If NASA wants to help humans boldly go where no man has gone before, the US space agency must work with other countries, say experts who fear budget constraints will keep astronauts stuck on Earth.

The fears were laid out in stark terms last week in a report by the National Academy of Sciences, which concluded that the space agency's $18 billion-a-year budget was simply not enough for it to fulfill all its missions.

Moreover, it said, there is a lack of national consensus on where exactly NASA should be spending its money in the first place.

Some think the agency should focus on getting humans back on the moon, four decades after the last astronaut landed there, as a stepping stone toward heading to Mars.

That's the "Constellation" program, promoted by then president George W. Bush in 2004, and later cancelled by President Barack Obama, who deemed it too expensive.

Obama instead has proposed to send humans to an asteroid by 2025 before launching a manned voyage to the Red Planet in the 2030s.

But NASA doesn't have the money to make this plan a reality, the report said. Nor is it clear anyone is really dedicated to making it happen.

"We've seen limited evidence that (an asteroid) has been widely accepted as a compelling destination by NASA's own work force, by the nation as a whole, or by the international community," said Albert Carnesale, a professor at the University of California at Los Angeles, who chaired the review committee.

Its congressionally-mandated report urged Obama to set an ambitious but technically feasible agenda for NASA, after consulting with potential international partners, in order to better align NASA's goals with its resources.

The White House has yet to respond, with the president mired in fraught negotiations over the broader federal budget, aimed at finding a deficit-slashing compromise with Republicans to avoid the so-called "fiscal cliff" looming just weeks away.

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To reach final frontier, NASA can't go it alone: analysts