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.