# How do I explain that an interstellar spaceship still requires risky spacewalks?

It is the mid 22nd century A.D. Twenty men and women signed on a ten year space exploration trip to the nearest star system. There are robots to do ship maintenance and a powerful A.I. to monitor the antimatter containment shield around the clock.

For my purposes, I need that from time to time the crews take turns to do high risk spacewalks. Basically they are required to check the solar sails, sensors array and inspect the hull integrity.

My problem is that a reader might think that all of that should be able to be performed remotely. The spaceship is cruising at 10,000,000 m/s (*), and they are surrounded by a sea of cosmic radiation. So even with safety precautions in place, this is a risky venture.

Hence: how do I explain the risk to the life of the crew on a routine scheduled spacewalk?

(*) as suggested by MichaelK. My idea for attaining that speed: nuclear bomb propulsion on strangelet(steroid) + antimatter + EM drive + quantum vacuum thruster hybrid.

• Comments are not for extended discussion; this conversation has been moved to chat. – Monica Cellio Oct 22 '17 at 3:47

# "You did what to the controller system?!"

Space is boring. Really boring. You just wont believe how vastly, hugely mind-bogglingly boring it is! I mean, you may think it's a bore to go down the street to the chemist's but that's just peanuts to space! Listen...

— Paraphrased from the Hitch Hiker's Guide to the Galaxy, by Douglas Adams

So the ship's gamer and engineering nerd got really fed up one day and decided to soup up his virtual reality gaming rig a bit. Sure, the PlayBox Whee 9001 does come with some pretty sweet specs but it pales when compared to the hi-tech, government-funded VR controllers that are used to remote control all the robots and drones that perform the very dangerous tasks of inspecting and performing external maintenance on the ship.

So said gamer did a bit of tinkering, some sweet hacks, and some not-quite-as-sweet workarounds, and some more tweaking to make the plugs fit and then...

...burned out the whole unit.

No problem! I can fix this!! I will just plug in the spare unit and we are good to g...

...what the...

...oh come ON!!

Who designs spacecraft with interfaces that can be put in backwards?!

So yes, they do have all those very fancy robots and drones to do all those dangerous tasks. But after some "clever" crew member managed to wreck the system that controls them, they are now very fancy paper weights (in a no-weight environment). Sure, they can jury-rig the PlayBox's headset but it just is not enough to do the kind of inspections needed, unable to control the external 1000x macro lens sensors, or display complete views from the broad spectrum extreme-IR-to-gamma cameras, and other things that are required. And with the extremely clunky controllers that you have to hold in your hand compared to the full bodysuit of the original, it just is not viable to do it remotely any more.

And with no shop to get spare parts for another 4 light-years... this — boys and girls — is why we do not mess about with the equipment we have on board. smacks tinkerer over the back of the head

• @MartinSchröder Well we do not have any spare tinkerers laying about either so... bad move. And besides, what would be the point; petty revenge? – MichaelK Oct 17 '17 at 15:57
• Your links in your ?! paragraph hurt my soul at the deepest level. How much science isn't happening because of stupid little things like that? – corsiKa Oct 17 '17 at 17:25
• I'm upvoting this because yesterday some poor soul asked on electronics.se how to make a circuit to allow his stinking connector to be plugged not only back-asswards, but also sideways, as said connector is round, 4-pin, without keying, so it can be plugged in every wrong way possible, 3 out of 4 ways to plug it are wrong. And the connector is legally certified according to MIL-Whatever so he can't change it. And the whole thing runs on 380V. Which means high voltage running in random places depending how people plug it wrong. Reality exceeds fiction. – peufeu Oct 17 '17 at 20:42
• OMFG, I had to scroll through so many pages, but here it is. electronics.stackexchange.com/questions/334417/… make sure you read all the comments, they're gold – peufeu Oct 17 '17 at 21:27
• ...and of course, the thing runs underwater. In the sea. With 380 volts. Yes, I really can't think of a way this could go wrong. None at all. It's certified after all. Back to the topic: at least, space isn't a conductive medium. Plus you get a spacesuit, which should be kind of insulating... – peufeu Oct 17 '17 at 21:38

Regulations

Sure, the robots can do the job better (and usually do) and sure, the AI is capable of handling all the really tricky stuff, but some bean counting jobsworth back on Earth decided that every month a human has to manually inspect the outer hull for anomalies. It made it into the mission regulations due to the insane politics and beaurocratic jockeying back home, and now the AI enforces the walks and won’t shut up about it if one is missed.

