A post-launch analysis by Boeing engineers shows three pieces of debris falling off the shuttle Columbia's external fuel tank one minute and 22 seconds after launch Jan. 16 slammed into the orbiter's left wing. While the Boeing team concluded Columbia could safely land despite potential damage to the shuttle's fragile heat-shield tiles, internal agency emails released today raised potentially troubling questions about how the damage was assessed within NASA and how widespread discussion of its possible impact really was.

A tracking camera captures the debris striking the shuttle's left wing during launch. Photo: NASA

"I am advised that the fact that this incident occurred is not being widely discussed," Mark Shuart, an engineer at NASA's Langley Research Center in Virginia, wrote in an email to two colleagues.

Worried more about the impact of re-entry heating on Columbia's left main landing gear tires than he was about a high-altitude catastrophe, Robert Daugherty emailed Shuart later that "we can't imagine why getting information is being treated like the Plague. Apparently, the thermal folks have used words like they think things are 'survivable,' but 'marginal.' I imagine this will be the last we hear of this."

In another email, Daniel Mazanek with Langley's Spacecraft and Systems Branch, questioned the Boeing team's assumption the debris was made up of low-density foam insulation falling from Columbia's external tank. Mazanek wrote a colleague the debris could have been ice or even hardware from somewhere else on the orbiter or tank.

"The debris may have been made of ice or some other material(s) and could be much more massive than the calculated 1.211 kg (2.67 lb)," he wrote. "If the photogrammetric measurements accurately measured the debris to be (20 X 16 X 6 inches), and it was made of solid ice, the mass could be approximately 28.7 kg (63.4 lb). The energy released from this impact could be almost 25 times greater than estimated. Other dense materials, such as aluminum, would make this impact even more damaging."

Mazanek suggested a re-analysis of the launch video to more accurately determine what the debris was and where it came from.

"As a reference, if the debris was 1.211 kg and assuming a conservative relative impact velocity of 457.2 m/s, the kinetic energy would have equivalent to a 500 lb safe impacting at 75 mph," he emailed. "If the debris was 28.7 kg, that would be the equivalent of a 500 lb safe hitting the wing at 365 mph."

Within days of Columbia's launching, a team of Boeing engineers began assessing the debris impact, using software to model various damage scenarios. While many observers, including this reporter, saw two pieces of debris while replaying launch videos in the days after Columbia's destruction, the report released today is the first citing three such pieces.

"At about 82 seconds into the flight, multiple pieces of debris were seen emanating from the ET (external tank) bipod area and later seen impacting the Orbiter lower surface," the Jan. 24 report states.

"Film analysis results indicate impact at about 1/3 of the wing from the vehicle centerline."

Three pieces of debris were observed, the report continued, with a maximum size estimated at 20 inches.

The report is one in a series of studies done in the wake of Columbia's launching to determine what sort of damage the debris might have caused. In two widely reported studies released earlier, the engineers said the loss of a single tile could cause serious damage, but would not result in a loss of the shuttle.

But those studies did not include analysis of damage that might remove or seriously erode multiple heat-shield tiles. They simply said that pending the completion of that analysis, the team believed Columbia could safely land.

The charts released today did not shed any light on that issue, although they do show the expected impact angles and velocities of the debris. For the worst-case scenario, one in which it was assumed a 20-by-16-by-6-inch piece of foam hit the left wing, the impact velocities averaged 643 feet per second, or 438 mph. Impact angles averaged 15.3 degrees for impacts on the leading edge of the wing and 9.3 degrees for the tiles just behind the edge.

But the charts released today do not explain what conclusions the engineers reached from that data or what those conclusions were based on.

"Three sets of charts, from Jan. 21, Jan. 23, and Jan. 24, were part of an analysis conducted during the STS-107 mission of the impact of external tank foam debris striking Columbia during ascent," NASA said in a brief statement accompanying today's release.

"The analysis was presented to the STS-107 Mission Management Team on Jan. 27 with the conclusion that the effects of the debris did not pose a safety of flight concern for Columbia. Mission managers concurred with that conclusion.

"These charts do not represent a comprehensive look at the analysis, which included extensive verbal communication, and took place over more than a week while Columbia was in orbit. All open work shown on these charts was completed and reported to space shuttle management."

The Columbia Accident Investigation Board has concluded an intrusion of superheated plasma from a breach in the left wing led to Columbia's destruction. But it is not yet known where the breach occurred or what caused it. Engineers are focusing on the leading edge area and the left main landing gear door and associated seals.

Wiring from sensors located at the back of the wing was routed to immediate left and then in front of the main landing gear wheel well. Those sensors dropped off line early in Columbia's descent, indicating plasma ate through the wiring. That's consistent with a breach in the leading edge area ahead of the wheel well. But a breach in the wheel well itself has not yet been ruled out, even though the temperature increases seen there were far below the temperature of the presumed plasma.

Regardless of the details, the post-launch engineering analysis of the debris impact - the assumptions made, how the conclusions were reached and how they were reported to senior management - will no doubt continue to receive attention. The charts released today shed little light on those matters.

But the email traffic that has surfaced in the wake of the disaster shows many engineers in the shuttle entry systems community were worried about the possible consequences of the presumed tile damage.

Dennis Bushnell, a Langley manager who participated in the shuttle's original flight certification back in 1980 and 1981, emailed a colleague on Feb. 5 that boundary layer turbulence, magnified by tile damage, could be a lethal combination.

"If the flow is turbulent at peak heating the heat shield would/could burn through the wheel well doors [even with undamaged tiles]," he wrote.

"More extensive tile damage, whether from external tank insulation or ice impingement would obviously add insult to injury and compromise TPS (thermal protection system) integrity AS WELL AS ACT AS A BOUNDARY LAYER TRIP. If the gouges (caused by debris impacts) were extensive enough then free shear layers form which have VERY LOW TRANSITION REYNOLDS NUMBERS [below a hundred thousand] AND large impingement HEATING PEAKS."

NASA, he said, "should have done more analysis of this whole situation/taken it more seriously."

That was an after-the-fact observation. Before Columbia returned to Earth, it's not clear whether anyone believed a catastrophe was imminent.

Shuart ended a Jan. 30 email by writing, "It will be interesting to see the extent of the damage after landing on Saturday."