Jim Bridenstine, administrator of the U.S. National Aeronautics and Space Administration (NASA), speaks during the World Agriculture Expo in Tulare, California.

Bloomberg | Bloomberg | Getty Images

Hours before the planned liftoff of SpaceX’s first crewed flight, NASA Administrator Jim Bridenstine told CNBC on Wednesday that the space vehicle for the mission is probably the safest to ever carry astronauts. 

Bridenstine said that while the administration had concerns at the start of its partnership with SpaceX about fueling the rocket while astronauts were aboard, he’s been assured of its capabilities over the last few years.

Elon Musk’s company designed and built the capsule, named Crew Dragon, that is scheduled to lift off at 4:30 p.m. ET Wednesday to carry NASA astronauts Bob Behnken and Doug Hurley to the International Space Station.

“Make no mistake, there were concerns about fueling while the astronauts were on board. But, over the last number of years, we have been made very comfortable. This is probably the safest vehicle astronauts have ever launched on,” Bridenstine said in an interview on “Squawk on the Street” from the Kennedy Space Center. “Of course, I’m saying that in theory because it hasn’t been tested and with humans on board at this point.”

The administrator’s comments came six hours ahead of what’s expected to be a historic moment for spacecraft and the partnership between NASA and Musk’s SpaceX. The mission, named Demo-2, is the first crewed mission in SpaceX history and the first launch of NASA astronauts from U.S. soil since 2011.

READ ALSO  New map shows where China's latest virus cases are clustered

“What NASA did, when we came up with this program to launch commercially: We did not say how to design your vehicle. We did not tell them how to do it,” Bridenstine said. “What we said is, ‘Here are the top-line requirements for capability and the top-line requirements for safety. And you need to prove to us, through your engineering prowess, you need to prove to us how you’re going to achieve those end states.'”

Since the space shuttle program retired almost 10 years ago, the U.S. has paid Russia more than $80 million per seat to fly NASA astronauts to the International Space Station. But in partnerships with SpaceX and Boeing under a program called Commercial Crew, NASA hopes it can reduce those costs by teaming up with private industry.

Though Bridenstine said Wednesday the launch was still likely to happen at 4:33 p.m. ET as planned, NASA is watching a weather system off the coast of Florida in case it needs to abort the mission due to safety concerns. 

Subscribe to CNBC PRO for exclusive insights and analysis, and live business day programming from around the world.