ULA delays launch of last Delta IV Heavy rocket carrying classified payload

March 28 (UPI) -- United Launch Alliance has delayed the last launch of a Delta IV Heavy rocket booster to Friday afternoon.

The rocket, which served as a workhorse for uncrewed American space missions for years, was set to launch from Space Launch Complex-37 at Cape Canaveral Space Force Base at 2:45 p.m. EDT, but flight control issued a hold at about four minutes to launch.

Launch Director Ton Heter confirmed the launch would not go forward as planned. The next launch attempt will be at 1:37 p.m. EDT on Friday.

The launch, dubbed NROL-70, is being carried out on behalf of the National Reconnaissance Office with a classified payload that is believed to be an Electronic Signals Intelligence Satellite.

The launch marks the end of the Delta IV's career and the retirement of the Delta family of rockets, which have carried payloads into space for the U.S. since 1960.

The first successful Delta launch placed the Echo 1A communications satellite experiment into orbit.

The Delta IV Heavy, developed by Macdonald Douglas and Boeing, following the 1997 merger between the two companies, and now built by the United Launch Alliance, was the platform that carried a large number of U.S. satellites over the past two decades.

The first Delta IV flew in 2002 and Thursday's launch will mark the 45th and final launch from the platform. Of the 44 prior launches, 16 were conducted via the Delta IV Heavy.

While most Delta IV Heavy launches have been on behalf of the U.S. defense establishment, the platform has also carried satellites for the National Oceanic and Atmospheric Administration and NASA itself.