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"Rubicon" - There's no turning back, on ABC's "Defying Gravity," SUNDAY, AUGUST 23 (10:00-11:00 p.m., ET). It's a humbling moment for the crew of the spaceship, as each of them deals with a personal point of no return, while the Antares itself crosses a critical boundary when going back to Earth is no longer an option, and the home planet becomes just another twinkling light in a vast canopy of stars. (ABC/SERGEI BACHLAKOV) TY OLSSON, LAURA HARRIS