Have you ever seen the launch of a space shuttle? If you have—even if it was in a movie—you would have heard something along the lines of: "T minus 10 seconds and counting." What does this mean? Well, to launch a space shuttle there is a launch series, or a sequence of tasks that must be completed in a certain order within a certain time frame for the shuttle to “blast off”!
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Let’s look at how NASA keeps track of time before and during a space shuttle launch. First, there are two clocks. The first clock, called the L clock, keeps track of the time remaining before the launch in real time. So if the L clock displays 8 hours, then that would indicate 8 hours until liftoff. However, the clock that you will hear about more often is the T clock. This clock also counts down time, but it has certain holds built into it. A hold is when time freezes for a certain period so that something that’s necessary for the launch can occur. Take a look at a portion of the launch series below (find the entire series at http://www.nasa.gov/mission_pages/shuttle/launch/countdown101.html):
T-9 minutes and holding (This is the final built-in hold, and varies in length depending on the mission.)
- Final launch window determination
- Activate flight recorders
- Final "go/no-go" launch polls conducted by NASA Test Director, Mission Management Team and launch director
T-9 minutes and counting
- Once countdown resumes, start automatic ground launch sequencer
- Retract orbiter access arm (T-7 minutes, 30 seconds)
- Start auxiliary power units (T-5 minutes, 0 seconds)
- Arm solid rocket booster range safety safe and arm devices (T-5 minutes, 0 seconds)
- Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds)
- Retract gaseous oxygen vent arm, or "beanie cap" (T-2 minutes, 55 seconds)
- Crew members close and lock their visors (T-2 minutes, 0 seconds)
- Orbiter transfers from ground to internal power (T-50 seconds)
- Ground launch sequencer is go for auto sequence start (T-31 seconds)
- Activate launch pad sound suppression system (T-16 seconds)
- Activate main engine hydrogen burnoff system (T-10 seconds)
- Main engine start (T-6.6 seconds)
You can see that there is a hold built into the launch series at T-9 minutes. At this point in time, the T clock pauses until the three tasks listed are completed. Once those "final 'go/no-go' launch polls" are conducted, the clock resumes its countdown and the series continues with the tasks listed under "T-9 minutes and counting." If you were one of the technicians in the launch crew, working with units of time would become quite effortless for you.
Think about what would happen if there were a delay in the launch series, an unexpected hold that was not built in. The times on the L clock would be off and would need to be refigured. While computers do most of the work today, it is still the human beings behind those computers that have the power to manipulate time when "timing" a liftoff!
Take a look at this video and listen for the countdown: http://www.youtube.com/watch?v=9-rnftkBA1s
You can learn all about the space shuttle program and other NASA missions at the first website below. Use the next link to practice adding and subtracting units of time.