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The countdown clock is one of the most-watched timepieces in the world. On this page, you'll learn how the countdown operates, and what milestones to watch for during our live launch coverage.
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. For the space shuttle countdown, built-in holds vary in length and always occur at the following times: T-27 hours, T-19 hours, T-11 hours, T-6 hours, T-3 hours, T-20 minutes, and T-9 minutes.
Here are some of the key events that take place at each milestone after the countdown begins. Note: Event times and lengths are approximate and subject to change.
T-43 hours and counting
The Shuttle Test Director performs the traditional call to stations and the countdown clock is activated.
- Begin final vehicle and facility close-outs for launch
- Check out backup flight systems
- Review flight software stored in mass memory units and display systems
- Load backup flight system software into the orbiter's general purpose computers
- Remove middeck and flight deck platforms
- Activate and test navigational systems
- Complete preparation to load power reactant storage and distribution system
- Complete flight deck preliminary inspections
This is the first built-in hold and typically lasts four hours.
- Clear launch pad of all non-essential personnel
- Begin loading cryogenic propellants into orbiter's power reactant storage and distribution (PRSD) system
- Begin operations to load cryogenic reactants into the orbiter's fuel cell storage tanks
This built-in hold typically lasts four hours, but may be extended if PRSD offload is required.
- Demate the orbiter's midbody umbilical unit
- Clean and vacuum crew module
- External tank nosecone purge
- Begin final preparations of the orbiter's three main engines for main propellant tanking and flight
- Fill launch pad sound suppression system water tank
- Resume orbiter and ground support equipment close-outs
- Close out the tail service masts on the mobile launcher platform
This built-in hold varies between 13 to 14 hours.
- Weather and engineering briefings
- Pad debris inspection and closeout
- Flight crew equipment late stow
- Move rotating service structure to "park" position
- Activate the orbiter's inertial measurement units and communications systems
- Perform ascent switch list
- Activate the orbiter's fuel cells
- Clear the blast danger area of all nonessential personnel
- Switch the orbiter's purge air to gaseous nitrogen
Image above: Sparks fly beneath space shuttle Discovery as engines ignite for the liftoff on mission STS-124. Credit: Photo courtesy of Scott Andrews
T-6 hours and holdingThis built-in hold typically lasts two hours, or one hour for a 24- or 48-hour scrub.
- Mission Management Team and launch director receive weather update
- Launch team verifies no violations of launch commit criteria before loading the external tank with propellants
- Clear pad of all personnel
- Chill-down of propellant transfer lines
- Begin loading the external tank with about 500,000 gallons of cryogenic propellants
- Finish filling the external tank with its flight load of liquid hydrogen and liquid oxygen propellants
This built-in hold typically lasts two-and-a-half hours.
- External tank loading enters stable replenish
- Perform inertial measurement unit preflight calibration
- Align Merritt Island Launch Area (MILA) tracking antennas
- Final Inspection Team proceeds to the launch pad to conduct a detailed analysis of the vehicle as the team walks up and down the entire launch tower
- Closeout Crew proceeds to the launch pad to configure the crew module for countdown and launch and assist the astronauts with entry into the orbiter
- Televised weather briefing
- Flight crew weather briefing
- Astronaut Support Person enters crew module and begins comm checks
- Crew departs for the launch pad and, upon arriving at the pad, begins entry into the orbiter via the White Room
- Complete close-out preparations in the launch pad's White Room
- Check cockpit switch configurations
- Astronauts perform air-to-ground voice checks with Launch Control (Kennedy Space Center) and Mission Control (Johnson Space Center)
- Close the orbiter's crew hatch and check for leaks
- Complete White Room close-out
- Close-out crew retreats to fallback area
A fish-eye view captures space shuttle Endeavour just after liftoff on mission STS-111. Credit: NASA
T-20 minutes and holdingThis built-in hold typically lasts 10 minutes.
- NASA Test Director conducts final launch team briefings
- Complete inertial measurement unit preflight alignments
- Transition the orbiter's onboard computers to launch configuration
- Start fuel cell thermal conditioning
- Close orbiter cabin vent valves
- Transition backup flight system to launch configuration
This is the final built-in hold, and varies in length depending on the mission.
- Final launch window determination
- Activate flight recorders
- Final "go/no-go" launch polls conducted by NASA Test Director, Mission Management Team and launch director
- Start automatic ground launch sequencer
- Retract orbiter access arm (T-7 minutes, 30 seconds)
- Start auxiliary power units (T-5 minutes, 0 seconds)
- Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)
- Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds)
- Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds)
- Crew members close and lock their visors (T-2 minutes, 0 seconds)
- Orbiter transfers from ground to internal power (T-50 seconds)
- Ground launch sequencer is go for auto sequence start (T-31 seconds)
- Activate launch pad sound suppression system (T-16 seconds)
- Activate main engine hydrogen burnoff system (T-10 seconds)
- Main engine start (T-6.6 seconds)
- Solid rocket booster ignition and liftoff!
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