UPDATE (1:47 p.m.) - Liftoff!

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UPDATE (1:44 p.m.) - ULA Launch Director Tom Heter III has confirmed that the Delta IV Heavy rocket is ready and NRO Mission Director Col. Chad Davis has given final permission to launcha readiness poll of the launch team verified all systems are "go."

T-minus 4 minutes and counting began at 1:43 p.m.
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UPDATE (12:20 p.m.) - Launch leaders have pushed back the scheduled launch time by one minute to 1:47 p.m.

According to the ULA website, the adjustment was made due to a Collision Avoidance (COLA) closure that prohibits liftoff at the opening of the launch window.

"COLAs are brief moments in time when the launch is not allowed to occur because the trajectory would pass too close to another object already in space. This analysis is based on a screening of known active and debris objects in orbit that could cause a conjunction with the ascending rocket and payload."
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(7:44 a.m.) - A Delta IV Heavy is scheduled to launch from Vandenberg Air Force Monday.

United Launch Alliance (ULA) says the launch is set for 1:46 p.m., adding that the latest weather forecast shows a 40 percent change of acceptable conditions for the launch.

Base officials say the rocket is carrying a National Reconnaissance Office payload and will lift off at Space Launch Complex-6.