SAN FRANCISCO — NASA’s dwindling corps of astronauts may not be large enough to staff the international space station, support the development of commercial crew vehicles and help space agency officials draft plans for future exploration missions beyond low Earth orbit.

That was the conclusion of a report, “Preparing for the High Frontier — The Role and Training of NASA Astronauts in the Post-Space Shuttle Era,” published Sept. 7 by the National Research Council (NRC). Panel members concluded that leaders of NASA’s astronaut office at the Johnson Space Center in Houston are doing a good job of ensuring that the space agency recruits and trains enough astronauts for the job they have been assigned to perform: staffing the international space station. However, NASA also will need to rely on astronauts to support the space agency’s plans to help private companies develop commercial crew taxis.

“The astronaut office has never been tasked with supporting the commercial entities that will depend on them or with helping the Federal Aviation Administration draft whatever policies, rules and regulations will have to be in place when commercial crew transportation begins in earnest,” Fred Gregory, a former astronaut and NASA deputy administrator who co-chaired the NRC panel, said. “The astronaut office has never been tasked with supporting exploration of the Moon or Mars.” Nevertheless, it is clear that astronauts will be needed to perform those jobs, he added.

As a result, NASA officials may need to reassess the number of astronauts the space agency employs. In 2000, NASA’s corps of astronauts peaked at 150. Currently, there are 60 NASA astronauts, NASA spokeswoman Kylie Clem said Sept. 8. An additional astronaut, Mark Kelly, has announced plans to retire in October to write a book with his wife, U.S. Rep. Gabrielle Giffords (D-Ariz.). Nine additional astronaut candidates who were selected in 2009 are undergoing training, the report said.

NASA officials have estimated that the space agency will need a minimum of 55 to 60 astronauts to perform their assigned jobs through 2016. That figure was determined by evaluating the number of astronauts needed to staff the space station, prepare for space station missions and recover post-flight. The resulting number is augmented by 25 percent to meet any unforeseen demands. Until 2010, NASA estimated the number of astronauts needed and augmented that figure by 50 percent to account for possible shortfalls. The 25 percent margin was adopted due to budget pressures, according to the NRC report.

“There are a huge number of jobs that are talked about and anticipated that are not incorporated in that model,” Gregory said.

There is also great uncertainty. Since the administration of President Barack Obama announced plans in early 2010 to cancel the Moon-bound Constellation program, NASA’s human spaceflight program has been in flux, making it difficult for space agency officials to determine how many astronauts will be needed to perform future missions. “Now as the shuttle is retired and the international space station enters its fully operational phase, the agency is undergoing a new and uncertain transformation which will also have implications for the Flight Crew Operations Directorate, the astronaut office and the astronaut corps,” the report said.

Astronauts require years of training. NASA will be unable to quickly augment its astronaut corps to respond to an emergency onboard the space station or any other demand, the report said. New astronauts typically train for a minimum of three years prior to their first space station mission, familiarizing themselves with the international laboratories, acquiring the knowledge and skills needed to spend months in orbit, and preparing to conduct various science experiments. While planning space station crews, NASA officials seek to pair inexperienced astronauts with veterans and to ensure that missions include people with diverse skills.

In addition, the NRC panel concluded that the additional physical and emotional demands of long-duration stays onboard the space station could prompt some astronauts to leave the space program. After flights, astronauts may be prevented from returning to spaceflight temporarily or permanently due to bone loss, vision problems, radiation exposure or physical injuries suffered during spacewalks, the report said.

The stress of leaving one’s family for several months may also limit the number of astronauts willing to make repeated trips into orbit. “How many times do folks want to go to the international laboratory for six months?” Gregory said. “Will they do it once, get the T-shirt and quit?”

The NRC report also evaluated NASA’s ground-based astronaut training and simulation facilities and approved the agency’s plans to retire some of the facilities designed specifically to support the space shuttle with one exception. The Shuttle Engineering Simulator Dome might prove useful in future efforts to train space station crews for rendezvous and docking, according to the report. The NRC panel said NASA’s ongoing use of T-38 jets was appropriate because it helps to prepare flight crews for the “fast dynamics, physical stress and risk found in spaceflight.”

Debra Werner is a correspondent for SpaceNews based in San Francisco. Debra earned a bachelor’s degree in communications from the University of California, Berkeley, and a master’s degree in Journalism from Northwestern University. She...