An invincible checkers-playing program named Chinook has solved a game whose origins date back several millennia, scientists reported Thursday on the journal Science's Web site. By playing out every possible move -- about 500 billion billion in all -- the computer proved it can never be beaten. Even if its opponent also played flawlessly, the outcome would be a draw. Chinook, created by computer scientists from the University of Alberta in 1989, wrapped up its work less than three months ago. In doing so, its programmers say the newly crowned checkers king has solved the most challenging game yet cracked by a machine -- even outdoing the chess-playing wizardry of IBM's Deep Blue.