It's bloody ace when you can see the hardware in a space photo it really reminds you that the probe is actually there! This is from Rosetta's fly by of Mars.

a) As has been suggested, interplanetary space is pretty empty. Very little dust around to adhere to the spacecraft. Certainly anything that keeps it’s distance from planetary bodies would not have anything visible to the naked eye. However, if you have a spacecraft touching down on the moon or an asteroid for instance, things can get pretty filthy (not in a good way). A key problem is charging. Spacecraft and planetary surfaces can become charged in an absolute sense and so when a lander touches down on the surface of the moon for instance there can be a significant electrostatic attraction and so fine, non-conductive dust can be attracted to the spacecraft and stick to the surface. The Apollo missions had a nightmare with it getting everywhere. You’re probably also familiar with the coatings of dust the Mars rovers build up after a time on their solar panels. For missions like Voyager, you will encounter micrometeorite impacts from dust, usually around planetary bodies which will somewhat pockmark the surfaces. The Cassini probe had to undergo a manoeuvre when it passed through Saturns rings to fly with the high gain antenna dish as a shield due to the relatively high density of material there. Spacecraft probably actually end up looking worse in low earth orbit where in addition to sub-millimetre space debris and micrometeorites there is also a problem with atomic oxygen which is highly oxidising (funnily enough!) and causes significant degradation of spacecraft surfaces in the ram direction. b) For radiation, no need for any special precautions. With spacecraft design in interplanetary space you’re primarily concerned with protons from solar flare events and galactic cosmic rays (protons/ heavy ions). Both are highly penetrating due to their energy and above a certain energy can’t really be shielded against. Around some planets there are trapped radiation belts of protons and electrons. Jupiter is a particularly severe case of this and the electrons leaking from Jupiter’s belts can be measured in high earth orbit! When the particles strike the spacecraft they release their energy into the material as a modification of charge states and can trigger a nuclear process (not a nuclear chain reaction!). The induced activity would be very low even for a highly irradiated spacecraft. Funnily enough I had a go at a paper exercise to see if it was viable to launch an RTG containing a semi-stable heavy isotope which could be induced to alpha emission under the proton exposure from the Earth’s radiation belt to improve safety and energy yield. I couldn’t get the particle fluxes and cross-sections to work. Orders of magnitude off a useful power output! Our main concern with nuclear contamination on earth is from particulates or deposition of the material from the reactor, bomb, etc. itself rather than induced radioactivity. Materials from reactor cores are different story…

Source: Space scientist/engineer with PhD in space radiation

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