Nasa 'will need more astronauts'
Nasa does not have enough astronauts, even though the space shuttle programme has ended, a report says.
The National Research Council, a non-profit group advising on science policy, said Nasa should increase the size of its space-flying crew.
It said astronauts were needed to staff the International Space Station (ISS) and pave the way for new exploration.
American astronauts currently have to travel on Russian spacecraft in order to reach the ISS.
The US space shuttle programme ended in July, after 30 years.
After many American astronauts retired or quit this year, there are only 60 left, down from a peak of 149 a decade ago.
Wednesday's report warns that Nasa's current projected astronaut corps size of 55-60 could pose a risk to crew replacements in case of illness or injury.
It said: "The committee concluded that the currently projected minimum staffing target size for the active astronaut corps poses a risk to the US investment in human spaceflight capabilities."
It added: "Given the range of potential crew assignment constraints and uncertainty in future requirements, the astronaut corps appears to be sized below the minimum required.
"The current plan for the size of the astronaut corps does not have the flexibility to accommodate commercial, exploration, and new mission development tasks, or unexpected increases in attrition."
The committee that worked on the report for Nasa was made up of 13 experts, including five former astronauts.
US astronauts currently fly on Russian Soyuz capsules to reach the ISS. A planned mission to the space station is on hold after a failed launch of cargo in August.
The station will become vacant temporarily at the end of November if Nasa cannot launch a new crew by then.