The countdown for STS-121, the second Return to Flight mission since the loss of Shuttle Columbia, has begun.
The countdown clock, three miles away from Discovery on the launch pad, came to life at 10pm UK time – although the chances of her launching on Saturday is subject to the weather.
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NASA are officially noting that there’s a 60 to 70 percent chance of a ‘No Go’ weather restraint, due to the threat of thunderstorms in the area. The unpredictable weather could continue until Tuesday.
Shuttle Discovery is currently set to make her first launch attempt at 3:50pm local time on Saturday.
For now, preparations are proceeding as planned, with the next major event being the retraction of the Rotating Service Structure (RSS) which has surrounded Discovery since arriving at the pad. The RSS protects the Orbiter from the elements, as well as providing access for Shuttle workers.
Those workers are currently carrying out the final closeouts of the Shuttle and the launch pad ahead of the launch event.
Discovery’s final preparations in the launch flow include the loading of her flight software for the mission into her General Purpose Computers (GPC) later on Wednesday night.
Thursday will see the pad cleared of pad workers in preparation of the loading of cryogenic reactants into Discovery’s fuel cells – located below the orbiter’s payload bay.
On Friday, critical activities include the filling of the Sound Suppression Water System tank with water – ready to be flooded on to the pad 16 seconds before launch to buffer the intense sound of the main engines of the Shuttle and the two Solid Rocket Boosters.
The actual countdown was started by controllers in Firing Room 4 at the Kennedy Space Center.
**Coming soon: ‘Infrared’ – STS-121 music video.**