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.

But analysts said the president can't ignore NASA forever.

In recent years, the White House has acted under the assumption that "things are stable and are not a problem. And as long that stays the case they are not paying much attention to it," explained John Logsdon, former director of George Washington University's Space Policy Institute, who consulted on the panel.

"This report could upset this equilibrium because it is kind of saying that the emperor has no clothes," and that "what you are doing (with NASA) makes no sense," he said.

Ultimately, Logsdon said, "What they say is if the US wants to remain a leader in space, the US should work more closely with other countries in defining the future that all can agree on."

And that's not what the US has been doing so far, he emphasized.

The report's conclusions are not surprising to people who watch the space industry closely, he said, "but what is surprising is that such a high level committee said it."

Scott Hubbard, a former NASA Mars program director and now a Stanford University professor, agreed that "NASA has been asked for decades to do more than it has funding to do."

"For a really ambitious major effort like a moon mission or, particularly, sending humans to Mars, I think almost without a doubt that has to be through international collaboration," he said.

He cited the International Space Station -- a $100 billion project that was built with contributions from 16 countries -- as the best example of a successful international space cooperation.

"I can imagine something like that being organized for deep space and ultimately for Mars," he said.

But some of the US's international efforts have been less fruitful.

Logsdon recalled that in February the US cancelled its contribution -- worth more than a billion dollars -- in the ExoMars project, a cooperation with the European Space Agency, citing budgetary constraints.

The project aims to send two probes to Mars in 2016 and 2018 to search for signs of life. ESA filled NASA's spot with the Russian space agency.

Hubbard said he'd prefer to see NASA's budget go up than to see the agency give up any of its ambitious plans. But he said most experts agree it would take $3 billion a year extra to make the difference.

"In a $3.7 trillion-a-year budget, $3 billion is not all that much money," Hubbard said, except that, "these days people argue over pennies."