Currently, NASA only flies astronauts to the International Space Station aboard SpaceX's Crew Dragon capsules and Russia's Soyuz vehicles.

"After reaching its peak of nearly 150 astronauts in 2000, the size of the corps has diminished with the end of space shuttle missions in 2011 and now stands at 44, one of the smallest cadres of astronauts in the past 20 years," officials wrote in the report.

"As NASA enters a new era of human space flight, including returning to the moon and eventually landing humans on Mars, effective management of its astronaut corps — the people who fly its space flight missions — is critical to the agency’s success.".

The current size of the corps is in part due to a surge in retirements — about 10 a year, according to the report — around 2011 when the agency grounded its fleet of space shuttles and flight opportunities starkly decreased.

"With a corps aligned to a single mission, as it is now with the ISS [International Space Station], the Astronaut Office is in a position to quickly reassign astronauts because all 44 have been selected and initially trained for the same mission," officials wrote in the report.

Artemis missions are expected to require about the same period of specialized training as space station missions, the report noted.

But while NASA has selected a group of 18 astronauts from which to pull Artemis crewmembers, it has not assigned any seats yet, nor has the agency developed the training program for its moon missions; the report warned that the agency may be running low on time for that process.

"While the Astronaut Office estimates training for the Artemis 3 and successor missions will require approximately two years, even with the projected delays to Artemis 2 and 3 launches the agency could be overestimating the time available to develop and implement the necessary training framework and regimen across key Artemis systems," the report noted.

Overall, the report conveys concern that NASA's astronaut corps will join the list of constraints on future missions along with factors like budgets, spacesuit supply and rocket manufacturing.

— International Space Station shines in gorgeous fly-around photos by Crew Dragon astronauts.

For example, as NASA pushes to diversify its representation in space, the agency needs accurate demographic information about its astronauts, the report noted.

Similarly, as missions head to planetary surfaces instead of low Earth orbit, tracking which astronauts have backgrounds in geology — currently just four astronauts — will be important, according to the report.