Many astronauts have retired or quit with this year's end of the shuttles. NASA expects to operate the International Space Station until 2020, however, and U.S. companies hope to provide spacecraft for crews.
NASA's astronaut corps totaled nearly 150 in 2000. It's now down to 60. NASA projects it will need 55 to 60 astronauts over the next five years.
The report released Wednesday by the National Research Council warns that may not be enough.
NASA had asked the council to look at the role and size of the astronaut corps during this transition time. A committee of 13 experts - five of them former astronauts - conducted the study.