Live At KSC: Astronauts Hope "Today Is The Day!"

Spaceship Endeavour is being gassed up at Kennedy Space Center this morning after engineers determined launch-pad lightning strikes did no damage to critical shuttle electrical systems.

Seven astronauts led by veteran mission commander Mark Polansky are relaxing in crew quarters at the KSC Operations and Checkout Building, obviously happy to hear their shuttle has been cleared for external tank fuel-loading operations.

"Just got the word that we are GO to fuel the external tank. Hope that today is the day!" Polansky said in a Twitter post. "Watching the Tour de France at the moment."

Endeavour and its astronauts are scheduled to launch at 7:13 p.m. -- the middle of a 10-minute window of opportunity to set sail for the International Space Station. The middle of the window is typically selected for optimum propulsion system performance.

The weather forecast for launch is good. Meteorologists say there is a 70-percent chance conditions will be acceptable for an on-time liftoff. The chief concern is a chance of showers or thunderstorms within 20 nautical miles of the pad at launch time.

Shuttle launch rules prohibit flight through precipitation or launching during storms.

You can check out the details in this Official Launch Forecast from the U.S. Air Force 45th Space Wing Weather Squadron.

With a two-day lightning analyses now history, NASA managers and engineers are turning their attention to fuel-loading operations. Engineers in particular will be watching for the type of dangerous gaseous hydrogen leaks that forced launch scrubs on June 13 and June 17.

Higher-than-allowable concentrations of gaseous hydrogen were detected around a leak vent line about two hours and 10 minutes into fuel-loading operations on both June 13 and June 17.

The seven-inch line -- shown in the still image, above left -- is designed to keep pressures with the external tank at proper levels by venting excess gaseous hydrogen to a nearby flare stack, where the highly flammable substance is safely burned away.

Liquid hydrogen at Minus 423 degrees Fahrenheit "boils off" -- turns from a liquid to a gaseous state -- as the external tank is filled.

The propellant-loading operation began at 9:48 a.m. and is expected to take three hours to complete.