An independent panel appointed by the White House has warned current plans to send manned missions to the Moon and later to land astronauts on Mars are not viable.
It says the United States space agency NASA's annual budget of $US18 billion needs to be boosted by $US3 billion if astronauts are to conduct meaningful missions, the BBC reports.
The group has given President Barack Obama a series of options to help him shape NASA's future as part of a preliminary report.
Five years ago, President George W Bush set out his vision of sending astronauts back to the Moon by 2020. It was meant to be the staging post for the next frontier - a man landing on Mars.
But the panel of space experts says that's pretty much pie in the sky. Their report to Mr Obama says the current US human space flight programme appears to be on an unsustainable trajectory.
It says the spaceship and rocket programmes being developed to replace the shuttle are not presently viable.
The panel will deliver its full report later in September. Mr Obama will consult NASA administrator Charles Bolden and chief scientist John Holdren before responding.
A new vision for the space agency is expected to be outlined in October.