CAPE CANAVERAL, Fla. — The NASA Artemis II mission has a countdown timeline.

A launch countdown includes “L Minus” to indicate how far away liftoff is in hours and minutes, and “T Minus” to show a sequence of events built into the launch countdown.

Pauses in the countdown, or “holds,” are built in 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 continues to advance.

Below are some of the key events that take place at each milestone after the countdown begins. All times are approximate.

L-49 hours 50 minutes and counting

• L-49H50M – Launch team arrives to stations
• L-49H40M – Countdown clock begins
• L-49H40M – L-42H30M: Liquid oxygen (LOX)/Liquid hydrogen (LH2) systempreparations for vehicle loading
• L-45H30M – L-44H: Orion spacecraft powered up
• L-42H20M – L-41H: Core stage powered up
• L-42H10M – L-40H30M: Interim cryogenic propulsion stage (ICPS) powered up
• L-39H45M – L-35H30M: Final preparations of the four RS-25 engines

L-35 hours and counting

• L-34H45M – L-34H10M: ICPS is powered down
• L-33H30M – L-29H30M: Charge Orion flight batteries to 100%
• L-31H30M – L-24H30M: Charge core stage flight batteries
• L-20H15M – L-18H45M: ICPS is powered up for launch

L-16 hours and counting

• L-15H30M – L-14H: All non-essential personnel leave Launch Complex 39B
• L-14H15M – L-12H05M: Air-to-gaseous nitrogen (GN2) changeover and rocketcavity inerting
• L-13H15M – L-11H45M: Ground launch sequencer (GLS) activation

L-13 hours and counting

• L-12H35M – L-9H50M: 2-hour 45-minute built in countdown hold begins
• L-10H50M – Launch team decides “go” or “no-go” to begin tanking
• L-10H50M – L-9H35M: Orion cold soak
• L-10H40M – L-10H35M: Core stage LOX transfer line chilldown
• L-10H40M – L-9H55M: Core stage LH2 chilldown
• L-10H25M – L-9H40M: Core stage LOX main propulsion system chilldown

L-10 hours and counting

• L-9H55M – L-9H25M: Core stage LH2 slow fill start
• L-9H50M – Resume T-Clock from T-8H10M
• L-9H40M – L-9H30M: Core stage LOX slow fill
• L-9H30M – L-6H40M: Core stage LOX fast fill
• L-9H25M – L-8H: Core stage LH2 fast fill
• L-9H05M – L-8H30M: ICPS LH2 chilldown
• L-8H30M – L-7H45M: ICPS LH2 fast fill start
• L-8H – L-7H55M: Core stage LH2 topping
• L-7H55M – terminal count: Core stage LH2 replenish
• L-7H45M – L-7H20M: ICPS LH2 vent and relief test
• L-7H20M – L-7H10M: ICPS LH2 tank topping start
• L-7H05M – terminal count: ICPS LH2 replenish
• L-6H40M – L-6H10M: Orion communications system activated (radio frequency to mission control)
• L-6H40M – L-6H05M: Core stage LOX topping
• L-6H40M – L-6H30M: ICPS LOX main propulsion system chilldown
• L-6H30M – L-5H45M: ICPS LOX fast fill
• L-6H10M – Stage pad rescue
• L-6H10M: – Closeout crew assemble
• L-6H05M – terminal count: Core stage LOX replenish

L-6 hours and counting

• L-6H – Flight crew weather brief
• L-5H45M – L-5H30M: ICPS LOX vent and relief test
• L-5H30M – L-5H10M: ICPS LOX topping
• L-5H10M – terminal count: ICPS LOX replenish
• L-5H10M – All stages replenish
• L-5H10M – Start 1-hour 10-minute built in hold
• L-5H10M – L-4H55M: Closeout crew to white room
• L-4H40M – L-4H10M: Flight crew deployment to pad
• L-4H: Flight crew board Orion
• L-3H40M – L-3H10M: Crew module hatch preps and closure
• L-3H10M – L-2H45M: Counterbalance mechanism hatch sealpress decaychecks
• L-2H20M – L-1H40M: Crew module hatch service panel install/closeouts
• L-1H40M – L1H30M: Launch abort system (LAS) hatch closure for flight
• L-1H10M – Launch director brief – rocket & thermal protection system scan results with the imagery console
• L-50M – L-40M: Closeout crew departs Launch Complex 39B
• L-50M – Final NASA test director briefing is held 

L-40 minutes and holding

• L-40M – Built in 30-minute countdown hold begins 

L-25 minutes and holding

• L-25M – Transition team to Orion to Earth communication loop following final NTD briefing
• L-17M – Launch director polls team to ensure they are “go” for launch
• L-15M – Flight crew visors down
• L-14M – Flight crew short purge verification 

T-10 minutes and counting

• T-10M – GLS initiates terminal count
• T-8M – Crew Access Arm retract
• T-6M – GLS go for core stage tank pressurization  
• T-6M – Orion ascent pyros are armed
• T-6M – Orion set to internal power
• T-5M57S – Core stage LH2 terminate replenish
• T-5M20S – LAS capability is available
• T-5M20S – NTD lets commander knows LAS capability is available
• T-4M40S – GLS go for LH2 high flow bleed check
• T-4M30S – Flight termination system armed
• T-4M – GLS is go for core stage auxiliary power unit (APU) start
• T-4M – Core Stage APU starts
• T-4M – Core stage LOX terminate replenish
• T-3M30S – ICPS LOX terminate replenish  
• T-3M10S – GLS go for purge sequence 4 
• T-2M02S – ICPS switches to internal battery power
• T-2M – Booster switches to internal batter power
• T-1M30S – Core stage switches to internal power  
• T-1M20S – ICPS enters terminal countdown mode  
• T-50S – ICPS LH2 terminate replenish
• T-33S – GLS sends “go for automated launch sequencer” command
• T-30S – Core stage flight computer to automated launching sequencer  
• T-12S – Hydrogen burn off igniters initiated  
• T-10S – GLS sends the command for core stage engine start 
• T-6.36S– RS-25 engines startup  

T-0

• Booster ignition, umbilical separation, and liftoff 

Inside the terminal countdown, teams have a few options to hold the count if needed.

• The launch team can hold at 6 minutes for the duration of the launch window, less the 6 minutes needed to launch, without having to recycle back to 10 minutes.
• If teams need to stop the clock between T-6 minutes and T-1 minute, 30 seconds, they can hold for up to 3 minutes and resume the clock to launch. If they require more than 3 minutes of hold time, the countdown would recycle back to T-10.
• If the clock stops after T-1 minute and 30 seconds, but before the automated launch sequencer takes over, then teams can recycle back to T-10 to try again, provided there is adequate launch window remaining.
• After handover to the automated launch sequencer, any issue that would stop the countdown would lead to concluding the launch attempt for that day.

More on Artemis II

• WATCH: Artemis II moon flyby mission launch
• Get to know the Artemis II astronauts
• NASA shares Artemis II moon flyby mission overview
• Artemis II moon mission set to launch from Cape Canaveral
• Artemis II astronauts arrive at Florida launch site for first moon trip in 53 years
• Apollo’s impatient old-timers are rooting for NASA’s return to the moon with Artemis II launch

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