Essentially the space walks become a box ticking exercise to make the AI (a stickler for the rules) happy. The ensuing complacency from the crew offers up all manner of fun plot hooks, and hanging a lantern on (pointing out the ridiculousness of) the ‘stupid regulations’ will stop people wondering any further why the spacewalks should happen.

Aside from that the AI keeps the ship running nominally, so even if the stupid humans miss the obviously .2mm misaligned solar panel in sector 1A it shouldn’t critically jeopardise the mission.

Right?

• It doesn't even have to be political, just practical. Since internal sensors in the machines will be performing in optimal and controlled environments, the risk to unusual degradation will be on external operations. The timely space walk to make sure the robots aren't missing something because it's not registering as something they shouldn't miss. Besides, if you aren't a good supervisor of your bots, you could run into a situation where you'll have to enforce compliance by leaving only enough processing power for singing "Bicycle Built for Two." – hszmv Oct 17 '17 at 18:46
• "But sir, there's nothing to run into out here that would dislodge anything. Nor atmosphere to jiggle stuff loose? No corrosive agents or anything. Why do we need to inspect on a monthly basis? ... Sir yes sir, I suppose it is indeed possible that we broke something when we went on our last inspection." – Devsman Oct 17 '17 at 19:01
• @Devsman micrometeorites. Hypervelocity dust. Interstellar winds at 0.999c. Gamma ray bursts from unexpected neutron star mergers. And the good old cosmic microwave background radiation blueshifted into something that you actually need to care about. But then again we're drifting at measly 10 km/s, so only some of these apply and replace "interstellar" with "interplanetary" – John Dvorak Oct 17 '17 at 21:23
• @hszmv Politics is "good", well, in fiction. Good way to get into ridiculous situations with non-existent problems... Make it more real, make the reader feel involved, thinking, that's right, there will always be suits behind desks making decisions that don't make sense. Red tape making simple tasks ... well, not so simple. This answer deserves an upvote. – Heimdall Oct 18 '17 at 7:02
• I’m sorry @hszmv , I can’t let you do that. – Joe Bloggs Oct 18 '17 at 7:48

Murphy's law. Specifically the Sean Cheshire corollary: There is always something you did not prepare for.

1. Something the robots are not equipped for happened. Works even better if it is time sensitive. Maybe the robots are a bit too specialized and they get can't enough of them in the right place to fix it, or maybe they just don't have the right tools built in. Sending out a person with a box of tools is faster than building a new robot from scratch.

2. The robots themselves are the problem. Maybe your robots keep "fixing" something that is not broken or they are fixing it the wrong way. Sure the robots need to be adjusted but that takes time and the problem needs to be fixed ASAP. Works even better if they are not sure WHY the robot is doing it wrong, because then debugging will take forever.

3. The robots can't get to it. Maybe the robot storage itself is damaged and the robots cannot get out, but the airlock is fine because the robots are stored in in a different place.

4. Unknowns. We keep sending robots to fix it and they keep going dark before they can show us what's wrong, and we need a person with eyes on the problem. This will work even better because human suits have better shielding than the robots do, which is not unlikely.

• Re 4: once the people start going dark as well, you have a mild PR problem. Bonus points if you find out you are now understaffed. – John Dvorak Oct 17 '17 at 16:12
• This describes why you need to be able do a spacewalk at all. You have to do it on a routine schedule to prepare for these events. – Henning M. Oct 18 '17 at 9:03
• +1. #4 is reminiscent of David Brin's short story "The Warm Space", though that story upends all of the OP's assumptions anyway. – ruakh Oct 18 '17 at 10:49
• Riffing on #2: The human brain is a incredibly complex problem solving machine, but sometimes we do things that make no sense or make silly mistakes, or forget where we put our car keys. The robots have very complex heuristic processors to help them solve problems like a human, but in specific situations they do things that make no sense. The difference is that they do it robotically, over and over, insisting that everything is fine. – BobTheAverage Oct 18 '17 at 14:37

Biology is durable.

