Spectators watch as the countdown clock freezes at 31 seconds to launch
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Shuttle launch: Dramatic last-minute delay to lift-off

Just seconds before the final Nasa shuttle launch, Mission Control halted the countdown over concerns about a piece of launchpad equipment.

There were worries that the gaseous vent arm had not been completely moved out of the shuttle's way.

British-born former astronaut Piers Sellers explained to the BBC's Pallab Ghosh what would happen if the launch had to be scrubbed at such a late stage.

But after controllers were satisfied there were no problems, the signal was given to proceed with lift-off.