About 80% of the methalox propellant used by the Raptor engine is oxygen. If you manage to scrub oxygen from the very low earth orbit (VLEO), and then bring it to LEO, you are in business. The Japanese Space Agency put a satellite in VLEO in 2019 [1].

[1]https://en.wikipedia.org/wiki/Super_Low_Altitude_Test_Satell...