Administrator Jim Bridenstine said he is considering switching to commercial rockets to keep the June 2020 launch date.
Mr Bridenstine told a Senate committee that two private rockets would be needed, one to launch the Orion crew capsule and its European-built service module, the other to launch an upper stage.
Orion would have to dock with the upper stage in orbit around Earth, before heading to the moon.
NASA’s SLS, or Space Launch System, rocket could do everything in one go. That is why it is “a critical piece of what NASA needs to build”, Mr Bridenstine told the Senate commerce, science and transportation committee.
At present, Orion does not have the capability to dock with anything in orbit. That outfitting would have to be completed between now and next year, Mr Bridenstine said.
“This is 2019,” senator Roger Wicker, the committee chairman, reminded Mr Bridenstine. He added: “I’d sure like to keep us on schedule.”
Mr Bridenstine noted this option might require more money from Congress.
NASA is pushing for a sustainable moon programme this time around, as opposed to the come-and-go Apollo lunar landings half-century ago. The goal is to have an outpost with astronauts near the moon to serve as a stepping-off point for lunar landings.
This first mission coming up – essentially a three-week test flight – would carry no crew and would not land. Rather, the Orion would come close to the lunar surface before taking a big lap around the moon.
Mr Bridenstine said Nasa will decide in the next couple weeks whether to stick with its rocket and delay – or go commercial for this one test flight.
If private rockets are used – and Mr Bridenstine did not list preferences or mention any by name – the SLS would make its launch debut for NASA’s second exploration mission by 2023. That mission would carry astronauts around the moon.
This first test flight originally was scheduled for this year.
“I want to be clear: NASA has a history of not meeting launch dates, and I’m trying to change that,” Mr Bridenstine said.
NASA already is using private companies to make International Space Station shipments.
Just last week, SpaceX successfully completed the first test flight of its new Dragon capsule designed for astronauts. It could begin flying crews to the station from Florida this summer.