Non-renewable resources and artifacts are items that have a fixed number for each player and which can never be received again after the completing of a mission. Non-renewable resources should be treated with care, especially those that cannot be extracted by a Mylarai Building Extractor.
Despite their rarity, artifacts can be recovered from planets of other players, if of course they can be extracted with a Mylarai Building Extractor. Another way to circumvent the limit is to reset your account, thus having the chance to receive once more the items you may have been careless with the first time around. This also allows you to have double the planetary artifacts, as those used on planets will remain there, while you have the option of getting another "batch" of the same type of artifact that would otherwise be limited to you.
To make sure you don't lose useful artifacts, do not, under any circumstances, scrap items received from missions unless you know what you're doing.
Compare Renewable Resources.