We designate as a natural satellite any body that is located in orbit around a planet of greater mass, which exerts a gravitational attraction on the satellite. To consider an object as a natural satellite of a planet, it is considered as a basic criterion that the center of mass is within the host object (the planet).
At present, the Solar System is made up of 8 planets, 5 recognized dwarf planets, comets, asteroids, and at least 146 natural planetary satellites. The best known is that of the Earth, called simply "moon", which is the only one that the planet has. The inner or terrestrial planets have few or no satellites, and in contrast, the other planets possess several satellites which, after their discovery, were designated by different names, some of which come from Greek and Roman mythologies.
Natural satellites remain in orbit around a planet because they are at an equilibrium point around it, that is, they balance the centrifugal forces (which tend to move a body away from the center of rotation) and the centripetal force (which tends to drag to the center). The dynamics of how this happens is by Newton's laws of celestial mechanics, where natural satellites are not actually "suspended" in space around a planet, but are continually "falling" on it, only at such a speed high that is the same that "descends" due to the curvature of the planet.
As we noted earlier, planet Earth has only one satellite, the Moon. In contrast, Mars has two, Phobos and Deimos. Jupiter is the fifth planet in the solar system and in its orbit there are a total of 64 satellites (Callisto, Io, Ganymede and Europa are the best known). With respect to Uranus, its satellites are Titania, Ariel, Miranda, Oberón and Umbriel.
The term natural satellite is opposed to the artificial satellite, the latter is an object that revolves around the Earth, the Moon or some planets and that has been manufactured by man. Artificial satellites are spacecraft manufactured on Earth and sent on a rocket that sends a payload into outer space. Artificial satellites can orbit moons, comets, asteroids, planets, stars, or even galaxies. After their lifespan, artificial satellites can remain in orbit as space debris.
