All eyes turn to the weather forecast. They’re calling for rain on launch day. And according to SPACE SHUTTLE WEATHER LAUNCH COMMIT CRITERIA , they cancel for almost anything.
There’s a 60 percent chance of conditions that could prevent a launch during a 10-minute window, according to the Air Force’s 45th Weather Squadron
Weather forecasts for Sunday are better, with a 70% chance of acceptable launch conditions.
And on that note, the countdown clock starts today at 1:00 p.m. EDT. There is a Call to Stations for the start of the STS-135 launch at 12:30 p.m. EDT, and then the official clock starts ticking down at T-43 Hrs. If you do the math, you’ll see there are much more than 43 hours until the launch. That is because of the built in holds in the countdown. The actual elapsed time with be 70 hours and 21 minutes. So we’ll be at T-43 hours and L-70 h 21 m.
For the Space Shuttle, weather forecasts are provided by the U. S. Air Force Range Weather Operations Facility at Cape Canaveral beginning at Launch minus 3 days in coordination with the NOAA National Weather Service Space Flight Meteorology Group (SMG) at the Johnson Space Center in Houston. These include weather trends and their possible effects on launch day. A formal prelaunch weather briefing is held on Launch minus 1 day which is a specific weather briefing for all areas of Space Shuttle launch operations.
Weather briefings go as follows:
L-24 hr 0 min: Briefing for Flight Director and astronauts
L-21 hr 0 min: Briefing for removal of Rotating Service Structure
L-9 hr 00 min: Briefing for external tank fuel loading
L-4 hr 30 min: Briefing for Space Shuttle Launch Director
L-3 hr 55 min: Briefing for astronauts
L-2 hr 10 min: Briefing for Flight Director
L-0 hr 35 min: Briefing for launch and RTLS
L-0 hr 13 min: Poll all weather constraints