Experts asked to review the US human spaceflight programme have given strong support to the use of commercial services to launch astronauts.
The Augustine panel published its final report on Thursday and said America could find cheaper, faster successors to the shuttle in the private sector.
The US space agency is developing two new rockets and a crew capsule.
But the committee has told President Barack Obama that these systems no longer meet the US's immediate needs.
Speaking at a press conference in Washington DC, lead members of the panel said that if crew transport services to the International Space Station were passed to the private sector, it would free Nasa to work on more difficult and more exciting objectives.
"We think this is a time to create a market for commercial firms to transport both cargo and humans between the Earth and low-Earth orbit," said Norm Augustine, the panel chairman and a former CEO of Lockheed Martin.
"While that is certainly not simple, it is much easier than going to Mars. We think Nasa would be better served to spend its money and its ability - which is immense - focusing on going beyond low-Earth orbit rather than running a trucking service to low-Earth orbit (LEO)."
The panel published its summary findings in September. The full report adds considerable detail.
It will be used by President Obama, his chief scientist Dr John Holdren, and the new Nasa Administrator Charles Bolden, as they seek to put the agency on a sustainable future course.
Nasa-watchers have long complained that the organisation is being asked to do too much on insufficient funds; and the Augustine panel has determined that only an additional $3bn annually will see America carry out meaningful space exploration beyond low-Earth orbit.
Nasa is currently working to produce two new rockets - known as Ares 1 and 5 - and a new crewship called Orion. These are intended to replace the shuttle but have the capability also to take astronauts back to the Moon.
However, the Augustine panel said this "program of record", although well managed and technically competent, no longer fitted to the timescales over which it was needed and to the budgets it had been given.
The committee cited what it saw as the paradox of Ares 1 and Orion entering into service in 2017, which, on current plans, was a year after the termination of the space station. And because no lunar-landing capability would be ready by then, the Orion spaceship would have nowhere to go.
But if the space station's life was extended (which the panel supported), the diverted funds would only further delay Ares 1 and Orion.
The panel said it had provided Mr Obama with "options or alternatives" not "recommendations", but it was clear from the Washington press conference that the committee had favoured paths for the president to pursue.
These included asking Nasa to oversee the development of a less sophisticated capsule than Orion that could possibly be launched on an existing, but upgraded, rocket. This course could see crew transport services enter into service in 2015 that cost the taxpayer some $5bn to develop.
This sum would be about the same as, or slightly less than, developing Ares 1 but the ongoing costs to Nasa for missions to LEO would be much more favourable.
Panel member and MIT engineering Professor Ed Crawley said: "You can interpret the report as saying that we're suggesting in this option a new way of doing business, a new form of partnership where Nasa is the anchor customer for this service, [where Nasa] would have to significantly incentivise its development because of the return on investment expected by any commercial investor, and [where Nasa] would continue to play an important role as the quality assurance and mission assurance agency for it."
Professor Crawley added that if the taxpayer carried some of the capital risk of development, commercial players could then make a profit by selling crew transport services to other nations and perhaps to space tourists.
The approach, Professor Crawley and Mr Augustine explained, would enable the agency to concentrate on developing a heavy-lift rocket and a high-specification crew-capsule for use beyond low-Earth orbit, but on a timescale that matched the available budget.
The two men championed what they termed a "flexible path" which had the eventual goal of putting people on Mars.
Under this scenario, Nasa would use technologies and equipments when they became available to visit progressively more interesting and more challenging targets. These might include missions to asteroids, a sweep around Mars or even a landing on one of the Red Planet's little moons.
"What this flexible path does is allow us to take some of the components that you would build anyway - the heavy booster and the capsule - and start exploring while we're building the lunar landing system and the lunar surface system, so that when they become available it's time to go to the Moon," Professor Crawley said.
"In the report, it shows we can leave low-Earth orbit in those scenarios in the early to mid-2020s."
Nasa moved a test version of its Ares 1 rocket to the launch pad on Tuesday in preparation for a demonstration flight next week. Mr Augustine and Professor Crawley said they still supported the launch.
The flight should provide data that is valuable to engineers no matter which rocket programme Nasa eventually follows.
President Obama is expected to give his response to the Augustine committee report in the coming weeks.