The launch of an oft-delayed U.S. spy satellite has been pushed back again, this time to no earlier than Jan. 6, according to the United Launch Alliance.

A ULA Delta IV Heavy rocket was scheduled to launch the clandestine satellite mission, called NROL-71, on Sunday (Dec. 30) from California's Vandenberg Air Force Base. The launch has been delayed several times due to rocket issues, most recently on Dec. 19 due to a suspected hydrogen leak on the booster.

The United Launch Alliance Delta IV Heavy rocket carrying the NROL-71 spy satellite for the U.S. National Reconnaissance Office stands atop its launchpad at California's Vandenberg Air Force Base ahead of a Dec. 19, 2018 launch attempt. (Image credit: United Launch Alliance)

"The United Launch Alliance Delta IV Heavy carrying the NROL-71 mission will launch no earlier than Sunday, Jan. 6, 2019," ULA representatives said in a statemement."The mission will launch from Space Launch Complex-6 at Vandenberg Air Force Base in California."

ULA's attempts to launch the NROL-71 mission on Dec. 7 and 8 were delayed by technical issues. An attempt on Dec. 18 was thwarted by high winds, leading to the Dec. 19 launch scrub.

Email Tariq Malik at tmalik@space.com or follow him @tariqjmalik. Follow us @Spacedotcom and Facebook. Originally published on Space.com.