Nasa had been hoping its Orion ship would take it back to the Moon
Nasa needs its annual $18bn budget boosted by $3bn if astronauts are to conduct meaningful missions like trips to the Moon and beyond, a panel warns.
The panel, convened by the White House to review human spaceflight plans, has delivered its summary findings.
It says the spaceship and rocket programmes being developed to replace the shuttle are not presently viable.
The group has given President Barack Obama a series of options to help him shape the US space agency's future.
But the panel, led by retired aerospace executive Norm Augustine, says only a funding increase can truly get Nasa back on track.
"The committee finds that no plan compatible with the [Financial Year] 2010 budget profile permits human exploration to continue in any meaningful way," it said.
"The committee further finds that it is possible to conduct a viable exploration program with a budget rising to about $3bn annually above the FY 2010 budget profile."
After the 2003 Columbia shuttle disaster, former President George W Bush proposed retiring the orbiter fleet in favour of using an Apollo-like capsule and rockets to take humans back to the Moon by 2020.
But this programme, known as Constellation, has had to battle technical and budgetary woes, meaning that its entry into service originally intended for 2012 will now be at least three years late.
And it is the Augustine panel's assessment that this delay will probably turn out to be longer still, perhaps leaving a gap of seven years between the final flight of the shuttle and the first mission of its successor.
One idea is to use a launcher more closely related to the shuttle
"The committee did not identify any credible approach employing new capabilities that could shorten the gap to less than six years," the summary report notes.
"The only way to significantly close the gap is to extend the life of the shuttle programme."
This is one of the five broad options presented by the panel to President Obama.
Most involve the cancellation or modification of Constellation as currently envisaged. Most see a greater role for the private sector in launching astronauts to low-Earth orbit.
And most also call for an extension of the International Space Station project to 2020, well beyond the currently planned end point of 2016.
Nasa already has spent some $8bn developing Constellation. The proposed Ares I rocket and its crew carrier called Orion are both at an advanced stage.
Orion has just passed its preliminary design review and a test version of the Ares 1 stands ready at the Kennedy Space Centre for a demonstration flight in October.
Many commentators have speculated that the Ares 1 will be cancelled, but they suspect Orion will live on, perhaps launching on a vehicle with a design which relies more heavily on space shuttle technology.
The Augustine panel will deliver its full report later this month. President Obama will consult his new Nasa Administrator Charles Bolden and chief scientist John Holdren before responding. A new vision for Nasa is expected to be outlined in October.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.