Engineers studying data from the shuttle Columbia before it broke apart Saturday say temperature readings in the ship's left-side landing gear wheel well may be indicating a catastrophic "burn through" in a different part of the wing, not the wheel well itself.

It is still not known what might have caused such a penetration through the heat-shield tiles protecting the shuttle's underbelly from the fierce heat of re-entry. But program manager Ronald Dittemore said today engineers have been asked to re-assess whether a large piece of insulation that peeled off Columbia's external tank 81 seconds after launch and crashed into the underside of the wing could have compromised the integrity of the thermal protection system.

"Certainly this debris is one of our primary areas of emphasis," Dittemore said. "We're completely redoing the analysis from scratch. We want to know if we made any erroneous assumptions. We want to know if we weren't conservative enough. We want to know if we made any mistakes, and so we're redoing the complete analysis.

"Secondarily, we have a team of engineers and managers and technicians that are ... making the assumption from the start that the external tank was the root cause of the problem that lost Columbia. That's our starting point when we look at the tank. And based on that assumption, what is the fault tree that would substantiate that particular assumption? And so we're attacking that. And that's a fairly drastic assumption and it's sobering."

Columbia was destroyed Saturday when it suffered a catastrophic failure around 9 a.m. EST while en route to the Kennedy Space Center to close out a 16-day science mission. All seven crew members perished.

But the problem first manifested itself in telemetry beamed back to mission control as the spacecraft approached and crossed over the coast of California. Dittemore provided more details today on just what that telemetry indicated (all times converted to EST).

• 08:52 a.m. - Three left main gear brake line temperature sensors show an unusual increase. "This was the first event, the first occurrence of a significant thermal event in the wheel well on the left-hand side," Dittemore said.
  
  
• 08:53 a.m. - A fourth left brake line strut actuator and uplock actuator (the mechanism that holds the landing gear stowed in the wheel well) temperature measurements rise 30 to 40 degrees over a five-minute period.
  
  
• 08:55 a.m. - A fifth main gear brake line temperature sensor shows an unusual temperature rise.
  
  
• 08:57 a.m. - Now passing over Arizona and New Mexico, upper and lower left wing skin temperature sensors fail off-scale low, indicating sensor failures.
  
  
• 08:59 a.m. - The shuttle responds to increasing left-side aerodynamic drag that is attempting to yaw the orbiter to the left. Columbia's flight control system sent commands to the wing elevons to counteract the drag and to move the ship back to the right. Data also indicates two of four right-firing rocket thrusters fired as well to assist. Moments later, contact with the shuttle was lost.
  
  

"The aerosurfaces were doing what they needed to do to counteract the drag on the left side of the vehicle, the right yaw jets had to kick in to help the aerosurfaces and it appears we were losing ground as far as the rate of attitude excursion. It was not long after that point we lost all data and communications with the crew."

Dittemore said the temperature spikes recorded by the wheel well sensors and others were not enough to indicate a catastrophic burn through of the landing gear doors.

"The outside temperature is above 2,000 degrees," he said. "Seeing an increase in the wheel well of 30 to 40 degrees seems to indicate that that's not the point of any large thermal excursion. That's reflecting something else."

In short, it now seems more likely the readings from the wheel well and the fuselage just above it were "reflecting an overall increase in temperature but were not the exact point of the penetration."

"I'm speculating a little bit here because I have no data or evidence to say that's really what happened," Dittemore said. "But I'm trying to think in my own mind how would the temperature increase in the wheel well and on the side of the fuselage and still end up with an event that lost the vehicle.

"It does not seem logical that the wheel well is the source of the problem because the temperature does not reflect it. We've had now five different temperature sensors all showing a 40- to 50-degree increase over five minutes, the side of the fuselage showing an increase. There's some other event, some other missing link that we don't have yet that's contributing to this temperature increase. And we've got to go find that."

Still in question is what might have caused the penetration, wherever it occurred.

While Columbia was still in orbit, engineers analyzed long-range tracking camera footage of the external tank debris striking the left wing of the shuttle. Running software designed to predict the effects of such impacts, they assumed the debris measured 20 by 16 by 6 inches and weighed 2.67 pounds. The ran the program using various impact angles to determine worst-case damage scenarios.

As reported early Monday, the engineers studied two such scenarios. In one, most of a single tile was assumed to be missing and in the other, it was assumed several tiles in a region measuring roughly 7 by 30 inches. In both cases, the engineers concluded, "possible localized structural damage" could have occurred, "but no burn-through and no safety of flight issue."

"For the loss of a single tile at the main landing gear door and for the other case where you have more acreage damaged, in both those cases the analysis predicted that even though you might have structural damage - and what I mean by structural damage is localized heating where you may have some effect on the basic structure in that area - even though you might have localized structural damage, you would not have damage sufficient to cause the catastrophic event nor impact the flying qualities of the vehicle," Dittemore said.

But engineers don't like coincidences and Dittemore has ordered a second debris review to determine if the original analysis stands up.