Delta 4 Heavy Launch Delay Likely

A delay of up to two hours appears likely for tonight's planned launch of a Delta 4 Heavy rocket from Cape Canaveral Air Force Station.

Fueling of the rocket's three core boosters has not begun yet, making the targeted 7:45 p.m. launch all but impossible, based on timelines for most recent Delta 4 Heavy launch in 2007.

United Launch Alliance is not commenting on the the status of the launch because of the secrecy of the National Reconnaissance Office mission.

Launch controllers announced earlier this afternoon that technicians were studying an issue with a gaseous nitrogen relief valve that must be resolved before cryogenic fueling can begin.

The launch window extends for four hours, until 11:45 p.m.

We'll update The Flame Trench as soon as a new target launch time is disclosed.

If the launch were delayed until Thursday, forecasts show a 70 percent chance of favorable weather.