The Ares 1 rocket programme is too expensive and too late, the report says
A White House panel has suggested that Nasa should scrap its investment in the Ares 1 rocket and use commercial vehicles to launch astronauts instead.
The option has been put forward by a presidential advisory panel as part of a review of the US space programme.
The suggestions come just days before a test version of the Ares 1 is due to make a demonstration flight.
The review says the Ares 1 is too expensive to replace the space shuttle, and will be ready too late.
The review also suggests US policy shift away from Moon-focused initiatives for human space exploration towards other destinations.
Panel Chairman Norman Augustine said that astronauts could aim for a nearby asteroid or one of the moons of Mars, with the ultimate goal being a landing on the Red Planet.
Nasa currently spends about half of its $18bn annual budget on human space exploration.
Some $8bn has already been invested in the agency's post-shuttle plans, which go under the name of the Constellation programme.
These plans include the proposed Ares I rocket, its crew carrier called Orion, and a heavy-lift rocket known as Ares 5.
The prototype Ares-1X rocket is one of a number of demonstration vehicles which will lead eventually to the introduction of the Ares 1
But the panel said the rocket would not be ready to serve as a launcher for space station crews until 2017, by which time the International Space Station might have been removed from orbit.
"The slippage has caused a mismatch between what Ares 1 is needed for and what it's going to be able to do," Mr Augustine said.
The panel puts forward the option of making greater use of the commercial sector to take astronauts to low-Earth orbit.
If private space taxis took over the ferrying of crews to and from the space station, the Augustine committee said, it would free the space agency to focus on more challenging projects such as getting humans out into the Solar System.
But the government would have to invest about $5bn in the private sector to help it develop, the advisory group added.
"It is very clear that no commercial entity could raise the risk capital to build a rocket and capsule and recover the costs in our lifetime," Ed Crawley, a committee member and professor at the Massachusetts Institute of Technology said.
Nasa would be the anchor customer for the taxis, but rides could also be sold to other countries, scientists and to space tourists.