This designation tracks the time left until the exact moment of liftoff, which is designated as time zero. Ground control uses it to synchronize thousands of automated and manual steps, from fueling to final systems checks. If a technical issue arises, the clock can hold at a specific mark, delaying the launch without resetting the entire sequence.
Heard on the job
We're holding at T-minus four minutes while the pad crew clears a sensor fault on the second stage.
Insider tip
The T stands for time. When the clock reaches zero, the count switches to T-plus to track the time elapsed since the event occurred.