Last Updated at 11:07 a.m. ET
KENNEDY SPACE CENTER, Fla. - NASA is "cautiously optimistic" that weather which threatened the launch of Atlantis will pass, allowing liftoff Friday morning for the space shuttle program's 135th and final mission.
NASA managers early Friday cleared engineers to load the shuttle Atlantis with more than a half-million gallons of rocket fuel and to press ahead for an attempt to launch the orbiter as scheduled, at 11:26:46 a.m. Eastern. With a 70 percent chance of stormy weather, mission managers held open the option of standing down later Friday morning, depending on actual conditions.
The updated forecast improves to 60 percent "go" on Saturday with a 50 percent chance of acceptable weather Sunday.
Commander Christopher Ferguson, pilot Douglas Hurley, Sandra Magnus and flight engineer Rex Walheim strapped in shortly after 8:00 a.m. to await launch.
"Mike Moses, the mission management team chair, said some people might call it silly to try and play in the rain this morning, but he said we're going to absolutely try for tanking," said countdown commentator Allard Beutel.
"But throughout the overnight and closer to the dawn early morning hours, between six and seven o'clock, they'll keep their eyes on the forecast, of course, all night long, but around that time, they'll start getting a very focused look on the weather and see whether it's the right thing to proceed towards launch or whether the forecast really is getting worse. We'll look at perhaps standing down at that point."
Hoping for the best, engineers began pumping super-cold liquid oxygen and liquid hydrogen through transfer lines to pad 39A at 2:01 a.m. Eastern. Routed through Atlantis' main engine plumbing to condition the hardware to cryogenic temperatures, the propellants then flowed into oxygen and hydrogen reservoirs in the shuttle's huge external tank. Three hours later, the tank was full and engineers transitioned to "stable replenish" mode.
Going into the final launch campaign, NASA managers said Atlantis had to be off the ground by Sunday to avoid a conflict with the launch next week of an Air Force navigation satellite.
But agency officials said Thursday it might be possible to extend the shuttle launch window through Monday if the Air Force agreed to delay the satellite launch by one day. But no final decisions have been made.