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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.

Follow along with our Live Countdown Coverage

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

"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.

Image of screen capture showing star buttons.

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

  • 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

  • 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

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

T-19 hours & holding

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

T-19 hours & counting

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

T-11 hours & holding

  • 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

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

T-6 hours & holding

  • 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

T-3 hours & holding

T-3 hours & counting

T-20 minutes & holding

T-20 minutes & counting

T-9 minutes & holding

T-9 minutes & counting

T-0 - Liftoff!

Page Last Revised Page & Curator Information
April 3, 2002 Online coverage by: Dennis Armstrong (NASA), Anna Heiney (IDI)
Web Development: Lynda Warnock & Debbie Barton (FDC)
Video Production: Chris Chamberland (Photobition)
Countdown Clock by: Jim Fitzgerald (FDC)
NASA Official: Dennis Armstrong (

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