The new system calls for a new rocket, Ares 1
The US Obama administration is taking a fresh look at what humans do in space and how they get there.
The White House has asked Norman Augustine, a former aerospace industry executive, to lead a review of Nasa's manned activities and report by August.
The US space agency is due to retire its shuttles next year and is working on a new crew transportation system, to be introduced in about 2014-15.
The replacement looks much like the old Apollo system in its architecture.
Work on the Orion capsule and its Ares launcher was initiated in 2005, following a directive the previous year from President Bush in his Vision for Space Exploration.
The intention was to give the US a crew-carrier and rocket technology that could go beyond low-Earth orbit to more distant targets, back to the Moon and further.
The Augustine panel will now assess whether this is the correct strategy. The White House's chief scientist, John Holdren, said "it would be only prudent" to review the human spaceflight programme given the scale of its ambition and "the significant investment of both funds and scientific capital".
Nasa is currently spending more than $250m (190m euros; £170m) a month on replacement activities, which go under the heading of the Constellation programme.
The expenditure will continue through the review. The agency is preparing to launch a test rocket that incorporates key features of the new Ares vehicle.
In a statement, the agency said: "The Review of United States Human Spaceflight Plans will examine Nasa development programmes and possible alternatives.
President Bush called for a return to the Moon
"The goal is to provide options that will ensure the nation's human spaceflight programme remains safe, innovative and affordable in the years following the space shuttle's retirement."
Critics have asked questions about the technical scope of Constellation and Nasa's ability to manage its cost. Some have called for the Ares launchers to be scrapped in favour of adapting existing rockets.
The announcement of the review came as Nasa's proposed budget for the financial year 2010 was given as $18.69bn.
This represents a $903.6m, or 5%, increase on the 2009 budget request. All up, President Obama is looking to add an additional to $2bn to Nasa's budget line for 2009 and 2010.
By way of comparison, the European Space Agency spends just over 3bn euros ($4bn; £2.7bn) a year on its activities.
President Obama has yet to name a Nasa administrator. Christopher Scolese continues to lead the agency in an acting capacity.
Speaking about the human spaceflight review, Mr Scolese told the Associated Press: "Clearly if we are on the wrong path, we should change. If you are asking me if I think we're on the wrong path, no, I don't."