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Several interviews with moon hunters and top theorists reveal that the definition of a moon is not clear and that there has been almost no discussion, professional or casual, about whether there should be any lower size limits set to separate real moons from miniature imposters. And nobody is in a rush to do anything about it.
Some take long, elliptical paths and stray far above and below the planet's plane of rotation. Few are round.
While irregular moons define the outer orbital reaches of a planet's domain in space, the next region in is dominated by regular moons. Examples include Saturn's moon Titan and Jupiter's Galilean satellites: Io, Europa, Ganymede and Callisto. These classic, large and round moons tend toward entirely stable, simple, nearly circular orbits. All move through a plane in space that is roughly equal to the planet's equatorial plane. This conformity leads theorists to believe the moons formed out of the same nebula of gas and dust that built the planet.
Irregular moons are of unknown origin. They might be captured asteroids or comets, or perhaps they're pieces of young planetary hopefuls that didn't survive to orbit at the big table of nine, researchers believe. Many irregulars travel in packs that indicate they were once parts of larger objects. The irregulars are typically small and orbit at great distances and often on odd trajectories. Their orbits are stretched and tilted in just about every way you can imagine. They soar high above and below the plane in which regular moons orbit. They come close to the planet and then zoom far away on elliptical trajectories. More often than not their orbits are retrograde -- opposite the direction of regular moons and of the planet's spin.
Inner moons, sometimes called ring moons because they often travel amidst other debris, are almost surely the youngest. They're also the most likely to disappear.
Outside the traditional categories of satellites are two sorts of objects that really stretch definitions. Ahead of and behind Jupiter are two packs of asteroids -- tens of thousands of them -- that orbit the Sun but are also gravitationally bound to Jupiter. Astronomers call them Trojan asteroids.
Originally posted by Zerra
That is pretty cool! Kind of makes me sad we only have 1 moon
A quasi-satellite is an object in a 1:1 orbital resonance with its planet that stays close to the planet over many orbital periods. A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity (usually greater), as shown in the diagram on the right. When viewed from the perspective of the planet the quasi-satellite will appear to travel in an oblong retrograde loop around the planet.
Earth currently has four known quasi-satellites: 3753 Cruithne, 2002 AA29, 2003 YN107, and 2004 GU9. These objects remain in quasi-satellite orbits for times of tens to hundreds of years.