Countdown 101

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

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

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

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

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

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

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

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!

NASA's John F. Kennedy Space Center

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.The Shuttle Test Director performs the traditional call to stations and the countdown clock is activated.This is the first built-in hold and typically lasts four hours.This built-in hold typically lasts four hours, but may be extended if PRSD offload is required.This built-in hold varies between 13 to 14 hours.This built-in hold typically lasts two hours, or one hour for a 24- or 48-hour scrub.This built-in hold typically lasts two-and-a-half hours.This built-in hold typically lasts 10 minutes.This is the final built-in hold, and varies in length depending on the mission.