The space shuttle will not fly today:
From NASA: NASA has scrubbed the launch of Discovery because of a problem with a fuel tank sensor designed to protect the Shuttle’s engines by shutting them down if fuel runs low.
I haven’t heard yet how deep within the bowels this sensor is buried. I do know they’ll have to untank before they start.
Whenever they get this fixed, the daily launch window will continue to be very small – only about 10 minutes. The shuttle will be trying to catch up to the International Space Station and the time they’re in proper alignment is brief.