Launch officials said they would try again Tuesday, on the Fourth of July, after giving the work force some rest and a chance to replenish the shuttle's on-board fuel. The weather was expected to improve by Tuesday, although rain was still in the forecast.
Launch director Mike Leinbach said it will be tight getting Discovery ready for a Tuesday afternoon launch — only the second liftoff of a shuttle since the 2003 Columbia disaster — and that any thunderstorms on Monday could put his team behind.
He halted the countdown just an hour after the seven astronauts boarded the fueled spaceship.
"Looking out the window it doesn't look good today," shuttle commander Steven Lindsey radioed from the cockpit. He noted that July Fourth would be "a good day to launch." It would be the first time NASA has ever launched a crew on the holiday.
The afternoon sky was considerably darker than on Saturday and left NASA with little choice but to call off the flight to the international space station. Thunderstorms were moving in quickly from the west, and lightning was detected within a few miles of the launch pad. The astronauts rode back to crew quarters in the rain.
The back-to-back delays cost NASA an estimated $2 million in overtime pay and fuel costs.
"After a year of preparation and after a very careful countdown, you don't want to get into a rush and do something that is not smart from a weather standpoint," said deputy shuttle program manager John Shannon, chairman of NASA's mission management team. "Nobody is going to remember that we scrubbed a day or two days a year from now. But if we go launch and get struck by lightning or have some other problem, that will be very memorable."
Shannon said he told his team, "What a great gift NASA could give to the nation to return the shuttle to operation on Independence Day."
"If the weather is good, that's exactly what we'll do," he said.
Among the invited guests who returned for the second launch attempt were family members of the perished Columbia astronauts. Vice President Dick Cheney was back in Washington after a brief visit to the space center Saturday.
Last month, NASA Administrator Michael Griffin approved launching the shuttle despite the concerns of two top agency managers who wanted additional repairs to the foam insulation on the external fuel tank. Columbia was brought down by a chunk of flyaway foam, and a piece broke off Discovery's redesigned tank last July, barely missing the shuttle.
"I've kind of steeped myself in this problem over the last month, and I am quite confident that we've got a very good chance of flying and flying safely," Griffin said Sunday in an interview on CNN's "Late Edition with Wolf Blitzer."
Griffin has repeatedly pointed out that if Discovery were damaged during launch, the astronauts always could take refuge on the space station until a rescue vehicle is sent up.
Sen. Bill Nelson, a Florida Democrat, who flew on a space shuttle in 1986 right before the Challenger disaster, defended Griffin's decision on Sunday and said it was an acceptable risk.
NASA's top safety officer and chief engineer recommended at a flight readiness review meeting two weeks ago that the shuttle remain grounded until design changes are made to the foam that insulates 34 brackets on the fuel tank. Without this foam, dangerous ice could form on the metal brackets once the tank is filled with super-cold fuel.
As it is, more than 35 pounds of foam were removed elsewhere in what NASA described as the biggest aerodynamic change ever made to the shuttle's launch system. Shuttle managers said they wanted to fly with only one major change at a time.
Once in orbit, Discovery's crew will test shuttle-inspection techniques, deliver supplies to the space station and drop off German astronaut Thomas Reiter for a six-month stay.
NASA hopes to add an extra day to Discovery's 12-day flight to test spacewalking methods for repairing possible damage to the ship's thermal skin. By replenishing the shuttle's on-board fuel now, the astronauts will have a better chance of getting that additional day in orbit.