Calls for the United States to return astronauts to the moon before the end of the decade have been increasingly loud and frequent, emanating from bipartisan lawmakers and science advocates alike. But underlying that drumbeat is a quagmire of epic proportions.

NASA plans to use SpaceX’s Starship — the largest rocket system ever constructed — for a key portion of the lunar journey, yet it’s still unclear whether the vehicle will work. And a fierce competitor is nipping at the agency’s heels.

“The China National Space Administration will almost certainly walk on the moon in the next five years,” Bill Nye, the entertainer of “Science Guy” fame and CEO of the nonprofit exploration advocacy group The Planetary Society, said during a recent demonstration against the Trump Administration’s plans to cut science funding. “This is a turning point. This is a key point in this history of space exploration.”

Starship is still in the nascent stages of a long and laborious development process. So far, parts of the vehicle have failed in dramatic fashion during six of its 10 test flights. Another prototype recently exploded during ground testing. SpaceX is set to launch its next test, Flight 11, as soon as 7:15 p.m. ET Monday from the company’s South Texas launch facilities.

The megarocket has yet to hit several key testing milestones. These include figuring out how to top off Starship’s fuel as it sits parked in orbit around Earth. Such a step is necessary given the vehicle’s design and enormous size — but it’s never been attempted before with any spacecraft.

Adding to the uncertainty is that no one knows exactly how many tankers full of fuel SpaceX will need to launch to give Starship enough gas for a moon-landing mission, which NASA has planned for mid-2027.

A SpaceX Starship prototype launches from Boca Chica beach on the fourth integrated test flight of the rocket system on June 06, 2024, from Starbase, Texas. Brandon Bell/Getty Images

One SpaceX executive estimated in 2024 that number “will roughly be 10-ish.”

But, more recently, engineers at NASA’s Johnson Spa