Live at the Cape: Delta IV Countdown Under Way

The countdown to the planned launch of a Delta IV Heavy rocket is under way and the Air Force is warning mariners to stay out of a launch danger zone of the coast between 5:30 p.m. tonight and 12:30 a.m. Sunday.

The 24-story United Launch Alliance rocket is slated to blast off from Launch Complex 37 at Cape Canaveral Air Force Station at 7:33 p.m. tonight. The launch window this evening will extend to 11:33 p.m.

Engineers soon will begin the lengthy process of loading liquid hydrogen and liquid oxygen into the behemoth booster, which is the largest unmanned rocket in the nation's stable of space launch vehicles.

The first stage of the rocket is distinguished by three common core boosters that are strapped together to form the triple-bodied vehicle. An upper stage sits atop the first stage and the stack is topped by a classified National Reconnaissance Office payload.

A security zone surrounds the launch complex and a Launch Hazard Area extends out into the Atlantic Ocean under the flight path the rocket will follow into orbit.

Check out a map here: Launch Hazard Area

Violators can be jailed for six years and fined up to $250,000.

Air Force meteorologists say there is a 90 percent chance conditions will be acceptable for launch. The prime concern is a chance of high winds or thick clouds over the launch complex.

You can check out the official forecast issued by the Air Force 45th Space Wing Weather Squadron here: Launch Forecast.

We'll have updates here in The Flame Trench throughout the day, so make sure to refresh this page for the latest news. We'll also webcast live United Launch Alliance television coverage of the terminal countdown and launch starting 25 minutes prior to liftoff. At that time, you'll see a video player embedded in the top item in the blog. Simply click it to launch live coverage.

We'll also send text message alerts on the status of the launch. Sign up here to receive those alerts on your cell phone.

Flying onboard will be a classified payload owned by the National Reconnaissance Office, the super-secret operates the nation's fleet of spy satellites. The launch attempt will be the fourth since Tuesday.