Live at the Cape: Atlas countdown under way

Countdown to the planned launch at twilight tonight of an Atlas rocket is under way at Cape Canaveral Air Force Station as United Launch Alliance gears up for a super-secret spacecraft-delivery mission.

The rocket and its cargo -- a payload owned by the National Reconnaissance Office -- are slated to blast off from Launch Complex 41 at 5:03 p.m. The exact launch window is classified but a launch danger zone off the coast of Cape Canaveral will reopen to mariners at 7 p.m.

Mission managers were briefed on the weather forecast about a half-hour ago. The forecast still calls for an 80 percent chance that conditions will be acceptable for launch. However, the launch weather officer was keeping tabs on a low cloud deck and rain showers moving toward the Cape from the south. The cloud deck created a ceiling at about 6,500 feet, but the showers are dissipating as the cross the coast.

Engineers now are preparing to load liquid oxygen into the Atlas and Centaur stages of the rocket. Propellant lines that route the supercold oxidizer into the tanks are beiong chilled down.

We aim to webcast the United Launch Alliance broadcast of the launch live here in The Flame Trench beginning about 20 minutes prior to liftoff. Click the link above to launch our Atlas launch viewer. We also hoping the NRO lifts a blackout on the live video feeds at the launch pad as well as the live video stream. That also should happen about 20 minutes before launch, so refresh this page for periodic updates.