President Bush's vision for human space exploration is doomed to fail without a major injection of funding and changes in space policy, according to a report.
Nasa wants to send humans back to the Moon and on to Mars
The paper, by two influential experts, says current US space policy "presents a paradoxical picture of high ambition and diminishing commitment".
The report identifies what it says are four key failures of national policy that hinder future success in space.
But others have drawn attention to the authors' past democrat associations.
The paper was written by George Abbey, director of the Johnson Space Center in Houston, Texas, from 1995-2001, and Dr Neal Lane, White House science adviser under President Bill Clinton from 1998-2001.
Steps for improvement
It has been released by the American Academy of Arts and Sciences in Cambridge, Massachusetts, and is based on workshops with space companies, international agencies and scientists.
The four failures of national policy projected to hit future space successes that were highlighted by the paper were:
The report expressed hope about Nasa chief Mike Griffin
Dr Neal and Mr Abbey said they were hopeful about Nasa's new administrator, Mike Griffin, who has a scientific background and has already begun to change the agency's much-criticised management culture.
- Inadequate planning for Nasa's future
- Erosion of international co-operation in space
- Congressional restrictions on the export of space technology that hinder sales to other countries and hamper information sharing across borders
- A projected shortfall in the future US science and engineering workforce, made worse by visa restrictions on foreign students
A Republican staff member of the House of Representatives told the New York Times that it was unclear whether the critique would have much impact in Washington.
He said the American Academy of Arts and Sciences did not have a high profile in the capital. The staff member added that, while respected, Dr Lane and Mr Abbey "have some baggage" as former officials in a Democratic administration.
But John Steinbruner, director of the Center for International and Security Studies in College Park, Maryland, said the report would have an impact because of the high-profile authors.
A Nasa spokesman said the agency could not comment on the report because officials had not seen it.
President Bush's vision, outlined in January 2004, has included a major shift in emphasis towards human exploration, with the intention of returning astronauts to the Moon and, possibly, taking them on to Mars.
Both authors of the paper now work for the James A Baker III Institute for Public Policy in Texas.