CAPE CANAVERAL, Fla. — A top NASA manager is casting doubt on the space agency's ability to land astronauts on the moon by 2024.

Kenneth Bowersox, acting associate administrator for human exploration, told a Congressional subcommittee Wednesday that NASA is doing its best to meet the White House-imposed deadline. But he says he wouldn't bet anything on it.

Bowersox — a former space shuttle and space station commander — says it's good for NASA to have "that aggressive goal." He says many things need to come together, like funding and technical challenges, for 2024 to stand a chance. And he acknowledges there's a lot of risk in making that date.

The Trump administration urged NASA in March to accelerate its moon-landing plans by four years to 2024.

Bowersox's testimony comes on the same day as a report released by the General Accounting Office, which found NASA needed to do more to plan for unforeseen issues in its major projects, including cost and production date issues. 

For instance, the GAO said NASA was understating the cost growth of its Space Launch System vehicle by more than $750 million. That rocket is supposed to take astronauts back to the moon and beyond.