NASA’s dwindling astronaut ranks may leave the agency unable to adequately live up to its staffing commitments aboard the International Space Station and put a half-century of U.S. human spaceflight experience in jeopardy, according a former NASA associate administrator and chairman of a new National Research Council report on the astronaut corps in the post-shuttle era.

Joe Rothenberg, the retired NASA associate administrator for space operations and a former director of Goddard Space Flight Center, chaired the 13-member panel that produced the study. “I think the country would like to think we have a certain capability in human spaceflight that is equal to if not better than the Russians and the Chinese at this point,” Rothenberg tells Aviation Week. “If we had to give up a position on the space station because we could not fill it, we would lose our human spaceflight pre-eminence.”

NASA needs additional flexibility in managing its shrinking astronaut corps to cope with the effects of long-duration missions aboard the station on human health, unanticipated attrition and the undefined role of astronauts in the development of commercial space transportation services and spacecraft for deep-space exploration, according to the Sept. 7 report.

The astronaut corps reached a peak of nearly 150 just over a decade ago, when NASA was launching four to five space shuttle missions annually and initiating continuous staffing of the International Space Station. The number fell to 61 this year, as the shuttle was retired in July and the major assembly and outfitting of the station’s U.S. operating segment concluded.

NASA forecasts further attrition—a drop to 56 personnel within five years—though policy makers have yet to settle on an operating strategy for emerging commercial crew transportation services to ferry astronauts to and from the station as well as a plan and timetable for launching U.S. astronauts on missions beyond low Earth orbit.

Current trends could stimulate demand for astronauts to staff the station and participate in new ventures at a level close to the number of available personnel within three years, according to the 102-page report.

The agency’s vulnerability surfaced twice in the past year. Weeks before the February launch of the STS-133 mission, shuttle astronaut Tim Kopra was sidelined by injuries from a bicycle accident. NASA scrounged to find a replacement with his spacewalk skills. Astronaut Mark Kelly elected to command the May STS-134 mission flight even though his wife, Rep. Gabrielle Giffords (D-Ariz.), was critically wounded during a Tucson shooting spree. The agency looked to drafting a astronaut from its management ranks as a possible replacement.

As policy makers huddle over where NASA goes beyond the shuttle, few doubt the U.S. can resume launching its own astronauts by developing new spacecraft if it chooses to commit the resources. But the inability to fill its mission commitments while it temporarily relies on the Russians for launch services would be troubling, according to Rothenberg.

“The minute we can’t fill a position, the image of the human spaceflight capability of the United States falls to a new low,” he says. “We not only can’t fly people up, we don’t even have the people to go when there is the opportunity. We can’t live up to our commitments.”

Currently, NASA assesses its astronaut personnel needs annually and adds a 25% margin to deal with uncertainties. Until 2010, the agency used a 50% margin but agreed to the reduction in response to budget controls. The margin should be increased, according to the report.

“Whether it’s 30 or 50 percent, we did not determine,” Rothenberg says. “But we did determine that at the 25-percent level they have some real challenges.”

The challenges NASA faces include training demands, which can stretch to five years between the time a new astronaut candidate is selected and launched on a first mission, according to the panel. In spite of rigid medical screening, six-month missions on the space station are producing some health surprises in addition to the expected atrophy of bones and muscles—most recently a vision problem that can affect the eye sight of some astronauts after they return to Earth. Limits on radiation exposure also restrict the number of times an astronaut can serve aboard the orbital outpost.

The report also recommends that NASA continue to train astronauts at Johnson Space Center in Houston and include in that training flight time in the agency’s fleet of T-38 supersonic trainers. The flight training provides the agency’s non-military astronauts with experience in managing risky operations, the panel says.

The number of T-38s is projected to fall from 31 a decade ago to 16 within five years. They were first acquired by NASA in 1964 and maintained at Ellington Field in Houston.

NASA spokesman Mike Curie characterizes the report as supportive of the agency’s space station plans, efforts to foster commercial space transportation services and future exploration capabilities.

“It also provides guidance on the use of existing NASA facilities and other resources to train for future missions,” he says.