A travelling space fleet found an old militarized asteroid, with weaponry, space suits, munitions, and food which had been sitting there for 5 years. Is it still usable and safe?
In general: Yes
En Detail: It depends on a lot of variables
Basically there are four major influences that take place:
- Temperature
- Absolute
- Temperature difference
- Change over time
- Radiation
- Soft (Light)
- Hard (all forms)
- Atmosphere (or lack thereof)
- Impacts of other bodies
Temperature
Like on earth storing stuff to hot or too cold might destroy it. This might be due chemical effects (breaking up of molecules), physical effects (breaking of crystal structure) or 'simple' mechanical effect due different rate of expansion/contraction of combined materials - like a recoil operation jamming when hot.
Equally damaging is temperature difference. With a gradient high enough even 'simple' materials - like a steel plate - will degenerate over time and ultimately break.
Cyclic change of temperature is most devastating on the long run, as it combines all effects over and over again. So even small ones will add up.
As a result, no matter how advanced a technology is, it might be advisable to keepas much technology at controlled non changing temperature.
Radiation
Already soft radiation, like intense light will change composition of complex molecules - just think of a a newspaper in bright sunlight. Depending on distance from a star light may have a great influence on durability of the asteroid at whole and the station in particular.
Then there is hard radiation, no matter if Alpha, Beta or Gama. All will add corrosion to uncovered surfaces as well as breaking their structures, making them ultimate crumble.
As a result, no matter how advanced a technology is, it might be advisable to not expose their devices as little as possible, use cover wherever possible - much like gun ports on sailing ships.
Atmosphere (or lack thereof)
To start with, there is no real air tight system Any station not maintained (manual or automatic) will loose pressure. It might only be small, a few liter a day, but with ageing structural integrity it'll gets more and more. Most important for your setting might be that the station will for sure have a lower pressure than expected - beside the effect that it might have had a lower to start with, as saving on pressure is saving on energy and supplies.
Equally the composition of the atmosphere plays a role. If the station is mothballed, a standard breathing one (with Oxygen) might have been replaced by a nitrogen only to reduce oxidation effects. This in fact may be as well in non mothballed stat be used for some less frequented sections.
Last but not least, pressure also adds to disintegration. So a mothballed station might again have a way lower pressure.
Then there is vacuum. While it can be assumed that any technology able to maintain a travelling space fleet and asteroid station has sufficient means of lubrication and sealing for every day operation, this might not always be the same for long term situations.
As a result, no matter how advanced a technology is, it might be advisable to keep as much as possible stored 'inside' at controlled pressure and atmosphere.
Impacts of other bodies
Space is empty ... but at the same time filled with tiny bullets chasing you installations every tiny meteoroid will leave a scratch mark on exposed surfaces. They are the low sped high mass equivalents of radiation. One may be not of great influence, over time they sum up.
As a result, no matter how advanced a technology is, it might, once again, be advisable to store whatever needs to stay intact inside and covered.
Taking all of the above into account - and considering that we're talking about a civilisation capable to maintain a travelling fleet in space - it is safe to assume that the station will be mostly usable if build proper and left in a mothballed state.
Though, it still, I wouldn't assume that it's done with simply switching on power. After all, that usually won't work with a car left in a carport (but on earth) for five years. Even less for a space station. Depending on size it will take days to level temperature (and pressure) up. Just think of it, heat isn't distributed magically around the station but in pipes. They get leaky, clogged up or simply malfunction in some valve. Be prepared to have at least one guy called Mario.
Until that is working, you crew will have to wear space suits - maybe not heavy suits that let one survive in outer space, but at least light 'indoor' versions like the new Boing ones, able to keep pressure and regulate temperature.
And then there is the nuclear plant. They are like the wet dream of plumbers and function in no way like with the animation shown to general public (no matter if fission or fusion). It isn't connecting a backup battery and pulling some rods. They as well degenerate over time. Control rods in a fission reactor are consumables (and Tschenobyl happened in part due stuck rods).
And so on...
Bottom line:
Either make your civilisation so perfect and high above, that the station is fully functional and still operating (in mothballed mode) or be prepared to invest quite some time to get it up again.
Food on the other hand shouldn't be a big deal, if preserved proper, like canned in a container good for the (inside) environment, it will bear all the same "advantages" of durable food known form Earth :)