Wednesday, 6 July 2011

The final countdown

There could hardly be a more appropriate soundtrack to events at Kennedy Space Center this week. ‘The Final Countdown’, a rock song originally released by the Swedish band ‘Europe’ in 1986, reached number one in 25 countries, including the UK. In the United States the song peaked at number eight.

The clock has been ticking for the Space Shuttle programme for some years now but yesterday the last ever countdown began in earnest when the launch clock at Kennedy Space Center — the giant digital display familiar to TV viewers the world over — began counting down from 43 hours. When it reaches zero, Atlantis will be on its way.

But during a countdown the time on the clock does not translate to actual time — considerably more than 43 hours will elapse between countdown and liftoff. In reality some 70 hours separate the start-up of the countdown clock at 1:00 pm yesterday and the planned launch of the Shuttle at 11:26 this Friday.

The reason for the difference in timekeeping is a series of planned ‘holds’ — built-in pauses that allow launch managers to take stock of the mission's status and respond to changing conditions.

In all, a Shuttle launch countdown contains seven holds, which take place when the countdown clock reads 27 hours, 19 hours, 11 hours, six hours, three hours, 20 minutes and nine minutes.

A specific list of safety checks (inspect external fuel tanks at T-minus 11 hours), launch-preparation tasks (clear launch pad and surrounding area of personnel at T-minus six hours) and decision points (final ‘go/no-go’ poll among launch managers at T-minus nine minutes) are addressed before ending the hold and proceeding into the countdown.

For this final Shuttle mission, designated STS-135, the planned holds range in duration from 10 minutes to 14 hours.

Only after the countdown clock resumes following the final hold at T-minus nine minutes will it reflect the actual amount of time remaining until liftoff.


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