Elon Musk: We'll hopefully demonstrate orbital propellant transfer next year.

“One of the other technologies that's key for Mars is doing orbital propellant transfer. So you can think of this like similar to aerial refueling for airplanes, but in this case it's orbital refilling of rockets, which has never been done before. But it is, you know, technically feasible.

I always feel like these things are a little NSFW sort of. Listen, you've got to transfer fluid somehow. This has got to be done. The two Starships would get together and one Starship would transfer fuel and oxygen. And actually, most of the mass is oxygen. It's almost 80% oxygen that gets transferred, a little over 20% fuel.

So, you send a Starship to orbit with full payload, and then you send a bunch of other Starships up and you would refill the propellant on that Starship. And once the propellant tanks are mostly full, then you can depart for Mars or the Moon. So, this is an important technology which we should hopefully demonstrate next year.”

From: SpaceX Update, May 29, 2025