Posted: Mon, Mar 9, 2009, 1:51 PM ET (1751 GMT)

The countdown for Wednesday night's launch of the shuttle Discovery on a mission to the International Space Station started as planned Sunday evening. The countdown for the launch of STS-119 began at 7 pm EDT (2300 GMT) Sunday. Liftoff is scheduled for 9:20 pm EDT Wednesday (0120 GMT Thursday), with excellent weather forecast for the launch and no major technical issues being addressed in the countdown. STS-119 will install the S6 truss segment on the station and deploy its solar arrays, and also exchange one member of the station's crew.