From: Duane Takahashi, of Los Altos, Calif.
To: Kelly Beck, flight director

Question: Why are there built-in holds during a countdown? Does it give the ground crew time to repair or recalibrate the system?

Answer: Duane,

Built-in holds are used during the shuttle launch countdown to allow time for completion of key milestone events. These events must be executed and verified prior to starting subsequent events, otherwise the shuttle will not be ready for liftoff at the designated launch time. Once these milestones are met, we know exactly how long it takes to complete the remaining prelaunch activities, and the countdown continues until the next built-in hold or launch.

Examples of milestones in the launch countdown include turning on all of the navigation and landing aids during the T minus 3-hour hold and transitioning the shuttle's computers from prelaunch software to launch software during the T minus 20-minute hold. Since we know how long it takes to complete each of these milestone events, built-in holds are put in place to allow for completion of these activities prior to continuing the launch countdown.

Thanks for your interest in our nation's space program!

Michael L. Sarafin
STS-111, Orbit 1 GNC Officer