Transcript: How large does a chunk of rock have to be to be called by a moon or a satellite? There is no simple answer to this question; it's largely a matter of definition. However, in general objects that are less than 10 kilometers across are too small to be called moons. Deimos, Mars' smaller of the two moons, is slightly larger than this number. All the moons between 10 and 100 kilometers in the solar system typically have names, and above 1,000 kilometers the moons are distinctive enough to have particular surface features and even atmospheres.