Live coverage: Kennedy Space Center to reopen Friday

The practice countdown began Tuesday night, kicking off a carefully choreographed series of steps to ready the world’s most powerful operational rocket for what amounted to a simulated launch Thursday at 8:42 p.m. EST. Controllers then carried out additional tests to make sure the team can recycle, hold and restart an actual launch countdown as needed to handle unexpected problems.

The United Launch Alliance Delta 4 rocket, standing 206 feet tall and and weighing 720,000 pounds, unleashes 1.1 million pounds of thrust from its main engine and two side-mounted boosters at 12:52 a.m. EDT (0452 today to launch the GSSAP No. 3 and No. 4 satellites for the Geosynchronous Space Situational Awareness Program from Cape Canaveral.

A Falcon 9 rocket lifted off from Vandenberg Air Force Base on the Pacific coastline northwest of Los Angeles on Friday night, sending 10 Iridium communications satellites toward orbit on a previously-flown first stage booster.