NASA Artemis I Launch Countdown 101

Before the Artemis I mission launches on its way around the Moon, the launch team at Kennedy Space Center in Florida and supporting teams across the country will begin the launch countdown about two days before liftoff.

The launch countdown contains “L Minus” and “T Minus” times. “L minus” indicates how far away we are from liftoff in hours and minutes. “T minus” time is a sequence of events that are built into the launch countdown. Pauses in the countdown, or “holds,” are built into the countdown to allow the launch team to target a precise launch window, and to provide a cushion of time for certain tasks and procedures without impacting the overall schedule. During planned holds in the countdown process, the countdown clock is intentionally stopped and the T- time also stops. The L- time, however, continues to advance.

During the wet dress rehearsal test ahead of Artemis I launch, the test team may hold during the countdown as necessary to verify conditions before resuming the countdown, or use the test window or extend beyond it, if consumables and resources allow them to complete test objectives. The timing for some events is approximate and may differ slightly during testing on account of several planned operational demonstrations tied to specific capabilities and test objectives. During the rehearsal, controllers will count down to T-1 minutes and 30 seconds and pause to demonstrate the ability to hold for up to 3 minutes, then resume until 33 seconds before when launch would occur, then pause the countdown. Then they will recycle back to ten minutes before launch and conduct a second terminal countdown to approximately 9.3 seconds before launch, then end the countdown. Teams may refine the coundown timeline ahead of launch based on experience with the rehearsal test.

Below are some of the key milestones that take place for the Space Launch System rocket, Orion spacecraft, and ground systems after the countdown begins.

L-45 hours and counting

• The launch team arrives on their stations and the countdown begins (L-45, 40 minutes hours)
• Fill the water tank for the sound suppression system (L-45 hours)
• The Orion spacecraft is powered up (L-40 hours)
• The core stage is powered up (L-34 hours, 45 minutes)
• Final preparations of the four RS-25 engines (L-30 hours, 50 minutes)
• The Orion crew module hatch is closed (L-30 hours, 5 min)

L-30 hours and counting

• Leak checks are completed on the Orion spacecraft and the hatch on the launch abort system is closed (L-28 hours, 50 minutes)
• The crew access arm is retracted (L-26 hours, 5 minutes)
• Side flame deflectors are moved into place (L-21 hours)

L-14 hours and counting

• The interim cryogenic propulsion stage (ICPS) is powered up (L-12 hours, 45 minutes)
• All non-essential personnel leave Launch Complex 39B (L-12 hours)

L-8 hours, 40 minutes and counting

• Built in countdown hold begins and lasts approximately one- and one-half hour (L-8 hours, 40 minutes)
• The launch team conducts a weather and tanking briefing (L-8 hours, 40 minutes)
• Launch team decides if they are “go” or “no-go” to begin tanking the rocket (L-7 hours, 40 minutes)
• Core stage liquid oxygen (LOX) chilldown (L-7 hours, 20 minutes)

L-7 hours and counting

• Core stage LOX slow fill start (L-6 hours, 25 minutes)
• Core stage LOX fast fill start (L-6 hours, 10 minutes)
• Core stage liquid hydrogen (LH2) chilldown start (L-6 hours, 5 minutes)
• Core stage LH2 slow fill start (L-5 hours, 55 minutes)
• Core stage LH2 fast fill (L-5 hours, 40 minutes)

L-4 hours, 30 minutes and counting

• ICPS LH2 chilldown (L-4 hours, 25 minutes)
• Orion communications system activated (RF to mission control) (L-4 hours, 20 minutes)
• Core stage LH2 topping start (L-4 hours, 30 minutes)
• Core stage LH2 replenish start (L-4 hours 25 minutes)
• ICPS LH2 fast fill start (L-4 hours)

L-3 hours, 30 minutes and counting

• Core stage LOX topping start (L-3 hours, 25 minutes)
• ICPS LOX chilldown start (L-3 hours, 20 minutes)
• ICPS LH2 validation and leak test start (L-3 hours, 15 minutes)
• Core stage LOX replenish start (L-3 hours, 18 minutes)
• ICPS LOX fill start (L-3 hours, 10 minutes)
• ICPS LH2 tank topping start (L-3 hours)
• ICPS/SLS telemetry data verified with mission control and SLS Engineering Support Center (L-2 hours, 50 minutes)
• ICPS LH2 replenish start (L-2 hours, 35 minutes)
• ICPS LOX validation and leak test start (L-2 hours, 30 minutes)
• ICPS LOX topping start (L-2 hours, 20 minutes)
• ICPS LOX replenish start (L-2 hours, 10 minutes)

L-40 minutes and holding

• • Final NASA test director briefing is held
  • Built in 30-minute countdown hold begins

L-15 minutes and holding

• The launch director polls the team to ensure they are “go” for launch

T-10 minutes and counting

• Orion ascent pyros are armed (T-6 minutes)
• Orion set to internal power (T-6 minutes)
• Core stage LH2 terminate replenish (T-5 minutes, 57 seconds)
• Core stage auxiliary power unit starts (T-4 minutes)
• Core stage LOX terminate replenish (T-4 minutes)
• ICPS LOX terminate replenish (T-3 minutes and 30 seconds)
• ICPS switches to internal battery power (T-1 minute, 56 seconds)
• Core stage switches to internal power (T-1 minute, 30 seconds)
• ICPS enters terminal countdown mode (T-1 minute, 20 seconds)
• ICPS LH2 terminate replenish (T-50 seconds)
• Ground launch sequencer sends “Go for automated launch sequencer” command (T-33 seconds)
• Core stage flight computer to automated launching sequencer (T-30 seconds)
• Hydrogen burn off igniters initiated (T-12 seconds)
• Ground launch sequencer sends the command for core stage engine start (T-10 seconds)
• RS-25 engines startup (T-6.36 seconds)

T-0

• Booster ignition, umbilical separation, and liftoff

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