The clandestine payload going up this time, known only by its launch identification number of NROL-32, is widely believed to be an essential eavesdropping spacecraft that requires the powerful lift provided by the Delta 4-Heavy to reach its listening post.

In an address to the Air Force Association conference in September, NRO Director Bruce Carlson, a retired Air Force general, said this rocket launch would carry "the largest satellite in the world on it."

The NRO has flown various types of communication-interceptors since the dawn of the space age, and analysts say it is virtually certain this Delta 4-Heavy is hauling another.

"I believe the payload is the fifth in the series of what we call Mentor spacecraft, a.k.a. Advanced Orion, which gather signals intelligence from inclined geosynchronous orbits. They are among the largest satellites ever deployed," said Ted Molczan, a respected sky-watcher who keeps tabs on orbiting spacecraft.

Essential U.S. Spy Satellite Launching Friday