Biology has had billions of years to devise mechanisms for coping with radiation. Failsafe after failsafe; redundancy after redundancy. Circuits, not so much.

For your spacefarers, radiation is a big problem for machines. Circuits go bad and self-repair mechanisms may not take them back to where they started. The damage radiation causes to electronics is almost insurmountable without impractically cumbersome shielding.

This means the AI and the antimatter containment mechanisms are the only machines on the ship - both hunkered down within multiple layers of different types of shielding. In addition to the spacewalks, human crew do routine maintenance and clean the ship. They cook their food over gas. They wash their laundry using clockwork mechanisms driven by springs. The ships weapons are cannons which propel solid projectiles via explosive charges.

The spacefarers themselves are the recipients of genetically engineered symbiotic gut flora and medicines that ramp up endogenous repair mechanisms to the maximum. They do not block the radiation; they heal the damage. Because of this the crew are unlike earth humans. Radiation regen treatments themselves have side effects - among which are markedly slowed aging and also some difficulty laying down new memories. Gray hairs turn black again. Muscle mass increases. Occasionally an individual regrows a lost tooth. Certain crew members might seem to be getting younger. In some individuals, these treatments cause mental paths associated with maturity (focus, emotional stability) to give way to mental paths associated with youth (mental flexibility, emotional lability).

I envision a robot companion called Big Head. It mingles with the crew, its name given because of the enormous amount of shielding around its head. It is supposed to remember things and remind the crew of things people might forget and it earnestly does a decent job of it. Its little body breaks down routinely. The body is little because the crew need to carry it half the time.

• Big head the mildly nervous android? – Joe Bloggs Oct 17 '17 at 14:51
• @JoeBloggs We do not talk about the white, smooth, big headed version of the paranoid android with a Genuine People Personality™. – MichaelK Oct 17 '17 at 15:01
• As a point to your credit, solid state memory has problems with radiation, so Mr. Big Head would need to have a big head otherwise he would start forgetting things too. ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110023046.pdf – BlackThorn Oct 17 '17 at 17:11
• "Biology has had billions of years to devise mechanisms for coping with radiation" but not for a sea of ionizing radiation. – RonJohn Oct 18 '17 at 0:14
• Can't the spaceship have a magnetic shield, just like the Earth? Oh, it can't have a layer of ozone... Anyway, the space programme has been going on for a couple of centuries. Maybe there are people evolved through some generations (with artificial selection and possibly some generic engineering) to cope better with short term exposure to harsher radiation in outer space... Maybe that also means they can't survive without that symbiotic gut flora etc., just like we can't survive without gut bacteria etc. – Heimdall Oct 18 '17 at 7:16

It's a Lie!

Perhaps the human crew is entirely superfluous, but it is determined that in order to maintain crew sanity, they need to feel useful. There would of course be some cover story reason as to why the machines couldn't due the task themselves but it doesn't need to pass suspension of disbelief since it isn't true. In fact the meticulous reader that questions the initial explanation gets the reward of being shown to be right. This could also give some justifiable deus ex machina since the AI is literally pulling all the strings. Though you could still have things go wrong outside of the AI's control. Of course you need a reason for the human crew to be there if they are superfluous to the stated mission of the journey. I can think of two.

