Here is a table of the 12 largest satellites in our solar system. In addition to the size of each satellite, its home planet, its median distance from that planet, and discovery information, its median distance from its home planet is given in terms of the median lunar distance from the Earth. Remarkably, Pluto’s moon Charon is just 0.05 lunar distances from Pluto, only 19,591 km. Only one other of the largest satellites orbits closer to its home planet than the Moon orbits around the Earth, and that is Neptune’s moon Triton at 92% of the Earth-Moon distance. At the other end of the scale, Saturn’s moon Iapetus orbits Saturn over nine times further away than the Moon orbits the Earth.
Now let’s look at the orbital eccentricity of each of the largest moons, and the orbital inclination relative to the equator of its home planet.
Our familiar Moon is really an oddball: it has the greatest orbital eccentricity of all the largest satellites, and, with the exception of Triton and Iapetus, by far the greatest orbital inclination relative to the equator of its home planet. Triton is the oddball among oddballs as it is the only large satellite in our solar system that has a retrograde orbit: it orbits Neptune in a direction opposite the planet’s rotation. Iapetus has an orbital inclination relative to Saturn’s equator almost as much as the Moon’s orbital inclination relative to the Earth’s equator, but this anomaly can perhaps be forgiven because Iapetus orbits so very far away from Saturn. Its orbital period is over 79 days.
Note that the Moon’s orbital inclination relative to the equator of the Earth varies between 18.33˚ and 28.60˚. This occurs because the intersection between the plane of the Moon’s orbit around the Earth and the plane of the Earth’s orbit around the Sun precesses westward, making an entire circuit every 18.6 years.
Saturn’s third largest moon, Iapetus (eye-AP-eh-tuss), was discovered at the then-new Paris Observatory in 1671 by Italian-French astronomer (and observatory director) Giovanni Domenico (Jean-Dominique) Cassini (1625-1712). Upon further observation, Cassini noted that he could only see Iapetus when it was on the west side of Saturn, never the east. His telescope was not large enough to detect Iapetus on the east side of Saturn because it was much fainter then. He correctly reasoned that, “it seems, that one part of his surface is not so capable of reflecting to us the light of the Sun which maketh it visible, as the other part is.” He also must have realized that Iapetus was locked in synchronous rotation—as is our Moon—with the same side facing Saturn all the time, with its rotation period being equal to its orbital period. Today we know these periods to be 79.3215 days.
The leading hemisphere of Iapetus has a visual albedo of only about 5%, whereas most of the trailing hemisphere is much brighter, having an albedo around 25%. Thus, when Iapetus is on the west side of Saturn, its apparent visual magnitude is around 10.2, but on the east side of Saturn Iapetus is 1.7 magnitudes fainter at 11.9. Without a doubt, Iapetus is one of the most outlandish places in the solar system, and the Cassini Saturn orbiter flybys certainly amplified the strangeness.
Cassini made one close targeted flyby of Iapetus on September 10, 2007, passing within 762 miles of the surface. Here are a few of the best photos of Iapetus from Cassini.
The dark material appears to have been deposited from elsewhere in the Saturnian system, but sublimation of water ice may also play a role. In any event, the dark material is a relatively thin veneer, significantly less than a meter thick in many places.
The warm day on Iapetus sees a surface temperature of -227° F on the dark terrain and an even colder -256° F on the bright terrain. Inhospitable, to say the least!