This month, we’ve met four of Jupiter’s moons. Four. Which means there are at least sixty-three other moons we haven’t met, and possibly more that have yet to be discovered.
It seems a little unfair to spend so much time on four moons and so little on all the rest, except those remaining sixty-three moons make up less than 1% of the total mass orbiting Jupiter.
Depending on who you ask (click here or here or here), the four Galilean moons pictured above constitute between 99.997 and 99.999% of the stuff in orbit around Jupiter. That includes all sixty-seven moons plus Jupiter’s rings.
Jupiter’s moons are divided into three groups. This might be hard to remember, but the four innermost moons are called the “inner moons.” Beyond the inner moons lie the four Galilean moons, and beyond them there’s a cloud of what astronomers call “irregular moons.”
Many of the irregular moons are in eccentric, inclined, or even retrograde orbit. Most if not all of them are either captured asteroids or debris from asteroid collisions. A few may only be temporary residents and might slip loose from Jupiter’s gravity sooner or later.
Compared to the Galilean moons, all these other moons look like pebbles. I don’t feel too bad about skipping them. Granted, the inner moons play an interesting role in shaping and maintaining Jupiter’s rings, but we’re going to be talking a lot about shaping and maintaining planetary rings very soon. I promise.