Ascent Timeline

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.

ULA Team Ready for Launch

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.

Final Built-in Hold Coming Up

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.