With a successful spacewalk repair job today, engineers believe the shuttle Discovery's heat shield is in good shape for re-entry and landing Monday. But one question mark remains: What, if anything, to do about a damaged insulation blanket just below commander Eileen Collins' left cockpit window.

Credit: NASA

As the Discovery astronauts were unwinding from their third and presumably final spacewalk today, mission control called Collins with word that a fourth spacewalk, this one to repair the blanket, is a possibility, depending on how the damage assessment turns out.

"We wanted you to know that they are looking at the remote possibility, but yet if indeed the damage on the blanket warrants it, that they might have to plan for a fourth EVA," astronaut Julie Payette radioed from mission control in Houston. "This is a small probability, but we wanted you to know about their thinking, and also get any comments from your crew and especially from the EV (spacewalk) crew members about this idea so we can put that in the pot for planning."

"OK, thanks for the heads up," Collins replied.

Cameras on Discovery's robot arm were used to photograph the torn, puffed-up blanket today, providing zoomed-in views showing what looked like impact damage on the slender segment. At a change-of-shift news briefing, lead flight director Paul Hill said engineers hoped to complete a thorough damage assessment by Thursday morning.

Unlike the consequences of the protruding gap fillers removed by spacewalker Stephen Robinson earlier today, the blanket does not pose a threat during the high-speed, high-heating part of re-entry. Rather, the concern is that when Discovery reaches the denser, lower atmosphere, a piece of the blanket could rip away, possibly hitting the back of the orbiter.

"It's not an entry heating concern," Hill said. "The concern is if something comes off and then flies aft and hits some critical surface, whether its a control surface, the front of an OMS (rocket) pod or something like that. So we're taking a look at this as a debris problem, not an entry heating problem."

Hill said engineers are trying to assess "how significant it would be if we made impact with the amount of material that could come off in the worst case."

"There are also folks off doing statistical analysis on where the most likely areas are that could be hit," he added. "We'll have more data tomorrow. ... But we are definitely looking at various areas on the vehicle aft and what it would mean to us if we were to hit it with the predicted masses that could come off."

Mission management team chairman Wayne Hale likely will address the issue at his daily news briefing later today.

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