Because robots with enough finesse to do external repairs and robust enough to withstand external conditions are cost prohibitive

10 000 meters per second is kind of lame... that is not even escape velocity from Earth. Make it 10 000 kilometers per second and you got something. :) Second... as long as they do not run into anything, the speed does not really matter. They could be moving at 100 000 km/s second and not notice any difference from moving at 0 km/s. Yay Einstein and general relativity. :) That which causes problems in space is acceleration/deceleration, and especially if it is jerky, i.e. uneven.

You will never be able to avoid all questions from your audience in the sci fi genre. Look how many plot holes (sci holes?) they have in Star Trek or Star Wars that can be deconstructed by an 8 year old, 2 of the most successful franchises out there. I believe that pointing towards such a problem by listing the problems one might suspect and then having some bs excuse that isn't really true for why you do it anyways is counterproductive. Space walks are ok because they make the story interesting period. Anyways, consider economic reasons (machines are expensive) or broken robots

The inspections described are much simpler than what you're already describing the robots doing. You could handle these with a remote-control vehicle with a camera on it.

@bgvaughan: the entire point of the question is to find a reason why they would not do that.

Right. But it directly conflicts with the premise. If you've already got robots that do maintenance, then it does not makes sense to have humans risk their lives to do the kind of maintenance that would be the simplest to automate. At this point, all the answers involve changing the premise.

Sounds like thy might be in for a bit of a surprise, when they figure out it's not 10, but more like 250000 year trip.

They could be moving at 100 000 km/s second and not notice any difference from moving at 0 km/s Sure, but this stops being true if you add another several zeros. If you are going fast enough, radio and even the CMB will be hitting you like hard X-rays or gamma rays.

@Shane That is already one third of the speed of light, please don't add more zeros!

@Shane "That is already one third of the speed of light" well, at least it'll make "signed on a ten year space exploration trip" reasonable...

@JiK That had me literally laughing out loud. People are looking funny at me. :D

@user6760 Do note that a leisurely acceleration of 10 m/s per second (1g, Earth gravity equivalent) gains you nearly 1 million m/s per day. In a mere couple of months they would be travelling so fast they are going at noticeably relativistic speeds. So you do not need to go nuts on the propulsion... just assume they can sustain 1g, and then a 10 year hop to our nearest neighbouring star is an easy and comfortable outing.

10M m/s is not fast enough, though. 1/30 of light speed, and the nearest star (apart from Sun) is 4 light years away, so out would take more like 100 years to get there. More than 100M m/s would do though, 1/2.5 of light speed. Sustaining g acceleration would be a problem. The faster you go, the more energy it takes for the same speed increase (kinetic energy is proportional to speed squared). Not sure how that works with relativity where there's no difference between steady motion and being still... Another thing is that close to the speed of light the speeds don't add up.

@MichaelK: speed boost below relativistic is additives but above is multiplicative, so it would takes a tiny amount of impulse to increase 100m/s to 200m/s but insane amount just to increase 99,999,999m/s to 100,000,000m/s!

@Heimdall: see my comment to MichealK.

@user6760 Now you are winging it madly. If you have an engine with fixed thrust, once you reach 1/3 of the speed of light, the acceleration has only gone down to about 89% of what you have at non-relativistic speeds. The Lorentz Factor squares the fraction of light-speed so it does not really become bothersome until you get up to about 70% of light-speed. By then you will have lost only 50% of the acceleration. It is only after that that the relativistic part of the equation really shoots up.

The main question is, “Why are you sending twenty men and women to the nearest star system instead of just robots?”. If you find an answer for that, e.g. “there are jobs a robot can’t do, «insert example here»”, you have also the answer to why you might need humans to do spacewalks.

@Holger Yeah but the assumption of the question is that readers will think "Hm... robots should be able to do that sort of maintenance, especially considering the extreme risk involved". When something is dangerous it puts a strain on the willing suspension of disbelief to have a situation that forces people to do it. The reader will immediately ask "Why didn't they solve that problem?!". OP hopes to get a well founded "Well because..." or "They did, but..." that makes the reader go "Oh... ok then, I'll buy that!" as an answer.

Relativity isn't really that bad here - you can go up to 14% of the speed of light and only encounter a 1% mass increase - plug k=1.01 into v = c * sqrt(1 - (1 / k^2) ) and you get 0.14037c. 30% speed of light is about a 5% mass increase. So really, not so bad. CMB blueshifting on you might be a problem, but the mass isn't going to be THAT bad. Even doubling the mass only happens at .86c!!

To short for an answer so I'll comment it: someone's got to scrape the barnacles off the hull.

I think you have used the South African meaning of motivate (definition number 3 here), which means "to explain", which is unknown in the rest of the world. If you change it to "explain", your question would make a lot more sense to most English speakers.

@rjmunro: edited ;)