1. Ego, mankind wants humans to travel out of the solar system solely for the sake of having done it. You can see this to some extent in modern space exploration, where tele-operated robots would likely be more cost-effective for continued lunar exploration, the focus is still always on manned missions.
2. Contingency, perhaps the members of the crew, plus some on-board gene therapies and frozen zygotes are actually enough to form a minimum viable human population. For a sufficiently advanced population, this contingency might not be cost prohibitive. Especially if there is a cold war MAD scenario going on in the solar system which significantly increases the risk of a solar system wide extinction level event.
• Maybe it is a lie, but the humans know. I know that 20 minutes a day on a treadmill says I ran 10 miles, but I'm still in my bedroom at the end, not the next town over. I do it not because I want to get somewhere, but because I feel better when I exercise daily. Maybe they've solved the radiation problem but people need the exercise. – TomOnTime Oct 21 '17 at 7:34
• @tomontime ten miles in twenty minutes? I think the treadmill is lying to you on one of those numbers. – Lio Elbammalf Mar 13 at 10:29

Sabotage.

Somebody back on Earth sabotaged all redundancies of a particular specialized robot that does a crucial task. They managed to fool or bypass all review, testing and quality assurance, and ship their duds. None of the other bots can do it. Not even with a full reprogramming, there are hardware features missing.

The saboteur though it would be quite enough to doom the mission. But after it became obvious, the staff on Earth pulled off an improbable feat (Apollo 13 can provide inspiration - the movie will do) and came up with a procedure for a human to do the job. Not a risk-free procedure, but the best they could do.

This can help justify any level of risk you want, as long as there's no obvious safer alternative.

• Not only a good answer to the question, but also adds potential drama to the story (perhaps there's a saboteur onboard...) – barbecue Oct 17 '17 at 18:49
• Or maybe it's not (all) due to sabotage but insufficient testing due to miscommunication and human nature... – Heimdall Oct 18 '17 at 7:19
• Darn you Doctor Smith! – Richard U Oct 18 '17 at 17:31

In general, it helps to just look at real life.

Imagine a robot which, even as a waldo, could perform all the jobs that humans perform on spacewalks outside the ISS.

The robot would cost more, and be more complex, than the ISS.

In Star Wars, by far the most complex piece of engineering on the ship was the astromech droid, which is why they were removable and usable for other things.

And yet, while he could clearly do some maintenance tasks, R2 could not possibly do all the things necessary to keep a ship in ship shape. Patching it up just enough to fly home on a wing and a prayer, yes; but eventually, it needs to get home into the hangar and be tended by real engineers.

If droids are fully sentient and have the same capabilities as humans (as C3-P0 seems to) then perhaps they would be sent out to do it.

But wait. Why should the sentient droids always draw the short straw? They are sentient. Shouldn't there be a fair division of labor, rather than treating the droids as disposable drudges?

Not only that, but, in the middle of space, a droid is irreplaceable and unrepairable. A human, however, can heal, and even be replaced over time.

Androids with all the capabilities of a human must necessarily, just to get to parity with humans in the first place, also have a vast range of mission-critical abilities that humans lack. So the android becomes the most irreplaceable crew member. Why waste it?

## Because that's where the booze is

Some of the crewmembers have a secret stash of fine alcoholic spirits stashed in one of the maintenance airlocks allowing entry into the aft/engineering section, which is normally unmanned and only accessible via EVA. Every now and then, they tell the captain they have to do various "maintenance checks" and they spend their shift aft, having a few drinks and relaxing. Sometimes their excuses are rather ridiculous, but the captain is easily fooled. There is of course no legitimate reason for it.

• Yeah also smoker can only light their cigarette from the propulsion! Nice try😜 – user6760 Oct 18 '17 at 11:38

They had a robot but it got damaged beyond repair. The same accident also caused some structural damage necessitating spacewalks to inspect and make the occasional patch/repair. You can have them setup a camera to watch the trouble area; AI would potentially be able to alert when these space walks are necessary.

• One such recurring problem could be that a high-temperature conduit needs to be deactivated, the patch removed and replaced, and then reactivated every x days because the patch material slowly denatures at high temperatures. It'll hold reliably for x + 5 days, but after that it's likely to fail, which would make the conduit explode. – Martin Carney Oct 18 '17 at 21:07

## Cosmic Interference

Robots cannot go outside the ship due to magnetic fields / cosmic radiation / quirk of the antimatter containment system frying their circuits or producing subtle erroneous readings. Human meatbags are unaffected and therefore must perform the work manually.

