STS-123 mission specialist Richard Linnehan participates in a spacewalk outside the International Space Station. Photo by NASA

A National Research Council study released Wednesday determined that NASA's current plans for staffing the U.S. astronaut corps “do not provide sufficient flexibility to reliably meet projected” International Space Station and other mission needs. 

The research stated, because the existing model NASA uses to predict minimum astronaut corps size requirements cannot fully account for uncertainties, the factor for uncertainty used in the formula  should be increased above the current 25 percent.

Potential constraints of and recommendations for the astronaut program identified in the 104-page study included:

-Retirements and medical disqualifications: As NASA transitions to only long-duration space station missions (generally three to six months for space station crews, compared to two or so weeks for shuttle crews), astronauts could become less suitable to perform repeat missions. 

-Development of future spacecraft for space station and deep space missions: Hiring a new class of astronauts would help NASA, but the most valuable personnel to retain includes veteran astronauts. Training for a long-duration space station mission currently lasts approximately two to three years. NASA “will not have the luxury of hiring new people to deal with a serious failure aboard the [space station], or in the early stages of the development and oversight of commercial spacecraft and spacecraft beyond [low-Earth orbit].” 

-An increasingly global space community: The shuttle's retirement has reduced certain training requirements, but NASA astronauts must now also be familiar with international modules and equipment aboard the space station. 

-Crew-related ground facilities, including the agency's fleet of T-38N training aircraft: They continue to play a pivotal role in sustaining the astronaut corps, and therefore should be maintained.

The panel of the NASA-sponsored study noted it was not asked to address whether or not the U.S. human spaceflight program should continue, or what form the program should take in the post-shuttle era. The recommendations were based on the assumption that human spaceflight would continue.

The space station is to remain in operation through at least 2020. Following the shuttle's retirement in July, NASA is relying on foreign spacecraft for access to and from the orbiting lab (Russian Soyuz capsules for crew transfers; other Russian and internationally supplied spacecraft for cargo transfers). U.S. commercial companies will primarily fulfill that role for NASA later this decade (crew-carrying vehicles are expected to become available around 2015; cargo-carrying versions are expected earlier). The agency, on the other hand, is focusing on developing its own system to take humans to farther destinations, such as to an asteroid and ultimately to Mars.

Next week, three of the six people currently living and working aboard the space station are scheduled to return to Earth. Remaining crew members are scheduled to stay aboard the complex until mid-November. However, due to the failed launching of a Russian Progress 44 cargo spacecraft in August, NASA and its international partners are evaluating whether to continue sending crews to the space station.

Reach Len Ly here. Twitter: @len_ly