WASHINGTON – NASA says a September launch of the Space Shuttle Atlantis is unlikely now that a week of intensive trouble-shooting failed to identify “any immediate easy fixes” to the foam-shedding problems seen during Discovery’s return-to-flight mission.

About two minutes into Discovery’s July 26 launch, a large chunk of insulating foam peeled away from the shuttle’s external fuel tank, missing the orbiter, but setting back NASA’s efforts to get on with assembly of the international space station.

All told, Discovery’s external tank shed larger-than-expected pieces of foam from four separate areas, including sections where NASA made modifications in the wake of the Space Shuttle Columbia accident. Columbia was doomed by a piece of external-tank foam that broke off during launch and punched a hole in the orbiter’s left wing.

NASA’s three remaining shuttle orbiters are grounded again until the foam problem is resolved.

Bill Gerstenmaier, NASA’s newly appointed associate administrator for space operations, said Aug. 11 that engineers investigating Discovery’s foam loss incidents so far have found no solutions to the problem.

“We didn’t find any immediate easy fixes here,” Gerstenmaier said during a teleconference with reporters. He said at least some additional tank modifications appear necessary before NASA can fly the shuttle again.

NASA officials had been holding out hope of resolving the foam issues in time to launch Atlantis before the end of September. But Gerstenmaier said that no longer appears realistic, given that at least some of parts of Atlantis’ tank will require “minor engineering modifications” before the shuttle can be cleared for flight.

“We will probably not make the September launch window,” he said.

NASA’s next opportunity comes in November. Gerstenmaier said if Atlantis is cleared in time to make that four-day window, NASA would remain on track for resuming construction of the international space station in March.

But Gerstenmaier cautioned that even with the additional external tank modifications to be made prior to Atlantis’ flight, the hardware likely will continue to shed at least smaller pieces of foam.

“Now we stand a chance this second time around to greatly improve,” he said. “But I would tell you frankly that next time we fly we would still expect to see a little bit a foam loss.”

The largest chunk of foam shed during Discovery’s launch — weighing just under half a kilogram — broke away from a so-called protuberance air load (PAL) ramp that smoothes the airflow around the cables and pressure lines that run the length of the tank.

Foam in that area was damaged slightly and repaired prior to the tank being shipped from its New Orleans manufacturing plant to NASA’s Kennedy Space Center, Fla., for launch. That revelation, confirmed by NASA officials the week of Aug. 1 , gave some shuttle watchers reason to hope that the most troubling foam loss seen during Discovery’s launch would be deemed an isolated incident, allowing NASA to more quickly clear Atlantis for launch.

Gerstenmaier said that while the repair was “probably a contributing factor,” it likely was not the root cause. “That alone probably would not be enough to cause the foam loss that we saw,” he said. “There is probably another underlying problem.”

Gerstenmaier said NASA eventually could decide to do away with the PAL ramp altogether, something agency officials considered prior to Discovery’s launch but ultimately rejected because they did not know what new troubles that might introduce. Discovery’s flight provided new information on the foam-shedding problem, Gerstenmaier said, adding that more data from future shuttle flights could permit NASA to remove the PAL ramp. But he characterized that option as a “longer-term” fix.

NASA still has more sleuthing to do on the PAL ramp and other suspect parts of the tank before it makes any additional modifications . While NASA may be able to resume flights with only minor changes , Gerstenmaier said his team of engineers will be looking for longer-term solutions to further reduce foam shedding.