Any loss, even from an "unavoidable" reason is unacceptable. That's what should drive every single one of us in our industry to work hard to better ourselves every day.
You might want to read
The Challenger Launch Decision and
Truth, Lies, and O-Rings. Both of them deal with some very interesting organizational factors that directly caused both Space Shuttle accidents.
Any loss of life is unacceptable. Both shuttles crashed because of negligence in NASA. That most certainly unacceptable.
God, we're a bunch of wusses.
Absolute boulderdash. First: There is no such thing as an operation that is zero-risk. Even when you strap on whatever regional airliner you fly, there's a chance, however remote (and it is extremely remote, nowadays), that you will not be coming home. A strategic safety goal of
zero preventable hull losses, serious injuries and deaths makes sense for common carriage. It makes a lot of sense for the space business too, but in manned spaceflight, we don't know what we don't know. We don't really know, I think, what is 'preventable' in that business. We're really good at mitigating a lot of airline flying related-risks, but this isn't an airline that we're talking about here. These guys are not operating on proven technologies, and so on.
We just haven't done that much of it. We're really good at flying airliners. We're excellent at it, in fact, in this country. But we haven't had
nearly that level of experience with manned spaceflight, and people are going to continue to die in the race to conquer the stars, just as they did in conquering the sky. It's not an excuse for malfeasance, criminal neglect, or things like the
Challenger/Columbia disasters, but we'd be wise to re-read Gus Grissom's quote.
Spaceflight is not the airlines. In many ways we don't really know what we're doing on spaceflight. Consider that 100 years ago we didn't know what we were doing in airplanes, either. I consider it relatively amazing (given the nature of the technologies and performance extremes involved and ignoring organizational hogwash that was responsible for the Shuttle accidents) that the casualty rate for our manned spaceflight programs hasn't been
higher. The Apollo system pushed the limits of the day's technology and human performance; that a single LM was not splattered across the surface of the Moon is truly remarkable.
Bureaucracy killed the Challenger/Columbia astronauts. I would call those accidents avoidable. But I'm really surprised there weren't more. (I mean, they signed the Shuttle off as "airworthy" after a grand total of four all-up flights. Four flights. Just four. Harumph. Experimental...)