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A United Launch Alliance Delta IV Heavy rocket continued to remain on its pad at Vandenberg Air Force Base on Wednesday, after its planned launch was scrubbed for the fourth time in the past two weeks.

Delta IV Heavy launch from Vandenberg pushed back to no sooner than Dec. 30

ULA announced the latest postponement around 5:40 p.m. Wednesday, about 19 minutes ahead of the scheduled liftoff. The decision to call off the launch for Wednesday was due to "indications of elevated hydrogen concentrations within the port booster engine section," ULA reported.

The company said it will attempt the launch again at 5:31 p.m. Thursday, Dec. 20.

The launch is set to be part of a mission, dubbed NROL-71, for the National Reconnaissance Office. The rocket's payload was not revealed.

The launch had initially been planned for Dec. 7 but was postponed that day "due to an issue with a redundant communication link between the control center and the launch site," according to ULA.

It was rescheduled for Dec. 8 but was postponed again that day with just 7 seconds remaining in the prelaunch countdown "due to a problem detected by the Terminal Countdown Sequencer Rack," ULA reported.

It was rescheduled again for the evening of Tuesday, Dec. 18, but was called off early that afternoon due to high high ground winds.

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Willis Jacobson covers the city of Lompoc for Lee Central Coast Newspapers. Follow him on Twitter @WJacobsonLR.