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
You can think of this very similarly to the [egg drop challenge](https://youtu.be/nsnyl8llfH4?t=317). In the video I linked, he drops an egg off of a bridge and it survives just fine — but dropping the egg by itself certainly would have broken it. Hell, I bet you dropping something close enough to the egg sitting on a table by itself might cause some hairline cracks.
Fundamentally, you are right: spacecraft are very, very fragile systems. However, they can be carefully designed to be *extremely* resistant to certain kinds of dangers. Let’s use the egg again — ever tried breaking an egg by squeezing it uniformly? Even though it’s very fragile overall, the egg can still resist massive distributed pressures because of its unique properties.
This is what makes designing spacecraft really hard. We take a whole bunch of things that are very fragile but also very powerful in some regard, and we have to find a way to strap them all together so that we exploit their strengths and protect their weaknesses. This is why you see spacecraft getting tested so much; we are checking every conceivable possibility and failure mode to make sure we understand how they behave. This is how we can be (reasonably) confident that everything will work in flight — we’ve tested our design to ensure that those vulnerabilities are properly protected.
(The slightly less ELI5 answer is that we’re never *fully* confident in these things, and usually choose to report them terms of probabilities and standard deviations. At some point in the lifetime of a program, everyone gets together and decides just how stringent the requirements need to be. This informs what is considered an acceptable level of risk, and further dictates how much modeling, simulation, and [FMEA](https://en.m.wikipedia.org/wiki/Failure_mode_and_effects_analysis) is required.)
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