In astronomy, an inner moon or inner natural satellite is a natural satellite following a prograde, low-inclination orbit inwards of the large satellites of the parent planet. They are generally thought to have been formed in situ at the same time as the coalescence of the original planet. Neptune's moons are an exception, as they are likely reaggregates of the pieces of the original bodies, which were disrupted after the capture of the large moon Triton. Inner satellites are distinguished from other regular satellites by their proximity to the parent planet, their short orbital periods (usually under a day), their low mass, small size, and irregular shapes.
Discovery
Thirty inner satellites are currently known, found orbiting around all four of the giant planets (Jupiter, Saturn, Uranus and Neptune). Because of their small size, and glare from the nearby planet, they can be very difficult to observe from Earth. Some, such as Pan and Daphnis at Saturn, have only ever been observed by spacecraft.
The first inner satellite to be observed was Amalthea, discovered by E. E. Barnard in 1892. Next were the Saturnian moons Epimetheus and Janus, observed in 1966. These two moons share the same orbit, and the resulting confusion over their status was not resolved until the Voyager 1 flyby in 1980. Most of the remaining inner satellites were discovered by spacecraft Voyager 1 and Voyager 2 during their flybys of Jupiter (1979), Saturn (1980), Uranus (1986) and Neptune (1989).
In more recent years, further inner satellites have been found orbiting Saturn by the Cassini spacecraft from 2005 to 2009, and orbiting Uranus (2003) and Neptune (2013) by the Hubble Space Telescope.
Orbits
All inner satellites follow nearly circular, prograde orbits. The median eccentricity is 0.0012, while the most eccentric inner satellite is Thebe with e=0.0177. Their inclination to their planets' equatorial planes is also very low. All but one have inclinations below one degree, the median being 0.1°. Naiad, Neptune's closest moon, is the exception, being inclined at 4.75° to Neptune's equator.
The innermost satellites orbit within the planetary rings, well within the fluid Roche limit, and only the internal strength and friction of their materials prevents them from being torn apart by tidal forces. This means that, if a pebble were placed in the part of the satellite furthest away from the planet, the tidal force outward is stronger than the satellite's gravity planetward, so the pebble would fall upward. This is why photos of these satellites show them to be completely clean of pebbles, dust and rocks.
The most extreme cases are Saturn's moon Pan, which orbits within the rings at only 70% of its fluid Roche limit, as well as Neptune's moon Naiad. Naiad's density is unknown, so its precise Roche limit is also unknown, but if its density were below 1100 kg/m3 it would lie at an even smaller fraction of its Roche limit than Pan.
Those satellites which have an orbital period shorter than their planet's rotation period experience tidal deceleration, causing a very gradual spiraling in towards the planet. In the distant future these moons will impact the planet or penetrate deeply enough within their Roche limit to be tidally disrupted into fragments. The moons so affected are Metis and Adrastea at Jupiter, and the majority of the inner moons of Uranus and Neptune − out to and including Perdita and Larissa, respectively. However, none of Saturn's moons experience this effect because Saturn is a relatively very fast rotator.