> 2) surely we'd have noticed if there was a space fuel depot
> 2b) if there was, why would it need to find it at a lower altitude?
Well, it's a space plane, so I was thinking of a more exotic variation of conventional air refueling, but upon further thought that seems very unlikely as if it was that easy to get from land to the refueling point and from the refueling point to space, we'd see that as the usual way to get to orbit instead of rockets (or rocket attached ships). I don't spend a lot of time thinking about rocketry and orbit unless in a discussion about it, so this wasn't immediately obvious to me initially.
That said, it's possible that some satellites actually contained extra fuel for future use when deployed, either in them or as a separate attached payload. In that case, there wouldn't be one large space fuel depot, but possibly many small ones.
> 2b) if there was, why would it need to find it at a lower altitude?
Well, it's a space plane, so I was thinking of a more exotic variation of conventional air refueling, but upon further thought that seems very unlikely as if it was that easy to get from land to the refueling point and from the refueling point to space, we'd see that as the usual way to get to orbit instead of rockets (or rocket attached ships). I don't spend a lot of time thinking about rocketry and orbit unless in a discussion about it, so this wasn't immediately obvious to me initially.
That said, it's possible that some satellites actually contained extra fuel for future use when deployed, either in them or as a separate attached payload. In that case, there wouldn't be one large space fuel depot, but possibly many small ones.