CRAWFORD, Texas (Reuters) - President Bush (news - web sites), in his big space policy announcement on Wednesday, will call for replacing aging U.S. space shuttles with a new generation spacecraft to get Americans back to the moon and on to Mars, officials said on Saturday. Seeking to give NASA (news - web sites) a new mission a year after the shuttle Columbia tragedy and to provide a space vision for his re-election campaign, Bush will set a goal of returning to the moon by the middle of the next decade and establishing a human presence there as a...