The "main idea" is not an exactly defined concept. It is also not an aspect of English grammar. So you will need to think about this sentence in context.
Could you omit either part without changing the main point of the paragraph? In this case, I suspect the author is talking about interplanetary travel. That would make "ice-rich asteroids could become interplanetary refuelling stations" the main part (and the first part is just an explanation of a detail).
It is possible (but unlikely) that the paragraph is about making fuel, in which case the main part is "Splitting water into oxygen and hydrogen makes spacecraft fuel" (and the second part is just an example of why this is important).