Space shuttle Discovery’s astronauts activated one of the ship’s three auxiliary power units and performed a checkout of the orbiter’s flight control surfaces. This standard day-before-entry test provides assurance to the crew and Mission Control that Discovery’s elevons and speed brake will provide control once the shuttle enters the atmosphere for Wednesday’s 11:57 a.m. EST landing.

At about 6:40 a.m., the crew will “hotfire” Discovery’s Reaction Control System thruster jets to verify their ability to steer the shuttle through its entry profile before it encounters the atmosphere.

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