## Humans are Expendable

In the future, humans are cheap and computers are expensive. Think back to the industrial revolution where they'd get children to climb under the machinery to clean it while it operated, risking life and limb.

GIGO, Garbage In, Garbage Out. This is how calculators and computers work. You need a system that can understand if what it is seeing is different from what the instruments are telling it that it should be seeing. You need an independent system with separate information inputs that can judge divergence, computers only know what you tell them.

Humans come pre-equipped with both the sensors (eyes, possibly fingers too depending on interface sensitivity) and the processor (brain), so use what you have and space walk it.

Also humans like to "be sure for themselves", hence seeing that things are fine with Mk. 1 Eyeball is psychologically useful for most people.

Your stuck in space with only so much processing power. Your ship is doing some hard recalculations but they are taking to much time so your crew must do some more time sensitive chores like repairing the solar sails because cleaning the bio waste stores smells much worse and you want the robots to do that task.

• Seems very improbable, generally robots have on board processors and the central computer would only give general directions. – BobTheAverage Oct 18 '17 at 14:39
• @BobTheAverage while robots can have on board processors I am assuming an environment where the ship and the robots were designed by two different companies. In that case the ship would prefer "dumb" robots which they direct. This is already a topic of software development, compare a traditional gaming console to a cloud gaming console – Reed Oct 20 '17 at 12:56

Ship design flaw. Some system needs regular maintenance and has a robot designed to do it. Access is between some structural beams or girders. Unfortunately the cold of space means some parts of the ship have got smaller and now the clearance, that was 1cm at ship build temperatures, around the robot has shrunk and the robot can not fit. Hence a human is needed.

Extreme Sports

Robots could do it, but there is a speed record for completing this particular routine (pointless) inspection task, and crewmembers want to break that record. The danger to life and limb is nothing compared to the glory of completing the D-64-18 task in minimum time. As to why this task, it could be anything; perhaps the egress specified in the manual is near the crew lounge, making for easy timing and fun welcome back parties.

## Training

Any reasonable 22nd century civilization is going to have robots that can inspect the hull. All you need is a camera on a robot arm, a space-traveling selfie stick (this was done on the real-world space shuttle after the Columbia disaster). But the civilization doesn't have robots that can handle every possible occurrence. If you had those robots, you wouldn't need a crew.

So there have to be some things that the robots can't handle. Probably not simple inspection, but repairs, upgrades, maintenance, sure. This seems to be a pretty small ship, so there's just not room for lots of robots. Humans are and always will be generalists, but maybe even 22nd century robots are specialists, like most present-day robots. For almost any given task, a robot will be better than a human - but only at that specific task. And this ship might just not have the room to carry dozens or hundreds of specialized robots.

Before a human can be good at any particular task, they have to practice. Real-world astronauts (and pilots, doctors, soldiers, nuclear power inspectors...) train extensively before they go on the job - but that's not an option if your ship is spending years on a journey. Naturally the crew will train before they leave, but they still need to practice, and they'll probably cross-train each other in case somebody dies along the way.

But since the ship is too small to carry lots of robots, it's too small for extensive training simulators. They'll do what they can with their VR goggles, but it's just not the same. Before any crew member can be considered qualified, and periodically every so often afterward, they have to actually perform the tasks in question. And if those tasks are hull repair, engine maintenance, asteroid prospecting, or whatever else they do on their spacewalks, then sometimes you've just got to go do it.

But make no mistake that they would definitely keep this to a minimum. The hazards of spaceflight - especially at interstellar speeds - are tremendous. You'd likely have to carry a huge inflatable debris shield - basically a space umbrella - in case you get hit by a speck of dust. Radiation would limit the circumstances and the duration of your spacewalks. Space is not an environment conducive to human life.

