NASA's Mission Management Team today officially approved a one-day mission extension for shuttle Discovery's crew, allowing the astronauts to stage a third spacewalk next week to test wing leading edge repair techniques. Piers Sellers and Mike Fossum plan to stage their first excursion Saturday, starting around 9:13 a.m., to test the ability of a long boom on the end of the shuttle's robot arm to serve as a work platform for future repair work.

With spacewalk preparations in high gear, engineers are still assessing the health of two leading edge panels on Discovery's right wing, along with a protruding gap filler just in front of a propellant feedline access door on the orbiter's belly. Mission Management Team Chairman John Shannon said today it might take another day or two before engineers can either give Discovery a clean bill of health or show the "regions of interest" represent potentially serious problems. While many engineers are optimistic - Discovery appears to be one of the "cleanest" shuttles ever launched - Shannon said he would not speculate on the possible outcome of the ongoing engineering analysis.

Asked if he could at least give reporters guidance as to how he thinks the analysis might play out, Shannon said simply, "No. I am completely withholding judgment until the analysts come back. Just like we said, we're going to give them time to do their work and they'll come back and tell us what we have."

It is a sensitive issue because the fate of the post-Columbia shuttle program clearly rides on successful launches and re-entries. A serious problem with Discovery's heat shield could force the astronauts to abandon ship and move into the international space station to await rescue. And that almost certainly would mean the end of the shuttle program.

Engineers are optimistic no such doomsday scenarios are actually in play, but Shannon clearly wants to give the team time to reach a technically sound conclusion before making any official announcements.

On a more positive note, initial worry about a potential hypervelocity impact on the shuttle's reinforced carbon carbon - RCC - nose cap has lessened, thanks to high-resolution imagery shot by the shuttle astronauts today as part of a so-called focused inspection. Whatever the white, circular marking is - and it could be a bird dropping - it is not thought to be a potentially dangerous puncture caused by the impact of high-velocity space debris.

"That really got our attention and that was the number one priority of the focused inspection," said Steve Poulos, manager of the orbiter projects office at the Johnson Space Center. But after looking at today's photos, it would appear "that's not the case."

"It's a white horseshoe-looking artefact," he said later. "I'll let you use your imagination on what it might be."

The shuttle's nose cap and RCC wing leading edge panels experience the most extreme heating during re-entry and any punctures or cracks would be cause for serious concern. Discovery's nose cap, at least, appears to be sound, although it remains officially classified as an open issue.

Likewise, engineers are still examining two RCC panels on Discovery's right wing - RCC panels 5R and 9R - that show apparent discoloration or other unexpected markings. Engineers are hopeful analysis of data from earlier laser scans and high-resolution imagery shot by the crew today will show no cracks or other serious defects exist. But given the high stakes involved, the RCC community is taking a thorough look at both panels.

Data from sensors behind Discovery's wing leading edge panels indicate six potential impacts, three on each wing, during the climb to space Tuesday, the most significant peaking at 1.6 times the force of Earth's gravity. During ground tests, however, impacts of 10 Gs or more were required to cause the kind of damage that might preclude a safe re-entry.

"But what this does do then is require our inspection team for the wing leading edge to spend more time and energy looking at these particular panels, these regions where we've identified potential impacts," Poulos said.

Engineers now believe what appeared to be a protruding gap filler just behind Discovery's nose cap is more likely a bit of cloth batting-like material extending slightly above the surrounding tile. As such, it would not appear to be a problem for re-entry. Likewise, a gap filler seen extending between tiles near left wing RCC panel 17 has been ruled acceptable as is for entry.

But a protruding gap filler sticking up about an inch just in front of a propellant feedline access door on the belly of the ship could cause heating problems during entry if it is not removed. Such protrusions in the otherwise smooth flow of air over the shuttle's belly can trigger turbulence in the so-called "boundary layer" that provides an insulating effect. If the boundary layer breaks down and transitions from laminar to turbulent flow at high velocities, higher downstream temperatures result, along with possible damage to the shuttle.

Along with analyzing the aerodynamic effects of the gap filler in question, engineers also must calculate the structural effects of downstream heating, a complex process that likely will take another day or two to complete. Poulos said the proximity of the access door was not an issue and the primary issue is damage that might have to be repaired after landing.

"We really have to go off and understand the load paths and the reduction in margins because it is going to be a little warmer than we would plan for," he said.

Astronaut Steve Robinson removed two protruding gap fillers during Discovery's last flight, plucking them from adjacent tiles with his gloved fingers. But reaching the gap filler in question on this flight would be more difficult and likely would require an astronaut to ride on the end of a long inspection boom carried by the shuttle's robot arm.

As coincidence would have it, Sellers and Fossum plan to test the arm-boom combination Saturday to evaluate its strength and usefulness as a work platform for future repair work. If those tests go smoothly, NASA could opt to use the boom to remove the offending gap filler during the crew's third planned spacewalk next Wednesday.

As of this writing, however, no such decisions have been made.

In other developments, Shannon said a large 12-by-14-inch sheet of foam that peeled away from Discovery's external tank in about a half-dozen pieces two minutes and 50 seconds after takeoff likely weighed just .11 pounds, or 1.8 ounces. Engineers believe foam debris must weigh .25 pounds - four ounces - or more to pose a serious impact threat and in any case, no tile damage has been seen in the area.

Shannon also said engineers have recovered Discovery's twin booster rockets and hope to deliver film from on-board cameras by Sunday. Those new views may show previously seen foam releases from different angles and help engineers improve their understanding of the timing of such events.

Finally, Shannon said part of the launch pad hydrogen ignitor system, used to burn away excess hydrogen gas just before main engine start, broke off, shot out and hit the nozzle of main engine No. 1 prior to Discovery's launch. Analysis of data from the engine, however, showed the powerplant worked normally throughout the ascent.

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