Countdown 101

The Countdown Clock is one of the most watched timepieces in the world, and our Live Launch Coverage is the only place online to get up-to-the-minute information during a countdown. On this page we explain how to understand and follow along with our launch coverage.

The Countdown Clock is activated for each mission at T-43 hours and counting. KSC's countdown coverage page is activated at the same time, with links to informative pages about the mission and crew, payloads, and any scheduled spacewalks (EVAs). (Until the clock is activated, the Live Launch Coverage link at the bottom of this page will take you to the most recent launch coverage.) The KSC web team begins live online launch coverage at T-6 hours and counting, when countdown milestones become more and more frequent as launch approaches.

Throughout our launch coverage pages, you will find several links to excerpts from the 1988 NSTS Shuttle Reference Manual. This manual has not been updated since it was originally published and contains some information that is now outdated. None of the thousands of Shuttle upgrades since that time -- like the Shuttle's new glass cockpit -- or organizational and managerial changes since then, are included. Even so, it is still -- by far -- the most comprehensive resource about the Space Shuttle program available online today.
 

Follow along with our Live Countdown Coverage

As each milestone is reached, we will activate the appropriate link, indicated by a green star (). Links already activated will be marked with a brown star ().

"T" stands for scheduled liftoff time, "-" stands for minus, "H" stands for hours, "M" stands for minutes and "S" stands for seconds.

In the example below, the T-3 Hours & Counting milestone has been passed and the link has been activated. The T-20 Minutes and Holding milestone is the most recent page to be activated.

Please note that timelines are projected and subject to change at any time during the countdown.
 

Countdown Event Guide

If the countdown clock reads T-26 hours and counting, that doesn't mean the Shuttle will launch in 26 hours. During every Shuttle countdown, there are several times when the clock is temporarily stopped. These periods are called "built-in holds," and they vary in length from as little as 9 minutes to as long as several hours. For this reason, hold times must be taken into consideration when watching the countdown clock.

The Shuttle team uses these built-in holds to synchronize with other countdown clocks, monitor weather events, make final adjustments to the launch time, and check all systems before proceeding.

Here are the key events that take place at each milestone after the countdown begins. Event lengths are approximate and subject to change.

T-43 hours and counting - Start Countdown Clock (event length: 16 hours)

• The countdown gets underway with the traditional call to stations
  by the NASA Test Director.
• Mid-deck and flight-deck platforms are removed.
• Navigational Systems activated and tested.

T-27 hours & holding (event length: 4 hours)

• Preliminary Flight Deck inspections completed.
• Complete preparation to load power reactant storage and distribution system and complete flight deck preliminary inspections

T-27 hours & counting (event length: 8 hours)

• Clear launch pad of all nonessential personnel.
• Begin operations to load cryogenic reactants into Columbia's fuel cell storage tanks

T-19 hours & holding (event length: 4 hours)

• Demating of the Orbiter's mid-body umbilical unit takes place.
• Final preparations for Shuttle's three main engines begins.

T-19 hours & counting (event length: 8 hours)

• Begin filling sound suppression systems water tank.
• Close out the tail service masts on the mobile launcher platform

T-11 hours & holding (event length: variable - normally 12-14 hours)

• Move Rotating Service Structure (RSS) to park position.
• Activate Orbiter's inertial measurement units.
• Activate Orbiter's communications systems.
• Perform ascent switch list

T-11 hours & counting (event length: 5 hours)

• Activate the Orbiter's fuel cells.
• Clear the blast danger area of all nonessential personnel.
• Switch Columbia's purge air to gaseous nitrogen.

T-6 hours & holding (event length: 1 hour)

• The Final Inspection Team conducts a detailed analysis of the vehicle as they walk up and down the entire launch tower.
• Begin loading the external tank with about 500,000 gallons of cryogenic propellants as we begin T-6 and counting.

T-6 hours & counting (event length: 3 hours)

T-3 hours & holding (event length: 2 hours)

T-3 hours & counting (event length: 3 hours, 40 minutes)

T-20 minutes & holding (event length: 10 minutes)

T-20 minutes & counting (event length: 11 minutes)

T-9 minutes & holding (event length: variable - normally about 45 minutes)

T-9 minutes & counting (event length: 9 minutes)

T-0 - Liftoff!

Live Launch Coverage | Mission & Crew | Payloads | Crew EVA