What is a word which means something difficult or close to impossible to achieve? For example,
A ten-percent growth rate is a __: it is certainly possible on paper, but very difficult to accomplish.
I can think of a castle in the air or an ideal prospect, but I would love to have a simple one-word substitute.

single-word-requeststag? I mean no dissent. – cornbread ninja 麵包忍者 Mar 26 '12 at 20:33