The wet dress rehearsal formally began on Wednesday evening and continued into Thursday morning, as teams powered up parts of the rocket and spacecraft and charged flight batteries. The main part of the test, however, will start once mission managers give the go-ahead to fuel the Space Launch System rocket.

The booster will then be filled with more than 700,000 gallons of cryogenic propellant, and mission managers will practice counting down to a simulated launch time of 8:30 p.m. ET.

As part of the test, NASA plans to conduct two walkthroughs of the final 10 minutes of the countdown, with a pause at T-minus 1 minute and 30 seconds. At T-minus 33 seconds, NASA plans to reset the clock back to T-minus 10 minutes and run through again.

These stops and starts are designed to demonstrate that the rocket’s systems are performing as expected during the final part of the countdown, when automated systems take over control of the booster. The pauses and resets also give mission managers a chance to practice different scenarios, such as situations in which issues crop up that need investigation or in which a launch has to be called off due to technical issues or bad weather.