This episode's headline: On the Brink; Whitewater Revisited; Conversation - Stepping Down; New Galaxies. ANCHOR: JAMES LEHRER; GUESTS: LEON ARON, American Enterprise Institute; ADRIAN KARATNYCKY, Freedom House; MICHAEL McFAUL, Stanford University; SEN. WILLIAM COHEN, [R] Maine; DR. ED WEILER, NASA; CORRESPONDENTS: ELIZABETH FARNSWORTH; KWAME HOLMAN; CHARLAYNE HUNTER-GAULT