“The team gets into the mode of ‘This is launch countdown,’ and that’s really the focus that everybody has,” says NASA test director Jeremy Graeber. “To do it one more time is a great feeling.”

The only cloud on the horizon for an on-time liftoff is, predictably, Florida’s thunderstorm-prone summer weather. With a front expected to move over the mid-Atlantic coast on Thursday, meteorologists with the U.S; Air Force’s 45th Space Wing are forecasting a 60% chance weather will delay launch.

“We’ll have real high moisture on Friday,” says shuttle weather officer Kathy Winters. “When we have high moisture, particularly in the low levels, we can pop those thunderstorms and showers early when the sea breeze forms.”

Specifically, the forecast calls for a chance of showers and thunderstorms within 20 nm of the Shuttle Landing Facility, a violation of NASA flight rules that protect for a contingency landing at the launch site, and cumulus clouds within 10 nm of Launch Pad 39A, where Atlantis stands poised for liftoff. The forecast improves for launch attempts on Saturday and/or Sunday, after which the Eastern Test Range schedule shifts to support a Delta 4 rocket launch on July 14 with a GPS satellite for the Air Force. The shuttle’s next launch window opens July 16 if the Delta launches on time.

The abridged, four-member STS-135 crew, headed by Navy Capt. (ret) Christopher Ferguson, arrived at the Florida spaceport Monday afternoon in a pair of T-38 training jets.

“It’s such a pleasure to come down here when you have a rocket on the pad and it’s got your stuff loaded on it,” astronaut Rex Walheim tells reporters gathered at the landing strip.

NASA pared the last shuttle crew from the normal six or seven members to four to accommodate an emergency return on Russian Soyuz capsules, as there is no second shuttle available to mount a rescue mission should Atlantis sustain significant enough damage during launch or while in orbit to prevent a safe re-entry. NASA has preserved a safe haven option for shuttle crewmembers aboard the International Space Station and shuttle rescue capability since returning the fleet to flight after the 2003 Columbia accident.

Ferguson, Walheim, pilot Douglas Hurley and mission specialist Sandra Magnus were all fitted for Soyuz Sokol spacesuits, though just Walheim’s is flying with the STS-135 cargo. The others would be flown to the station as needed.

“The plan would basically change the sequence of when people would come down,” Walheim said in a preflight interview. “Some of the folks on the space station would stay longer than they anticipated, and then as spots free up we’d cycle our crew down one by one. [Russia] also will launch Soyuz spacecraft with just two people instead of three, which leaves a spot for them to come down with one of our crewmembers. We will kind of methodically do that until everybody’s rotated down.” Walheim would be the first to return, followed by Ferguson, Magnus and Hurley.

The goal of NASA’s final shuttle flight is to deliver a year’s worth of food, clothing, supplies and equipment to the space station to buy time in case NASA’s commercial cargo resuppliers, Space Exploration Technologies and Orbital Sciences Corp., encounter technical delays with their programs.

-aviationweek.com