Fans of GSC Game World’s S.T.A.L.K.E.R.: Shadow of Chernobyl who haven’t yet had their fill of shooting radioactive mutants will be pleased to have installed Steam today, as the prequel has been announced as exclusive to the digital distribution platform. S.T.A.L.K.E.R.: Clear Sky will still be coming to retail shelves early this year, but it won’t be available online outside of Steam, a nice get for the boys and girls at Valve. For a look at the first-person shooter’s ability to render grass and all things abandoned, check out a baker’s dozen screen shots in the gallery below. Official word on the game is after the jump.