T-minus 39 minutes and counting. The two-piece service structure which enclosed the Soyuz rocket is being retracted to a horizontal position soon. The towers protected the rocket and provided workers and the Soyuz crew with access to the spacecraft since the rocket arrived at the launch pad Monday morning.
Several other umbilical arms connecting the rocket to the ground will be retracted at various times later in the countdown.
T-minus 28 minutes and counting. The emergency escape system is being armed. The system would be employed if a major malfunction occurs, propelling the Soyuz capsule off the top of the rocket to safety.
T-minus 20 minutes and counting. The Soyuz rocket will fly northeast from the Baikonur Cosmodrome, jettisoning its launch escape tower 1 minutes, 54 seconds, after liftoff. Four seconds later, the rocket's four liquid-fueled strap-on boosters and their RD-107A engines will separate as the core RD-108A engine, also known as the Block A second stage, continues firing.
The Soyuz launch shroud will release at T+plus 2 minutes, 37 seconds, followed by ignition of the third stage RD-0110 engine and separation of the Soyuz second stage at T+plus 4 minutes, 45 seconds.
The third stage's RD-0110 engine will fire for four minutes to inject the Soyuz spacecraft into orbit. Shutdown is set for T+plus 8 minutes, 45 seconds, followed by separation of the capsule in orbit three seconds later.
A series of communications and navigation antennas will deploy moments later as the Soyuz begins its six-hour, four-orbit chase of the International Space Station.
Docking is set for 9:23 a.m. EDT (1323 GMT).
T-minus 17 minutes. At the time of launch, the International Space Station will be flying 251 miles over northeast Kazakhstan. When the Soyuz MS-04 spacecraft deploys from the Soyuz rocket's third stage, it will begin a six-hour rendezvous with the space station, which will be about 2,175 miles ahead of the capsule when it arrives in orbit.
04/20/2017 09:54 The crew is completing leak checks of the Sokol launch spacesuits at this point in the countdown
04/20/2017 09:56 Now in the launch count, realignment of the Soyuz rocket's trajectory control system and checks of internal batteries should be complete. The Soyuz telemetry system will soon be activated and monitoring of Soyuz's thermal control system also will begin.