But he already has an idea for how to get around the problem.
SpaceX CEO Elon Musk is well aware of a potentially fatal flaw in the company's plan to use a Moon base as a refueling station, but he's still hopeful they'll find the carbon needed to create fuel if they dig deep enough.
"Big challenge for Starship refueling on the moon is finding sources of carbon," he tweeted on Monday. "Probably some pretty big [carbon] deposits in craters from meteorites. Same goes for hydrogen & oxygen, also in (shadowed) craters."
The idea behind a lunar refueling station is that it would allow SpaceX to send its Starship into space with just enough propellant to reach the Moon. That would cut down on the spacecraft's weight — and, crucially, the cost of lifting it out of Earth's immense gravity well.
Then, Starship could fuel up on the Moon for the return to Earth – or for a flight to Mars or beyond — and take advantage of the weak lunar gravity.
Can You Dig It?
Still, planetary physicist Phil Metzger cautions that reaching the carbon below the Moon's surface won't be easy.
"Here on the Earth, to set up a mine, it can take 20 years — and that’s on the Earth," he recently told The Verge. "So when you talk about setting up a mine on the moon, it’s harder, especially because we have less understanding of the resource, and we have zero experience in doing mining operations in that environment."
READ MORE: SpaceX: Elon Musk explains a "big challenge" facing Starship's moon base [Inverse]