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

Let’s pretend we have a coathanger. One of those old wire coathangers. You can hang a coat on it pretty easy, hence the name. You might even be able to hang a really heavy coat on it. But if you were to push on the hook on that coathanger, it would twist and you could unwind it pretty easily. Then it wouldn’t hold a coat anymore. It might not be able to anything at all!

Now, let’s pretend this computer has a switch on it, and a switch got shaken loose. When it comes to to remotely press that switch, what if it just… doesn’t go?

Pretty specific example, right? It’s not the same thing, but here’s a case of a faulty switch [causing a crash](https://www.nature.com/articles/news041018-1) in a spacecraft. This one wasn’t installed properly and the entire mission ended in rapid, unplanned deconstruction of the craft and its samples.