Tenemos varios satelites naturales, lo que pasa es que son algo pequeños y no se denominan lunas 
Me llamó la atención el comentario de ken 2 y me puse a buscar info de que se considera como "luna". Pues como ahorita estoy algo ocupado la fuente más rapida fue wikipedia
The definition of a moon
Comparison of Earth and the Moon
There is not an established lower limit on what is considered a moon. Every body with an identified orbit, some as small as a kilometer across, has been identified as a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets. Small asteroid moons, such as Dactyl, have also been called moonlets.
The upper limit is also vague. When the masses of two orbiting bodies are similar enough that one cannot be said to orbit the other[citation needed], they are described as a double body rather than primary and satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon. Some authors consider the Pluto-Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether the barycentre is below the surface of the larger body, though this is somewhat arbitrary, as it relies on distance as well as relative mass.
http://en.wikipedia.org/wiki/Natural_satelliteA pesar de que no encuentro un concepto bien claro, veo una tendencia que cuando el satelite natural es muy pequeño entonces se le denomina en Ingles "moonlet" o en su defecto simplemente así "satelite natural", pero no luna.
Incluso en una de la fuentes que cita el que creó el tema dicen que es un poco exagerado denominarle luna.
Given that these tiny captured orbitals are only a meter or two in diameter, it may seem a stretch to officially call them "moons" --
Para mi que no es una luna solo es un pequeño satelite natural orbitando la tierra