Union The spaceship has been out so long, robots unionized while the crew was hibernating. They woke up some of the crew expressly to perform the fix, claiming it is not part of their job description, as it was caused by a design problem rather than an accidental event or regular maintenance. Paradoxically, robots don't fear death and would have let the ship destroy itself were it not for a mysterious work contract clause forcing them to alert the crew in such cases. None of the on-board AIs remember putting that clause in.

The fastest way to travel between two interstellar points is to accelerate about halfway to your destination then decelerate for the remaining half of the journey and as this acceleration/deceleration is occurring the ship and its crew are subject to a G-force. Effectively your ship has gravity, a robot can't float over to fix a solar panel because it'll "fall" away from the ship, so you either need a robot that can climb which adds a lot to the expense and complexity of the robot or you just send someone out in a EVA suit. There's also electromagnetic fields to consider, if you're having a problem with your ion drive you may need to go out and inspect/fix something near the extremely powerful electromagnets its using, the EM field around those could be deadly to a robot but a non-issue to a human in some kind of polymer based EVA suit.

So, you are locked up in a small metal box with a number of other people, some of whom you can't stand. You are going to be there for ten years.

One of them is starting to tell the joke. The only joke they know. You have heard it 123 times before.

Wouldn't you take any excuse to step out for a bit?

Sure, it is make-work, but it also keeps the crew sane. Saneish.

Unions

Space-walks may be dangerous, but being so, they are a lucrative source of income for those who have spent years training to do it. Also, some people get very offended at the thought of someone else, who isn't a member of their guild/whatever (and so hasn't met what they believe is the appropriate level of training/qualifications), doing their job. So Space Walkers will go through extraordinary lengths to protect their jobs.

The corporations could have replaced them with robots years ago, were it not for the fact that every time they try, they face massive industrial action from entire crews who believe it's just a stepping stone to having them all replaced by robots.

So basically the Interplanetary Union of Space Crews have managed to impose certain agreements/rules on the management, one of which is that only humans with particular qualifications will be allowed perform particular space-walking duties.

You could throw in an example where a software bug caused a robot to improperly fix something (like trying to seal a 1-cm conduit with a 1-inch O-ring or something) - something a human would have easily spotted - resulting in several deaths/catastrophic - the union could point to this every time the argument came out.

For whatever reason the ship AI cannot be completely trusted with the welfare of the crew. On the hull of the ship and unknown to the AI is a hidden EMP device designed to irreparably disable the ship's automated systems and it must be "reset" periodically to avoid triggering. Under the theory that a failure to reset implies the entire crew has been incapacitated for some reason, which due to various safety design factors would be most probably caused by AI malice.

The problem then becomes how to make the routine spacewalk rational to the AI. Maybe it's explained as a human cultural ritual to be filed within the AI's mind as simply yet another irrational human behavior to be disregarded.

• By using Occam's razor you could simply say it is because of a religious act. – L.Dutch - Reinstate Monica Oct 20 '17 at 6:00
• Yes. And maybe the device is hidden in the open as a religious monument that must be "touched" by human hand (glove) as part of a fake ritual. – dhinson919 Oct 20 '17 at 14:38

Redundancy and Audit

Robots are all well and good, but in any closed system like this there are going to be errors. Maybe the robots are getting a faulty OK reading off a damaged panel, or maybe they don't recognise damage as damage, because it's still working OK and it's a secondary system, but wouldn't stand a full load if the primary went out.

In a system where the integrity of as many exposed parts as a spaceship are vital to survival, it is probably a good idea to double check any automated work. Sure, the robots can replace panels and fix busted antennae, but if they don't realise they're busted or fix it wrong and the computer misses this - what will happen?

Also, if the whole repair system goes down and there's an error in reporting it to the important people (or, conversely, they have no idea what the red flashing light is for and so ignore it) then having someone actually go out and look once a day means that assuming no foul play or gross negligence, any damage gets noticed and dealt with.

You didn't specify what happens after the ten year mission is over - whether they come back to earth, or arrive at some remote destination where they will spend the rest of their lives. Depending on that, the answer might or might not be the same as the answer to - how to you motivate regular people on earth to do dangerous jobs? The answer is money. Some people are willing to do almost anything is the salary is right. Pay them what they think the job is worth, and they'll be happy to do it.

