Nasa cannot continue on its present path, which includes staffing the International Space Station and returning astronauts to the moon by 2020, and fulfill its ultimate goal of getting people to Mars, according to the US space agency's new chief.
Nasa is trying to finish construction of the €70-billion space station and retire the shuttle fleet next year. It also is developing two new rockets and a capsule-style craft that can ferry crews to the moon and other destinations in addition to the station, which orbits 360 kilometres above earth.
"We cannot continue to survive on the path that we're on right now," Charlie Bolden, a retired Marine Corps general and four-time shuttle astronaut, said in a speech to Nasa employees.
Mr Bolden said he believes the space programme's long-term objective is sending people to Mars.
"The challenge for us in the next few months is to figure out what the single most efficient, most cost-effective path is to get there," he said.
Mr Bolden was appointed Nasa administrator by President Barack Obama' administration in May, about five years after his predecessor, President George W. Bush, set a goal for the agency to retire the shuttle fleet by 2010, resume sending humans to the moon by 2020 and then push on to Mars.
Returning astronauts to the moon was estimated in 2005 to cost about €77 billion, but that figure has since been cut by about €28 billion. Nasa spends about half of its €12.6 billion annual budget on human spaceflight.
A presidential panel is reviewing options for the US human space program and is expected to issue its report next month. Mr Bolden told workers the review is "nothing to be afraid of".
Concurrently, a second review that encompasses all areas of space - military, commercial, civil and scientific - is under way by the national security advisor, James Jones, a retired four-star Marine general, Bolden said.
"There needs to be a coherent policy and so President Obama has asked General Jones to put together a group to take a look at the national space policy," Mr Bolden said. "That's already under way to a limited extent but we hope to be participating in that as a full member."
Mr Bolden's address to Nasa employees came seven days into space shuttle Endeavour's 16-day construction mission to the International Space Station.