among those who would rather see more affordable robots - rather than astronauts - exploring the lunar and Martian surfaces.
"The cost of a manned enclave on the moon, I think, is going to make the space station look cheap. That's the only good thing about it," said Stanford University's Mr Douglas Osheroff.
In any event, "I think we're still 30 years from going to Mars and if there's any reason to do that, I don't know," Mr Osheroff said.
Mr Bush envisions preparing for a Mars expedition more than a decade from now, one administration official said.
The White House has been looking for a new revitalising role for Nasa for months, with vice-president Dick Cheney leading the interagency task force since the summer.