An artifact is an unique magic item of divine (or equivalent) power
Mechanically speaking, "artifact" is special category of magic item. In addition to being the absolute highest level of rarity, even rarer than "legendary", there are also special rules that apply to all artifacts, most of which are detailed in Chapter 7 of the DMG. The most relevant to this question are the rules for destroying an artifact:
An artifact must be destroyed in some special way. Otherwise, it is impervious to damage.
The special way to destroy a given artifact is detailed in its item description (though this information might not be known to the player characters). So, unless "throw it into a sphere of annihilation" just happens to be the special way to destroy a particular artifact, that artifact cannot be destroyed, or even damaged, by a sphere of annihilation nor any other mortal magic, no matter how powerful. This kind of exception is common for artifacts. For example, Antimagic Sphere suppresses all magical effects and spellcasting within its area, except for "those created by an artifact or a deity".
Lore-wise, most (all?) artifacts are items created directly by a deity or something of equivalent power, which explains why they are impervious to things that would suppress, damage, or destroy most magic spells and items. They are, in the most literal sense, "god-tier magic items".