Author: Anna Heiney

One minute to go. Eastern Range is ‘go.’

Two minutes remaining in the countdown. The Atlas V rocket is on internal power. The launch team’s primary communications channels are quiet as the final minutes tick by.

When the countdown clock reaches zero, the Atlas V booster engine and single solid rocket booster ignite and the flight begins. The vehicle reaches Mach 1 after about 57 seconds, then flies through “Max Q,” the area of maximum aerodynamic pressure, about 12 seconds later. The rocket’s single solid rocket booster will be jettisoned after two minutes and 19 seconds.

Watch for a rapid chain of critical events beginning just after the flight passes the four-minute mark. The Atlas V booster engine cuts off, an event known as BECO, then the booster separates from the vehicle, followed by the Centaur main engine’s first start, called MES-1. Shortly thereafter, the payload fairing is jettisoned. This entire sequence takes place in less than 30 seconds.

The Centaur upper stage will perform two separate burns tonight in order to send OSIRIS-REx off on its journey to Bennu. This first burn, MES-1, will last about eight minutes. Its first cutoff is called MECO-1. After a 21-and-a-half-minute coast phase, the engine will reignite for MES-2, a nearly seven-minute burn that will cut off at MECO-2 – about 40 minutes after launch.

OSIRIS-REx will separate from the Centaur 15 minutes later.

The countdown has resumed and there are four minutes remaining until the 7:05 p.m. liftoff of the United Launch Alliance Atlas V rocket carrying NASA’s OSIRIS-REx spacecraft.

The switch to internal power is complete. Standing by to pick up the countdown at 7:01 p.m.

Launch Conductor Scott Barney has checked in with his ULA team members and they confirmed they are go to resume the countdown at the T-4 minute mark.

The hold will be released at 7:01 p.m. Liftoff is on schedule for 7:05 p.m. EDT.

OSIRIS-REx is switching from ground-based to internal power.

Countdown clocks are holding for 15 minutes in the final built-in hold of the night.

Although the countdown will pause at the T-4 minute mark, the team’s work will not. During this 15-minute planned hold, we can expect to hear final readiness polls as NASA Launch Manager Tim Dunn and ULA Launch Conductor Scott Barney verify OSIRIS-REx, the Atlas V rocket and the Eastern Range are ready to proceed.

Weather conditions remain favorable for liftoff at 7:05 p.m. EDT.

“All [launch commit criteria] are ‘go’ and expected to remain ‘go’ for the remainder of the countdown,” Launch Weather Officer Clay Flinn told controllers.