Natural Satellites are celestial bodies that orbit a Planet or any other Celestial body and which are not Man-Made. The most well known Natural Satellite is the Earth's Moon. The term Moon is often synonymous to Natural Satellites as they are generally orbiting a Planet. Generally speaking, all masses that are part of the Solar System, including the Earth, are Satellites either of the Sun, or Satellites of those objects, such as the Moon. For an object to be considered as a Satellite - the centre of mass of the two objects has to be inside the primary object. 6 Planets of the Solar System i.e. Earth, Mars, Jupiter, Saturn, Uranus and Neptune have Natural Satellites. The Earth has 1 Satellite, Mars has 2, Jupiter has 63, Saturn has 62, Uranus has 27 and Neptune has 13 Natural Satellites. Other bodies in the Solar System like Dwarf Planets and Asteroids, also have Natural Satellites. As of July 2009, 336 bodies have been formally classified as Moons. These include 168 orbiting the 6 Planets, 6 orbiting 3 Dwarf Planets, 104 Asteroid Moons, and 58 Satellites of Trans-Neptunian objects.
Classification / Types of Natural Satellites
Regular Moons or Regular Natural Satellites - Regular Natural Satellites have Prograde orbits i.e. they orbit in the direction of their Planets' rotation and lie close to the plane of their equators. They are considered to have formed in orbit about their primary Planet. There are at least 55 Regular Satellites of the 8 Planets : 1 at Earth, 8 at Jupiter, 22 at Saturn , 18 at Uranus and 6 at Neptune.
Irregular Moons or Irregular Natural Satellites - Irregular Natural Satellites are those whose orbits can be Pro or Retrograde i.e. they orbit against the direction of their Planets' rotation and lie at extreme angles to their Planets' equators. Irregular Moons are probably Minor Planets that have been captured from surrounding space. Most irregular Moons are less than 10 kilometres (6.2 mi) in diameter.
Inner Moons or Inner Satellites - Inner Satellites are those Satellites which follow a Prograde, low inclination orbit inwards of the large Satellites of the parent Planet. They are considered to have been formed in situ at the same time as the coalescence of the original Planet. Inner Satellites can be distinguished from other Satellites by their proximity to the Parent Planet, their short orbital periods (usually under a day), their low mass, small size, and irregular shapes. There are about 30 Inner Satellites orbiting around all 4 of the Giant Planets namely : Jupiter, Saturn, Uranus and Neptune. The Inner Satellites are small in comparison with the major Moons of their respective Planets. The orbit of Inner Moons is synchronous with their rotation so that they only show one face toward their parent Planet. Their long axes are typically aligned to point towards their Planet. Inner Satellites of Jupiter are Metis, Adrastea, Amalthea, Thebe. Inner Satellites of Saturn are Pan, Daphnis, Atlas etc.
Characteristics and Physical Features of Natural Satellites
Regular Orbits - Satellite orbits are called Regular or Prograde, if they are in the same direction as the Planet's rotation otherwise they are called Irregular or Retrograde (The term irregular can also refer to the shape of a satellite). Most of the major Moons in the solar system have Regular orbits (Triton being the exception) while most of the small Moons have Irregular orbits.
Tidal Locking - The regular Natural Satellites in the Solar System are tidally locked to their primaries, meaning that the same side of the Moon always faces the Planet. The only known exception is Saturn's Moon Hyperion, which rotates chaotically because of the gravitational influence of Titan. In contrast, the Outer Moons of the Gas Giants (irregular satellites) are too far away to have become locked. For example, Jupiter's Moon Himalia, Saturn's Moon Phoebe, and Neptune's Moon Nereid have rotation period in the range of ten hours, while their orbital periods are hundreds of days.
Trojan Satellites - Two Moons are known to have small companions at their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions are called Trojan Moons, as their orbits are analogous to the Trojan Asteroids of Jupiter. The Trojan Moons are Telesto and Calypso, which are the leading and following companions respectively of Tethys; and Helene and Polydeuces, the leading and following companions of Dione.
Asteroid Satellites - The discovery of 243 Ida's Moon Dactyl in the early 1990s confirmed that some Asteroids have Moons; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are Double Asteroids with two comparably sized components.
Satellites of Satellites - No Moons of Moons or Satellites are known. In most cases, the tidal effects of the primary would make such a system unstable. However, calculations performed after the recent detection of a possible ring system around Saturn's Moon Rhea indicate that Rhean orbits would be stable. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with shepherd Moons.
Size - The largest Natural Satellite, Jupiter's Ganymede, is 3,268 mi (5,262 km) in diameter, and it and Saturn's Titan are both larger than the Planet Mercury. In comparison, some Satellites are quite small, e.g., Deimos, the outer satellite of Mars, is c.4 mi (6 km) in diameter.
Some of the Major Moons or Natural Satellites of the Solar System
Origin and Evolution of Natural Satellites
Natural Satellites orbiting relatively close to the Planet on Prograde orbits are generally considered to have been formed out of the same collapsing region of the Protoplanetary Disk that gave rise to its primary. In contrast, Irregular Satellites are thought to be captured Asteroids possibly further fragmented by collisions. The Earth-Moon and possibly Pluto-Charon systems are exceptions among large bodies in that they are believed to have originated by the collision of two large proto-planetary objects. The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting Moons. As opposed to Planetary-sized bodies, Asteroid Moons are thought to be commonly formed by this process. Several Moons are thought to be captured foreign objects, fragments of larger Moons shattered by large impacts, or a portion of the Planet itself blasted into orbit by a large impact.