Before launching the Artemis I mission to its orbit around the Moon, the launch team at the Kennedy Space Center in Florida and support teams across the country will begin the launch countdown about two days before takeoff. The start countdown contains the times “L Minus” and “T Minus”. The “L minus” indicates how far we are from takeoff in hours and minutes. Time “T minus” is a sequence of events integrated into the start countdown. Countdown or “standby” pauses are incorporated in the countdown to allow the startup team to target an accurate startup window and provide a timeline for certain tasks and processes without affecting the overall schedule. During scheduled suspensions in the countdown process, the countdown clock is intentionally interrupted and time T also stops. Time L, however, keeps going. Members of the Artemis I launch team participate in a countdown simulation inside the Firing Room 1 of the Launch Control Center at NASA’s Kennedy Space Center in Florida on February 3, 2020. Credits: NASA / Kim Shiflett During the wet costume rehearsal test prior to the release of Artemis I, the test team may hold during the countdown as necessary to verify the conditions before continuing the countdown or using the test window or extending beyond that, if its consumables and resources allow it to complete the test objectives. The time for some events is approximate and may vary slightly during the test due to the many scheduled business demonstrations associated with specific test features and objectives. During the rehearsal, the testers will count down to the T-1 minute and 30 seconds and stop to show the ability to hold for up to 3 minutes, then continue for up to 33 seconds before the start and then will stop the countdown. They will then recycle again up to ten minutes before launch and perform a second terminal countdown to approximately 9.3 seconds before start-up and then end the countdown. Teams may improve the pre-release countdown schedule based on experience with the rehearsal. View of the Artemis I (SLS) space launch system and the Orion spacecraft just before sunrise at Launch Complex 39B at NASA’s Kennedy Space Center in Florida on March 23, 2022. Credits: NASA / Ben Smegelsky Below are some of the key milestones that take place for the Space Launch System rocket, the Orion spacecraft and ground-based systems after the countdown begins. L-45 hours and continue

The launch team arrives at its stations and the countdown begins (L-45, 40 minutes) Fill the water tank for the sound suppression system (L-45 hours) Orion spacecraft activated (L-40 hours) The basic stage is activated (L-34 hours, 45 minutes) Final preparations of the four RS-25 engines (L-30 hours, 50 minutes) The hatch of the Orion crew unit is closed (L-30 hours, 5 minutes)

The Moon serves as the backdrop for the Artemis I SLS and Orion spacecraft at Launch Complex 39B at NASA’s Kennedy Space Center in Florida on March 21, 2022. Credits: NASA / Ben Smegelsky L-30 hours and continue

Leak tests on the Orion spacecraft are completed and the hatch on the launch cancellation system is closed (L-28 hours, 50 minutes) Crew access arm retractable (L-26 hours, 5 minutes) Side flame deflectors move into place (L-21 hours)

L-14 hours and we count

Temporary Cryogenic Propulsion Stage (ICPS) Activated (L-12 hours, 45 minutes) All non-essential staff leave Launch Complex 39B (L-12 hours)

L-8 hours, 40 minutes and continue

The built-in countdown starts and lasts about an hour and a half (L-8 hours, 40 minutes) Launch team updates weather and tank (L-8 hours, 40 minutes) The launch team decides whether to “leave” or “ban” to begin tanking the rocket (L-7 hours, 40 minutes) Basic Liquid Oxygen (LOX) Stage Chilldown (L-7 hours, 20 minutes)

L-7 hours and we count

Basic LOX stage slow start filling (L-6 hours, 25 minutes) Basic LOX stage fast start of filling (L-6 hours, 10 minutes) Start of Liquid Hydrogen (LH2) Basic Stage Relaxation (L-6 hours, 5 minutes) Basic stage LH2 slow start filling (L-5 hours, 55 minutes) Fast completion of LH2 basic stage (L-5 hours, 40 minutes)

Standing on top of the mobile launcher, NASA’s SLS rocket is photographed at Launch Complex 39B at the organization’s Kennedy Space Center in Florida on March 18, 2022. Credits: NASA / Kim Shiflett L-4 hours, 30 minutes and continue

Chilldown ICPS LH2 (L-4 hours, 25 minutes) Activated Orion communications system (RF in dispatch control) (L-4 hours, 20 minutes) Start of LH2 basic stage peak (L-4 hours, 30 minutes) Start of LH2 basic stage replacement (L-4 hours 25 minutes) Quick start of ICPS LH2 filling (L-4 hours)

L-3 hours, 30 minutes and continue

Start of top-level LOX kernel (L-3 hours, 25 minutes) Start of ICPS LOX relaxation (L-3 hours, 20 minutes) ICPS LH2 validation and start of leak test (L-3 hours, 15 minutes) Start of LOX basic stage replacement (L-3 hours, 18 minutes) ICPS LOX filling start (L-3 hours, 10 minutes) ICPS LH2 tank filling start (L-3 hours) ICPS / SLS telemetry data verified by dispatch control and SLS Mechanical Support Center (L-2 hours, 50 minutes) Start of ICPS LH2 replenishment (L-2 hours, 35 minutes) ICPS LOX validation and start of leak test (L-2 hours, 30 minutes) ICPS LOX coating start (L-2 hours, 20 minutes) Start of ICPS LOX replenishment (L-2 hours, 10 minutes)

L-40 minutes and hold

The final update of the NASA test director is taking place The built-in 30-minute countdown begins

L-15 minutes and hold

Launch director votes for team to ensure they ‘go’ for launch

T-10 minutes and count

Orion climbing firearms are armed (T-6 minutes) Orion set to internal power (T-6 minutes) LH2 Core Terminal Replenishment (T-5 minutes, 57 seconds) Auxiliary power unit starts in the basic stage (T-4 minutes) Basic LOX phase of replacement (T-4 minutes) ICPS LOX finish replenishment (T-3 minutes and 30 seconds) ICPS switches to internal battery power (T-1 minute, 56 seconds) The main stage switches to internal power supply (T-1 minute, 30 seconds) ICPS enters terminal countdown mode (T-1 minute, 20 seconds) Refill ICPS LH2 terminal (T-50 seconds) Ground launch sequencer sends “Go for automated launch sequencer” (T-33 seconds) Basic Stage Flight Calculator in Automated Launch Sequencer (T-30 Seconds) Hydrogen combustion igniters started (T-12 seconds) The ground launch sequencer sends the command to start the engine in the basic stage (T-10 seconds) Starting RS-25 engines (T-6.36 seconds)

T-0

Enhancing ignition, umbilical separation and lifting