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Artemis I Launch Countdown 101

Launch team at Launch Control Center.
Members of the Artemis I launch team participate in a countdown simulation inside the Launch Control Center’s Firing Room 1 at NASA’s Kennedy Space Center in Florida on Feb. 3, 2020.
NASA/Kim Shiflett

Before the Artemis I mission launches on its way around the Moon, the launch team at Kennedy Space Center and supporting teams across the country will begin the launch countdown about two days before liftoff. Teams have and incorporated lessons learned from the wet dress rehearsal testing and have refined the launch timeline accordingly.

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.  

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

L-47 hours 40 minutes and counting  

Orion spacecraft on top of Space Launch System.
A view of the Artemis I Space Launch System (SLS) and Orion spacecraft just before sunrise at Launch Complex 39B at NASA’s Kennedy Space Center in Florida on March 23, 2022.
NASA/Ben Smegelsky
  • The launch team arrives on their stations and the countdown begins (L-47H, 40M) 
  • The countdown clock begins (L-47H10M)
  • Fill the water tank for the sound suppression system (L-47H – L-42H) 
  • Liquid Oxygen (LOX)/Liquid Hydrogen (LH2) system preparations for vehicle loading (L-47H – L-38H)
  • The Orion spacecraft is powered up if not already powered at call to stations (L-43H – L-41H30M)  
  • The interim cryogenic propulsion stage (ICPS) is powered up (L-39H30M – L-36H30M)
  • The core stage is powered up (L-38H – L-37H20M)  
  • Final preparations of the four RS-25 engines (L-37H20M – L-32H) 

L-32 hours and counting  

  • Core stage composite overwrapped pressure vessel (COPV) Pressurization to Flight Pressure (L-32M – L-23H) 
  • The ICPS is powered down (L-31H – L-30H30M)
  • Charge Orion flight batteries to 100% (L-31H – L-27H)
  • Charge core stage flight batteries (L-28H – L-22H)
  • The ICPS is powered-up for launch (L-19H30M – L-16H30M)

L-15 hours and counting  

  • All non-essential personnel leave Launch Complex 39B (L-13H – L-11H)
  • Ground Launch Sequencer (GLS) activation (L-11H15M – 10H15M)
  • Air-to-gaseous nitrogen (GN2) changeover and vehicle cavity inerting (L-11H45M – launch) 

L-10 hours, 40 minutes and counting 

Space Launch System on launch pad.
The Moon serves as a background for the Artemis I SLS and Orion spacecraft at Launch Complex 39B at NASA’s Kennedy Space Center in Florida on March 21, 2022.
NASA/Ben Smegelsky
  • 3.5-hour built in countdown hold begins (L-10H40M – L-7H10M) 
  • Launch team conducts a weather and tanking briefing (L-10H40M – L-9H50M) 
  • Launch team decides if they are “go” or “no-go” to begin tanking the rocket (L-9H40M) 
  • Core stage LOX transfer line chilldown (L-9H15M – L-9H) 
  • Core stage LH2 transfer line chilldown (L-9H15M – L-8H45M)

L-8 hours and counting 

  • Core stage LOX main propulsion system chilldown (L-9H – L-8H20M) 
  • Core stage LH2 slow fill start (L-8H45M – L-7H50M) 
  • Core stage LOX slow fill (L-8H20M – L-8H5M) 
  • Core stage LOX fast fill (L-8H5M – L-5H15M) 
  • Core stage LH2 fast fill (L-7H50M – L-6H10M) 
  • Engine bleed kick start (L-7H40M – L-7H20M)
  • Core stage LH2 topping (L-6H10M – L6H5M) 
  • Core stage LH2 replenish (L-6H5M – launch) 
  • Core stage LOX topping (L-5H15M– L-5H5M) 
  • ICPS LH2 ground support equipment and tank chilldown (L-5H20M – L-5H) 

L-5 hours and counting  

  • Core stage LOX replenish (L-5H5M – launch) 
  • ICPS LOX main propulsion system chilldown (L-5H5M– L-4H45M) 
  • ICPS LH2 fast fill start (L-5H5M – L4H5M) 
  • Orion communications system activated (RF to mission control) (L-4H20M – L-3H45M) 
  • ICPS LOX fast fill (L-4H55M– L-4H10M) 
  • ICPS LOX validation and leak test (L-4H10M – L-3H40M) 
  • ICPS LH2 validation and leak test (L-4H – L-3H40M) 
  • ICPS LH2 tank topping start (L-3H40M – L-3H25M) 
  • ICPS LH2 replenish (L-3H25M – launch) 
  • ICPS LOX topping (L-3H40M – L-3H20M) 
  • ICPS LOX replenish (L-3H20M – launch) 
  • ICPS/Space Launch System telemetry data verified with Mission Control Center and SLS Engineering Support Center (L-2H55M – L-2H45M) 
  • ICPS LOX validation and leak test (L-2H55M – L-2H30M) 
  • ICPS LH2 replenish (L-2H50M – launch) 
  • ICPS LOX topping (L-2H30M – L-2H10M) 
  • ICPS LOX replenish (L-2H10M – launch) 
  • ICPS/Space Launch System (SLS) telemetry data verified with Mission Control Center and SLS Engineering Support Center (L-3H – L-2H50M) 

L-50 minutes and counting

  • Final NASA Test Director briefing is held (L-50M)

L-40 minutes and holding  

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

L-15 minutes and holding   

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

T-10 minutes and counting  

  • Ground Launch Sequencer (GLS) initiates terminal count (T-10M)
  • GLS go for core stage tank pressurization (T-6M) 
  • Orion ascent pyros are armed (T-6M) 
  • Orion set to internal power (T-6M) 
  • Core stage LH2 terminate replenish (T-5M57S) 
  • GLS is go for flight termination system (FTS) arm (T-5M)
  • GLS is go for LH2 high flow bleed check (T-4M40S)
  • 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-4M)
  • ICPS LOX terminate replenish (T-3M30S) 
  • GLS is go for purge sequence 4 (T-3M10S) 
  • ICPS switches to internal battery power (T-1M56S) 
  • Core stage switches to internal power (T-1M30S) 
  • ICPS enters terminal countdown mode (T-1M20S) 
  • ICPS LH2 terminate replenish (T-50S) 
  • GLS sends “go for automated launch sequencer” command (T-33S) 
  • Core stage flight computer to automated launching sequencer (T-30S) 
  • Hydrogen burn off igniters initiated (T-12S) 
  • GLS sends the command for core stage engine start (T-10S) 
  • RS-25 engines startup (T-6.36S) 


  • 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.