Reason? A leak in the vent line from the main external tank out into the launch pad. You see, when they load that large red rocket with hydrogen, the gas that rises to the top has to go somewhere --which is the reason for those "vent lines". This way, the Shuttle won't explode on the pad instead of going upward into the atmosphere & Space. They had to wait a day to get close to the tank because of the hydrogen gas leaking out. (One spark and no more Discovery! We don't need that!)*
So the mission (STS-119, as you can see by going to the Shuttle section of NASA's website) may be cut short, but hopefully, things will get done - such as the last section of the ISS (International Space Station).
I've been interested in Space since I was about 6 years old; that's when I saw the only night launch of the Apollo missions - from across the state. I remember watching on CBS as Man walked on the Moon. I remember when the shuttles Atlantis and Discovery made their first flights. Now I understand what goes into NASA - and how it benefits not just the communities, but the counties where each facility is.
Hm....thought of going up there to actually view the launch... but then, how would I keep myself occupied all day? I'd have to get up there at 4am just to get a good spot (and no, I kid you not. You don't get up early, forget about finding a spot on the beach within 20 miles of KSC on Launch Day - let alone a hotel! And the Tickets get snapped up quick; average time is two hours after they announce when the Box Office is open). But nah. I have to record this, as I've done since I've gotten to this place. I'm glad NASA-TV is part of the Basic Package on my cable! (If you have a satellite, check with your provider - especially if you have a HDTV box. You may be able to get that channel for a small yearly fee.)
So when is it scheduled to go up? Well, if you didn't click on the first link, or on KSC's link, it's 7:42pm US Eastern Daylight Time on Sunday. I'll probably start recording when I know they're coming out of the last hold - the "9 minute hold". That means that it'll be Nine minutes before Launch when that clock starts up at that point. But that's not the only thing I record; I also record the replays from all the cameras they use to watch the Shuttle go up.
You can do something before and during the launch: Pray - pray that nothing goes wrong. If you remember what happened to Challenger and Columbia, you'll know why I said that.
*Thank you WESH reporter Dan Billow, for explaining that!