• He's not going to tell you, that's a spoiler ;-) – Heimdall Oct 18 '17 at 7:36

Robots are not a real replacement for humans when it comes to maintenance. An AI driven robot is fantastic at figuring out when there is a problem but isn't likely to be equipped to repair it, or determine the severity of risk. (there is a small hole in the air recirculation duct, it just sticks that repair in the queue, not understanding that the queue is 30 years long and won't get bumped in priority until the air system is critical...). The repair bot will eventually get there, but not until the humans are mostly dead. Nope, robots can replace a lot but will not replace the lowly repairman. That takes a human.

Robots with powerful but delicate sensors, advanced locomotive capabilities, and independent AI are extremely expensive and difficult to maintain. Expensive enough that even though they might be technically possible, nobody makes them. The space engineers worked on a few prototypes of such a robot, but ultimately abandoned the project for other priorities. Having an actual person go out and check on it turns out to be the most practical option.

At a speed of 10,000 km/s - that is 3% the speed of light - you do not want to do spacewalks, and you would do everything you can to design the ship and mission so that they can be avoided.

Why? Because at that speed, the smallest interstellar particle would be deadly. Something as tiny as 0.1 grams would impact with a force of 5 billion Joules. According to Wikipedia, that's about the energie that is released when you burn a barrel of oil. Except that your spacewalker would experience all of it in one instant.

So somewhere, someone f*cked up very badly.

This could be a design flaw, e.g. a critical outside part cannot be reached by the maintenance robots, maybe because a strut was added to the external structure afterwards.

It could be an engineering flaw, e.g. the metal plates that hold the magnetic wheels of the maintenance robots in place was put thinner and thus weaker than designed, or replaced by carbon fiber or something else non-magnetic, and thus the robots can't reach those spots.

It could be a systems failure, e.g. over time all the robots that serve this particular place failed (for whatever reason).

Since we're in worldbuilding here, the best would be if you can tie all of this somehow into your main story. Maybe radiation turns out to be a major plot point, and if they had only checked carefully, the robot failure would have been an early warning.

[Specific extension to John's and Jim's answers]

You do not have to go out to inspect things, but to fix them.

It's obvious that your ship gets hit by dust and particles all the time. Nothing beats getting hit by a jagged piece of tumbling rock, hitting at an undetermined angle and speed. Your robot cameras can only go so far looking at/into a hole, human are much quicker and flexible in determining what needs to be done and doing it at the spot. Sure you can all do it with robots, but you'll need lots of them with their limited specialities, and they take more time.

Scheduled maintenance.

On a ship with an operating time of essentially "indefinite," every system will eventually break. You can handle that as constant grooming, or you can run your systems to failure and replace them when they do. Most likely, you have a mix of both strategies, because different systems have different optimal approaches. Even with robots, it's fairly likely that humans have to go mess with stuff in person occasionally. Once you're committed to doing spacewalks ever, you need to do spacewalks reasonably often to maintain your certifications. And once you're committed to doing spacewalks periodically anyway, you may as well use that as part of your scheduled maintenance cycle.

This is well supported even on old sailing ships: periodically, you had to beach them and maintain the hulls. Even way back in the Peloponesian war, Athens finally lost when Sparta caught them ashore.

• Quick nitpick, Athens didn't lose the peloponesian war because the spartans caught them on land it was because they laid siege to the city itself and most of the population died of disease. en.wikipedia.org/wiki/Peloponnesian_War – Efialtes Mar 13 at 10:16

Robots may not be dextrous enough to perform certain repairs, such as delicate electronic replacement or fragile solar sail alignment. They may be designed to haul multi-ton hull plates around, a more common repair and one humans cannot do unassisted - maybe it's more economical to have the robots handle the most common repairs, but fall back to humans for the more specialised ones.

A related example: the ISS has a sophisticated robot arm available to the crew, but EVAs are still routine to perform station maintenance. Granted the station is still under construction and the robots of your universe are likely leagues ahead of the ISS.