Answers Your Questions
From: Duane Takahashi, of
Los Altos, Calif.
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.