The twice-postponed launch of a spy satellite by United Launch Alliance is scheduled to take place at 5:57 p.m. on Tuesday, Dec. 18, out of Vandenberg Air Force Base near Lompoc.

The liftoff of a Delta 4-Heavy rocket will be broadcast live by ULA beginning 20 minutes before takeoff at ULALaunch.com.

The mission, at the behest of the U.S. National Reconnaissance Office, has already been scrubbed twice, the last time coming on Sunday, Dec. 9 — just seven seconds before takeoff. The exact reason for pulling the plug at the last minute has not been reported. Officials only said that it was due to an unexpected condition. The day before, the launch was cancelled due to an issue with circuitry.

The Delta 4-Heavy is the largest rocket in the Delta 4 family by ULA, a conglomerate of Lockheed Martin and Boeing. Unlike the other class of Delta rockets, the Delta 4-Heavy includes three rocket boosters.

The launch scheduled for Tuesday, just an hour after sunset, stands to produce a dramatic display as the rocket plume emerges in sunlight at higher altitudes.