As I was reading the Cubesat standard, one of requirements is low outgassing "to prevent contamination of other spacecraft". It also gives a link to a NASA-approved list of low outgassing materials.
Is this an actual problem that can be a hurdle and has caused problems in the past, or is it a mere formality like "don't send stuff that will boil off in vacuum, spraying everything around with droplets"?
Its not a mere formality it is unfortunately one of those things that makes satellites expensive. However I wouldn't be surprised if all kinds of nominally incompatible materials have already been exposed in cubesat missions. EDIT This report might be useful background: NASA contamination control report 1996
It could in principle affect other parts of the cubesat or in the shorter term other cubesats in the dispenser or other systems outside a cubesat dispenser. A consequence of the dispenser being an enclosure is that there will still be some leak paths and these become the main out-gassing vent routes. The time for this to occur might limit the worst of the hazard to the host cubesat.
The mechanisms of adhesion to other surfaces are varied and include condensation on cool surfaces and also UV ionisation and electro-static return to a surface not on a direct flight path. EDIT The link I added above has a short section on this, section 2.3.1.2.2.2, figure reproduced below:
Note however the reference goes on to say that this particular mechanism is believed to be much more of a problem at GEO altitudes because the plasma screening distance is much greater than at LEO.
Outgassing products can be water vapour, most solid objects have a dose of trapped moisture, or something more destructive. See this article http://space.skyrocket.de/doc_sat/hs-702.htm regarding ouggassing from solar arrays. It says:
The first version of the 702 used solar arrays with concentrators. These concentrators tended to early fogging, as due to an inherent design flaw the outgassing of the solar cells was higher than expected. This fogging lead to much reduced lifetime.
The latter satellites all weighed several tonnes and I suspect cost over a billion $ between them.
There are a few recorded incidents of spacecraft contamination caused by outgassing.
On Cassini, some haze was noted in test photographs after launch; this was thought to be due to some outgassing material near the camera itself. It was removed after several attempts using the camera's heaters:
Cassini Imaging Science report, 2004, p.475-476.
On Stardust, the navigation camera became contaminated after launch. Again spacecraft heaters were used to eventually clear the problem:
NASA page on Stardust's cameras
Also the Stardust "Lessons Learned" report mentions this and an additional problem with sample collection:
...However, despite these precautions the Stardust spacecraft outgassing was sufficient to degrade camera operations, and the aerogel capture media was significantly contaminated during manufacture. We also never completely solved the problem of defining useful limits for organic contaminants of spacecraft hardware, which haunts us as we rather unexpectedly captured primitive cometary organics.
How serious a problem can outgassing of satellites be?
Serious.
Outgassing is discussed in the following questions and/or their answers.
I'm going to propose that this is not a link-only answer as these are not external links. To try to summarize all these different serious problems related to outgassing would be a several thousand word copy/paste montage that would not be updated when information in all these other posts may be revised, updated, or new answers emerge.
So I'm going to propose that the sheer size of this list of other posts in this site as support that outgassing can be a serious problem.
