Evaporation and sublimation into a vacuum is called outgassing. All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. In man-made systems, outgassing has the same effect as a leak and can limit the achievable vacuum. Outgassing products may condense on nearby colder surfaces, which can be troublesome if they <answer>obscure optical instruments</answer> or react with other materials. This is of great concern to space missions, where an obscured telescope or solar cell can ruin an expensive mission.
Why can outgassing products ruin a space mission?
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