NASA's next space shuttle will likely launch in March 2006 and not be the Atlantis orbiter as previously planned, space agency officials said Thursday.

Instead, the Discovery orbiter - which returned to Earth last week after concluding NASA's 14-day STS-114 mission - will be the next to fly, which will ease future launch schedules and allow engineers additional time to complete troubleshooting and repair work on external tanks to prevent foam shedding during launch, shuttle officials said.

"We think, really, that March 4 is the timeframe we're looking at," Bill Gerstenmaier, NASA's foam investigation lead and newly-appointed associate administrator for space operations, told reporters during a Thursday press conference. "It looks like we're going to have to do some repair...on the tank."

Gerstenmaier said the decision to push toward a March 2006 launch date is not final, and is pending another two weeks of troubleshooting efforts by tank engineers. The delay would pass over launch windows in November and January 2006. Last week, Gerstenmaier said it was unlikely that Atlantis would make a four-day launch window beginning Sept. 22.

Engineers are once again poring over external tank foam debris after observing a nearly 1-pound piece separate from Discovery's external tank during the shuttle's July 26 launch. The foam did not strike Discovery, but was the same type of debris concern that doomed Columbia's STS-107 mission in 2003.

Columbia's left wing was struck by a 1.67-pound piece of foam insulation during launch, critically wounding the spacecraft and compromising its heat shield. The resulting damage cased the orbiter to break apart during reentry on Feb. 1, 2003, killing its seven-astronaut crew.

NASA spent two years trying to prevent such foam loss from endangering shuttles again before launching Discovery spaceward last month. After observing the foam shedding during Discovery's launch, shuttle officials pledged not to launch another orbiter until the issue was solved.

Gerstenmaier said engineers at NASA's Michoud Assembly Facility in New Orleans, Louisiana, where the external tanks are built, plan to dissect portions of completed tanks to better understand how to make repairs for the next shuttle flight.

The delay comes one day after the release of a final report from Stafford-Covey Return to Flight Task Group, an independent panel that watched over NASA's effort to resume shuttle flights after the 2003 Columbia disaster. That report also included a minority report of personal observations from some task group members, including a scathing critique of the space agency's return to flight effort that faulted, among other things, NASA's launch date setting practices as unrealistic.

"Going all the way out to March [is] an effort to give people some time for a planning horizon," said Michael Griffin, NASA's top administrator, told reporters during the press conference. "We are trying to insert the necessary conservatism in this and are giving ourselves what we hope is plenty of time."

Griffin maintained that the delay will not hinder efforts to complete construction of the International Space Station (ISS) and meet NASA's obligations to its international partners.

"Absent major problems, we believe we can essentially complete the International Space Station by the end of [the shuttle's] retirement," Griffin said.

NASA's three remaining orbiters are slated for a 2010 retirement.

"It doesn't seem to be a major impact at first look," Gerstenmaier said of the March 2006 launch target.

Gerstenmaier served as NASA's ISS program manager before being appointed to head the agency's space operations effort.

Shuttle swap

Gerstenmaier said that by targeting March 2006 for the next shuttle flight, shuttle engineers now have time to turn Discovery - which is expected to return to NASA's Kennedy Space Center in Florida from its California landing site on Saturday - around for the next flight.

Under the previous plan, Atlantis was tapped to launch its STS-121 mission - NASA's second return to flight mission - then turn around and launch STS-115, a construction flight to the International Space Station (ISS). But the new schedule should be more manageable, shuttle officials said.

"We can now go Discovery, Atlantis and then back to Discovery," Gerstenmaier said. "What that gives us is not just one flight opportunity, but several flights."

NASA's third remaining orbiter, the shuttle Endeavour, is currently in the middle of a major modification period.