Follow along with
the countdown! The Virtual Launch Control Center is the only
place online to get breaking information directly from NASA's
Kennedy Space Center in Florida. Live countdown coverage will
begin about six hours prior to launch and will conclude about
10 minutes after liftoff, when orbit insertion is complete.
Coverage includes frequent updates on the countdown status and
videos of key events.
 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.
Image to left: Spectators gather on the
grounds in front of the countdown clock during a space shuttle
launch. Credit: NASA
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
|  Image to right: The space shuttle launch
team seated in Kennedy's Firing Room 1.
Credit:
NASA
T-27 hours and
holding
This is the first built-in hold and
typically lasts four hours. |
- Clear launch pad of all non-essential personnel
|
| T-27 hours and counting
|
- Begin operations to load cryogenic reactants into
the orbiter's fuel cell storage tanks
|
T-19 hours and
holding
This built-in hold typically lasts four
hours. |
- Demate the orbiter's midbody umbilical unit
|
| T-19 hours and counting
|
- 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
|
T-11 hours and
holding
This built-in hold varies in length, but
typically lasts 12 to 13 hours. |
- 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
|  Image to right: The rotating service
structure is rolled slowly into the "park" position, revealing
Space Shuttle Atlantis as the launch countdown for STS-98
enters its final hours. Credit: NASA
| T-11 hours and counting
|
- Activate the orbiter's fuel cells
- Clear the blast danger area of all nonessential
personnel
- Switch the orbiter's purge air to gaseous nitrogen
|
T-6 hours and
holding
This built-in hold typically lasts two
hours. |
- 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
|
| T-6 hours and counting
|
- Finish filling the external tank with its flight
load of liquid hydrogen and liquid oxygen propellants
- 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
|
T-3 hours and
holding
This built-in hold typically lasts two
hours. |
- Perform inertial measurement unit preflight
calibration
- Align Merritt Island Launch Area (MILA) tracking
antennas
|
| T-3 hours and counting
|
- 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
| 
T-20 minutes and
holding
This built-in hold typically lasts 10
minutes. |
- Shuttle Test Director conducts final launch team
briefings
- Complete inertial measurement unit preflight
alignments
| Image to right: Space Shuttle Discovery
waits to launch on mission STS-103. At the top is the external
tank gaseous oxygen vent arm system with the vent hood
(sometimes called the "beanie cap") poised above the external
tank. Extending toward the cabin of the orbiter below is the
orbiter access arm, with the White Room at the end. Credit:
NASA
| T-20 minutes and
counting |
- 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
|
T-9 minutes and
holding
This is the final built-in hold, and
varies in length depending on the mission. |
- The Launch Director, Mission Management Team and
Shuttle Test Director poll their teams for a go/no go
for launch
|  Image to right: A fish-eye view captures
Space Shuttle Endeavour just after liftoff on mission STS-111.
Credit: NASA
| T-9 minutes and
counting |
- 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)
|
| T-0 |
- Solid rocket booster ignition and liftoff!
|
NASA's John F. Kennedy Space Center |
