The U.S. Air Force on Sunday successfully launched the Delta IV Heavy Rocket from Cape Canaveral, Fla., which carried a classified National Reconnaissance Office (NRO) satellite. Called NROL-32, it is widely believed to be a spy satellite.

"This mission helps to ensure that vital NRO resources will continue to bolster our national defense," Brig. Gen Ed Wilson said in a statement. Wilson is the commander of the 45th Space Wing, the arm of the Air Force that oversees the mission. "The spectacular evening launch showcases how the 45th assures access to the high frontier and supports global operations," he said.

NRO director Bruce Carlson in September called the NROL "the largest satellite in the world." Since it's a classified mission, few details have been disclosed about what the satellite will actually do, although most U.S. reports have said that will likely be used to monitor enemy activity.

"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," Ted Molczan, an established satellite tracker told Spaceflight Now. "The satellite likely consists of sensitive radio receivers and an antenna generally believed to span up to 100 meters to gather electronic intelligence for the National Security Agency."

The vessel was originally supposed to take off on Thursday, but was delayed 24 hours due to an issue with a wiring issue on the launch pad system. It was postponed again after engineers uncovered a malfunction with the temperature readings in Delta IV's boosters. Both problems were fixed.

It's the fourth trip for the United Launch Alliance's (ULA) Delta IV rocket, an unmanned booster with two million pounds of thrust that carries other vessels into space.