How are spacecraft parts both extremely fragile and able to stand up to tremendous stress?

Question

The other day I was watching a documentary about Mars rovers, and at one point a story was told about a computer on the rover that almost had to be completely thrown out because someone dropped a tool on a table next to it. Not on it, next to it. This same rover also was planned to land by a literal freefall; crash landing onto airbags. And that’s not even covering vibrations and G-forces experienced during the launch and reaching escape velocity.

I’ve heard similar anecdotes about the fragility of spacecraft. Apollo astronauts being nervous that a stray floating object or foot may unintentionally rip through the thin bulkheads of the lunar lander. The Hubble space telescope returning unclear and almost unusable pictures due to an imperfection in the mirror 1/50th the thickness of a human hair, etc.

How can NASA and other space agencies be confident that these occasionally microscopic imperfections that can result in catastrophic consequences will not happen during what must be extreme stresses experienced during launch, travel, or re-entry/landing?

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EDIT: Thank you for all the responses, but I think that some of you are misunderstanding the question. Im not asking why spacecraft parts are made out of lightweight materials and therefore are naturally more fragile than more durable ones. Im also not asking why they need to be 100% sure that the part remains operational.

I’m asking why they can be confident that parts which have such a low potential threshold for failure can be trusted to remain operational through the stresses of flight.

In: 3487

Answer 1

If you’ve heard of “military-grade” as a descriptor of things, there is also Space-class. I used to work at a lab that did destructive physical analysis (I was an IT guy, not on the testing floor). But some products had to pass mil-spec, but another set of products had to be space class, so a group of parts would be tested for things like acid bath, thermal shock (dry ice, basically, followed by heat), die shear (being hit). The parts would be graded after the test and the rest of the lot would be assumed to have the same tolerances.