Well, wouldnt only the magnets have to be cooled? And they have gotten superconductors up to 177K according to that link. Not saying it would be very practical, just questioning whether it is completely off the table. (Not that I consider the asteorid belt to be the limit for that, as we are fast approaching a level of robotics where asteorids could be remotely mined. But that is a digression I guess.)