Saboteur

Analysts are still studying whether the two strips of cloth called gap filler that are poking out from the bottom of the orbiter could cause uneven heating during re-entry that may constitute a threat to the craft and crew, the mission's lead flight director, Paul Hill, said in a press briefing this morning. If the analysts decide that the gap fillers do pose a threat, he said, the mission manager may call for a risky spacewalk and repair maneuver in which astronauts try to pull the tough material the rest of the way out, push it in or cut it off.

Mr. Hill said a spacewalk repair was not likely but that his engineers and analysts were looking closely at the issue and that he could not rule one out.

Gap fillers, as their name implies, fill the gaps that NASA leaves between some shuttle tiles to allow for expansion and contraction of the shuttle's body from the extremes of heat and cold that it is exposed to. The fillers themselves are heat resistant, and are made of alumina-borosilicate fiber. Having gap filler poke its way out up from between tiles is not uncommon, but it could be a concern because it causes an uneven surface and can lead to unusual patterns of heating during re-entry. If a filler pokes out too much, especially if it is far forward on the shuttle so that its downstream heating affects a larger part of the shuttle's belly, it could be a problem, Mr. Hill said.

NASA, through a long examination of all previous landings with protruding bits of gap filler, had found a comfort level with protrusions in the same areas that stick out a quarter of an inch, he said. But the two pieces on this flight are one inch and six-tenths of an inch, he said.

This shuttle and the space station have been outfitted with more cameras and sensors than ever before, and so they might be detecting something that has happened many times in the past; it is possible that the feltlike material burns down during re-entry, and that longer protrusions have occurred in the past. But one quarter inch is within "our conventional wisdom," Mr. Hill said, and so the little strips of cloth have become an intense focus of aerodynamic analysis, he said.

Aerodynamics experts have been studying the size and the position of the two gap filler protrusions, Mr. Hill said. Some members of the engineering community are concerned, he said, while others believe "this is not a big deal."

If mission managers do order a spacewalk, it is likeliest to take place on Wednesday, during an already-scheduled walk that was originally planned to focus on installing an external equipment cabinet on the station's airlock. It is also possible, but even less likely, that the task would take place in an entirely new spacewalk that could take place during the extra day of the mission that NASA has added to allow even more supplies from the shuttle to be loaded on to the station.

An important goal of this mission is to test the extensive modifications that NASA has made to the external fuel tank and its foam, which tends to shear and pop off during the stress of launching and can strike the orbiter.

The loss of the shuttle Columbia and its crew occurred because a 1.67-pound piece of foam hit the left wing during the ascent, creating a hole that let in superheated gases during entry.

NASA has said that, thanks to extensive modifications to its external tank, Discovery has sustained 25 nicks and dings, one sixth the number that shuttles generally return to Earth with. But a .9-pound piece of foam fell off the tank and narrowly missed the oribiter. That compelled officials to suspend further shuttle flights until the problem is resolved. Perhaps three other pieces that were larger than NASA's safety rules allow fell off the tank as well.

The analysis of the shuttle's delicate tiles and the heat-resistant blankets that coat the top