(25 Aug 1997) English/Nat The challenge of saving Russia's Mir space station from the junk heap of space history is only the latest example of the difficulty of living, and surviving, in space. But scientists at America's NASA space agency say the effort is worth the risk and cost, if humans are to have an on-going presence in space. The effort to salvage Mir, they say, is the first step toward a more permanent base for humans -- the international space station. And NASA is hoping that new