The launch of a giant Delta IV Heavy from California was scrubbed for a fourth time Wednesday evening, with the clock reset for 5:31 p.m. on Thursday.
“Launch Alert: we have declared a scrub for today’s launch of the #DeltaIVHeavy. We will set up for a 24 hour recycle and the next launch attempt will be 12/20 at 5:31pm PST,” rocket builder United Launch Alliance said in a tweet.
The problem was reportedly a hydrogen leak in one of the rocket’s three main engines.
The 233-foot-tall rocket was to blast off from Vandenberg Air Force Base near Santa Barbara after three earlier delays carrying a classified satellite for the secretive National Reconnaissance Office.
The Delta IV Heavy consists of three Delta IV rocket cores strapped together and topped with an upper stage. It is the largest rocket launched from Vandenberg — capable of lifting over 60,000 pounds into orbit — but smaller than the new SpaceX Falcon Heavy that flies from Cape Canaveral.
When the rocket finally launches, San Diegans near the coast could see the two boosters fall away and then the upper stage separate from the main core to continue the mission into space.
The payload was identified as NROL-71 but no other information was released. Space enthusiasts speculated online that it is the first of a new generation of KH-11 electro-optical reconnaissance satellites.
United Launch Alliance is a joint venture of Boeing and Lockheed Martin that launches most American classified payloads.