Before NASA sends its astronauts Reid Wiseman, Victor Glover, Christina Koch, and CSA (Canadian Space Agency) astronaut Jeremy Hansen on their Artemis II mission around the Moon, the launch team at NASA's Kennedy Space Center in Florida and teams across the country will begin counting down about two days before liftoff.
A launch countdown contains "L Minus" and "T Minus" times. The "L minus" indicates how far away liftoff is in hours and minutes. The "T minus" time is 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, however, continues to advance.
Below are some of the key events that take place at each milestone after the countdown begins.
All times are approximate for when these milestones occur.
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) system
preparations 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 rocket
cavity 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 decay
checks
- 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.
Launching the Artemis II Moon rocket will lift off the agency's first crewed mission under the Artemis program, testing the systems that will return astronauts to the Moon for an enduring presence, and paving the way to human exploration of Mars.
