NASA announced Tuesday that a fateful hailstorm the previous evening had caused such damage to space shuttle Atlantis' external liquid-fuel tank that the scheduled March 15 launch of mission STS-117 would have to be pushed back to at least late April.
Golf-ball-sized hailstones from the sudden thunderstorm left hundreds of small dings in the massive orange structure sitting on the launch pad.
It also crushed some foam along wedge-shaped brackets, an area where the shuttle in the past has shed foam — a potential danger. It did some cosmetic damage to more than two dozen tiles along the shuttle's left wing.
"This constitutes, in our evaluation, the worst damage we have ever seen of hail on the external tank," said Wayne Hale, manager of the space shuttle program.
The entire launch assembly — shuttle, liquid-fuel tank, twin solid-fuel rocket boosters and mobile launcher platform — now has to make the slow crawl back to the Vehicle Assembly Building, more than three miles away.
If repairs can't be made at the Kennedy Space Center, the tank will have to be shipped back to its manufacturer in New Orleans.
NASA has another external fuel tank on hand, but it's being readied for another mission in June.
The damage is concentrated in the upper third of the enormous external tank, a section which holds liquid oxygen propellant .
The launch of Atlantis would have to be after a Russian Soyuz vehicle completes a mission to the international space station in the first part of April, putting the next opportunity likely between late April and late May, officials said.
The three-member crew of the space station, who had been preparing for their visitors next month, will have to rearrange their work schedules.
"We're a little bit bummed out with the fact that (the space shuttle) isn't going to be here," U.S. space station crew member Sunita Williams said in an interview from space Wednesday morning.
During their 11 days in space, Atlantis' astronauts are to deliver a 35,000-pound addition to the international space station, the heaviest ever, along with a new pair of solar arrays.
Crew members also plan to unfurl the solar arrays, fold up an old pair and conduct at least three spacewalks.
The hail Monday was up to 2 inches in diameter; the National Weather Service considers three-quarters-inch hail to be severe, said David Sharp, a meteorologist with the weather service.
"Most people didn't see thunderstorms, let alone severe thunderstorms," Sharp said. "It only occurred in one location, and that was NASA's Kennedy Space Center complex."
In 1999, hail from a storm made 650 dings in space shuttle Discovery's external tank, forcing NASA to delay a launch and return the spacecraft to the Vehicle Assembly Building.
In 1995, space shuttle Discovery was sent back to the Vehicle Assembly Building because of fuel-tank damage caused by a pair of woodpeckers that drilled about 200 holes in the rust-colored foam insulation, apparently in an attempt to roost and build nests.
Hail also hit the external tank of Atlantis in 1990, causing minor damage.
The insulating foam on the external tank is of special concern to NASA since foam flew off space shuttle Columbia during liftoff in 2003 and struck the orbiter. The damage allowed fiery gases to penetrate Columbia during re-entry, breaking up the craft and killing its seven astronauts.
NASA redesigned the external tank, removing large amounts of foam, before last year's three successful shuttle missions. The space agency plans another design change to the tank before the shuttle program ends in 2010.
NASA managers had hoped to fly five shuttle missions in 2007, the most ambitious schedule in five years. Atlantis' flight was set to be the first of the year; the second was set for June.
Hale said he was confident the goal of five flights could still be met. He said, "There might be some small effect on a couple of later flights, but by the time we roll around to the end of the year, I expect we would be fully able to catch back up."
The Associated Press contributed to this report.