Shuttling to Death

BBC reports that that Space Shuttle return to flight launch has been delayed to July:
Although the space shuttles have been showered by small pieces of debris thousands of times during the 24 years that they have been flying, Nasa can no longer naively close its eyes to the possibility of danger.

After 113 flights of the space shuttle only one has had a fatality loosely linked to tile damage. I say loosely linked because in this situation it is impossible to come up with conclusive evidence that ice chunk A caused tile C to knock off. The CAIB report specifically identified a foam ramp as the likely cause of the ice that they believe struck the wing of the Space Shuttle knocking off one or more tiles that left a section of the shuttle unprotected during re-entry. It seems like there really is no way to give conclusive evidence of anything sometimes in Engineering.
Last week, shuttle program managers told reporters that engineers had assessed 170 sources of potential debris and cleared all but a handful as a potential threat to shuttle safety. A follow-up review was held at the Kennedy Space Center on Tuesday and Wednesday, but managers could not dispel their concerns.

Again I'll say it is interesting that in spite how sure NASA seemed to spin that the ramp foam was definitely the cause of the failure, they still felt the need to comb through 170 other possible causes. I'd like to know specifically how they cleared those other potential sources. Did they do impact tests? If not that what testing was done to clear them?

The other thing that bothers me is that when I look at footage of Shuttle launches I wonder how they can be so sure of themselves. The amount of debris coming off of everything in those launches is too numerous to count. How can they be so sure that only a handfull of them will cause such a failure in the future? Even the first Shuttle launch had shown some tiles to fall off.

The only way to ensure no future failures is to have some sort of system that can validate the existence of every tile on the Space Shuttle once it is in orbit. They could either do this by visual photos from the ISS, or they have onboard sensors to detect the presence of tiles. That way once in orbit mission control would have map of missing tiles and could determine within minutes the situation. I'm thinking they could install magnetic sensors inside the hull to detect the presence of the tiles. That is assuming of course that the tiles can be magnetized enough to show a difference in the sensor output. I'm sure if not that they could use a host of other sensors to achieve the same goal. But I'm bloviating right now. I'm sure there are other Engineers out there with a much better sense of what NASA is doing than I have.

Also I think about how ridiculous this all is. NASA has known about this danger for years. They considered it an acceptable level of risk. All of sudden a fatality occurs after 113 successful flights and it becomes a serious issue?

I have to ask the question what other factors does NASA not consider a serious level of risk in Space Shuttle launches?

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