SpaceX’s Starship is powered by methane. Its Falcon 9 runs on highly refined kerosene known as RP-1.

But if a new tweet means anything, SpaceX CEO Elon Musk already has his eyes set on a much higher-tech rocket fuel: antimatter.

In a reply to a post about antimatter rockets — hypothetical spacecraft that would be powered by antimatter — Musk tweeted just two words: “Ultimately, yes.”