As seen from Earth, the large satellites of the giant planets only subtend angles of one arcsecond or less. Although they are comparable in size to the Earth’s Moon - some of them are even larger — they appear more than two thousand times smaller due to their vast distances from the Earth. Telescopes on Earth can't see details smaller than a few tenths of a second of arc, so Earth-bound astronomers were forever barred from seeing these satellites as anything more than fuzzy disks. However, within our lifetimes, robotic spacecraft have transformed these satellites from tiny, blurry images to exciting worlds with unique “personalities.”
Jupiter and its moons Callisto and Europa. Click here for original source URL
Most people know of the nine major planets, but our Solar System also includes another dozen substantial worlds, many with spectacular landscapes. In effect, the Solar System contains not just the nine major planets, but dozens of world-class objects that are large enough to have individual geological and astronomical characteristics.
Ariel casting a shadow on Uranus. Click here for original source URL.
The Voyager mission discovered many new satellites in each system. Additional discoveries using telescope data and images from other missions turned up even more satellites, bringing the total to at least 67 for Jupiter, at least 62 for Saturn, at least 27 for Uranus, and at least 14 for Neptune. There are constant discoveries of moons around gas giants still being made. "Moonlets" have been spotted on the edge of Saturn's rings, and data from the Cassini mission continues to make new discoveries.
Collage of Jupiter's moons. Click here for original source URL.
Each planet is surrounded by a spherical volume of space called its sphere of gravitational influence. Within this region, the planet’s gravity has a bigger effect on satellite motions than the Sun’s gravity does. For example, if you tried to put a satellite in orbit around Jupiter, but outside Jupiter's sphere of gravitational influence, the satellite would eventually drift away from Jupiter into an orbit around the Sun, because the Sun would dominate its motion. But inside Jupiter’s sphere of influence, the satellite’s motion would be determined almost completely by the planet. Jupiter’s gravity keeps its satellites in Keplerian orbits - for example, the closest satellites orbit the fastest, in accordance with Kepler’s laws.
Permanent satellites occur only inside a planet’s sphere of gravitational influence. Inside this volume, as a rough rule, each giant planet's family of satellites can be divided into four groups:
1. Countless mini-moonlets and dust particles that make up the planet’s ring system.
2. Small satellites close to the planet, on the outskirts of the ring system (or in some cases inside the ring system). These are sources of ring material, and they play a large part in shaping the rings themselves. Most are too small to be seen from Earth, so they weren’t discovered until interplanetary probes photographed them.
3. Large satellites at intermediate distances from the planet. These are worlds in themselves, and many have distinctive, active geology.
4. Small satellites on the outskirts of the planet's sphere of gravitational influence. These satellites are probably not native to the planet, but rather they were passing interplanetary bodies that the planet's gravity captured into orbit. There are several satellites that orbit their planets in a "backward" retrograde direction — opposite to the direction of all planets and most other satellites. These are most likely captured interplanetary bodies.
Comparison of sizes of selected moons in the solar system. Click here for original source URL.
While the smallest satellites in the solar system are merely the largest instances of ring particles, the largest satellites are bigger than the planet Mercury. How big does a hunk of rock have to be to be called a moon or a satellite? There’s such a broad range of sizes that there’s no simple answer to this question. It’s a matter of definition, rather than science. However, objects less than ten kilometers across are usually considered too small to call true satellites. (Deimos, the smaller of Mars’ two satellites, is just barely larger than this.) Most of the named satellites in the solar system are between 10 and 100 kilometers across. Some satellites are even larger than 1,000 kilometers across, and have their own atmospheres and other distinctive features.