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?
​
​
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
What you’re illustrating here is the absolute insane levels of planning an engineering that go into every single tiny aspect of space flight. Testing and simulating every little aspect of the mission allows for a very high degree of understanding of the tolerances necessary to complete a mission. But, at the end of the day, things can always go wrong oh, there’s always the possibility that somebody made a mistake somewhere. That’s why there’s always such a large celebration in the mission control room when major milestones have been reached: the anxiety everyone had knowing the thousands of things that could go wrong.
Computer was probably not fully assembled, and was missing the covers or the padding that will protect it during actual flight and landing.
Takeoff is indeed very bumpy, but mostly predictable, and they do make sure there are absolutely no lose objects bouncing around the cabin. One they are in orbit, lot of things get take out, unfolded, clipped into place, etc.
Hubble mirror is glass or metal, vibration will not do anything to it as long as it is attached with soft padding.
It’s not that the tool damaged the computer, but the tool violated the pedigree for the computer. Since the pedigree is required to launch the computer, it would have been very expensive to disassemble the computer, test every part, and assemble it to be **sure** that no damage had occurred. To be 99.9% sure that nothing bad could have happened isn’t sure enough to pass launch criteria.
The Hubble mirror is an interesting example. The mirror was made extremely precisely, albeit wrong. That allowed it to be corrected for later. There was a plan to test the Hubble mirror, but the schedule was compressed. Then the Challenger Disaster delayed the launch many months, but NASA didn’t want to spend the money on the Hubble test, because they were worried about their budget because of the disaster.
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.
>he 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.
Lets talk about where it is used and what happens if there is a flaw that needs fixed. If this is your home computer, it is actually pretty easy to get a technician there to fix it. If it goes to mars, there is no way to get a ‘fix’ to that device.
That is the first issue – we cannot simply fix it if it breaks. The second is cost.
Lets assume another situation – a one time available use on earth vs mars. Same issue of no-technician being able to fix it. I it costs $100 to get it there on earth, it may not be as big of an issue to send another. If it costs 2 billion and 8 months to get it another planet, it is no so easy to simply send another one to replace it.
So with Space hardware, the lack of servicability coupled to the extreme costs and time delays to get items to the location make it all the more important they are perfect or at least as perfect as we can make them.
The last part is we may design items to withstand specific forces, but we don’t want to expose them to these forces without reason. The windows on your home are impact resistant. We don’t regularly hit them with a hammer to check. Same idea here.
With that computer, it may cost $25,000 to replace it on earth with one that didn’t have that issue occur. On a 2 billion dollar mission where failure is extremely expensive, it can be worth it to replace the item rather than risk an extremely unlikely failure.
>I’ve heard similar anecdotes about the fragility of spacecraft.
This is also somewhat true.
Spacecraft have huge weight limits given the energy required to lift something into space. We simply cannot armor something like a battleship.
The second item is energy. We are used to thinking about impacts in earth terms, with wind resistance. In space, the speed differentials can be huge. We are talking about speeds in the thousands and tens of thousands of miles an hour. 200km is 17,000mph orbital velocity
Imagine a baseball sized object coming at your spacecraft with a differential speed of 2000 mph. It is this huge energy level based on speed that makes even small low mass items dangerous to spacecraft.
In earth terms, imaging dropping a bowling ball on a piece of plywood from waist height. Will it punch through? Now image a bullet from a gun. Will it punch through? In space, the bullet analogy is actually pretty good. A typical handgun shoots a bullet around 800mph. A hunting rifle – around 2000mph.
That is the power of speed.
You’re brain can’t handle being hit very well. Luckily, it rides around inside a cushioned cage. Most space equipment is sort of the same way: sensitive bits crammed into a durable package.
More importantly… putting stuff in space is REALLY expensive. Any minor mishap on the ground is a big deal because you want to know 100% that it didn’t cause some problem that you can’t see but nonetheless makes it not work after you spend $100 million putting it in space and out of reach of people that can fix it.
The Hubble mirror was not a minor imperfection, it was a serious flaw because the lens system used for measuring the mirror surface was built incorrectly. They ended up polishing the mirror to the wrong shape. “1/50 the thickness of a human hair” is about 2 microns, which is a HUGE error for a telescope mirror. Even an amateur Newtonian telescope you can buy for <$500 is polished to better than 0.2 micron accuracy.
Anyway, spacecraft parts do need to be extremely lightweight and still survive launch. This is achieved through extensive analysis and testing. Everything is modeled in the computer to predict the stresses and make sure it can survive the expected environment. Then every component & sub-system is tested on a vibration table, and in thermal-vacuum chambers, etc. Then the entire spacecraft is put through the same tests. Even something as large as the Space Shuttle was put on a vibration test stand and [shaken](https://www.nasa.gov/centers/marshall/history/this-week-in-nasa-history-space-shuttle-program-s-first-mated-vertical-ground.html).
Also, heritage is very important in the space industry. We try to use components and designs that have flown successfully before, because we know they work. We keep track of every component and assess its heritage; if any component is identified as a new design that hasn’t flown before, it will be replaced with something that HAS flown, or will undergo extra scrutiny.
>Apollo astronauts being nervous that a stray floating object or foot may unintentionally rip through the thin bulkheads of the lunar lander
I don’t think that was ever a concern. What they may have worried about was tearing off the thermal protection, which was a thin reflective film, like a mylar balloon. The bulkheads and structural components of the lander were quite durable.
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.)
It’s not that they are fragile it’s that they are designed and tested based on certain assumptions. If those assumptions are violated then it ‘might’ not work. When you are putting things in space you don’t want to leave it at night.
Latest Answers