Jupiter: How Many Moons?

23 November 2012 by Troy McConaghy, posted in Astronomy

Jupiter and its biggest moons

Jupiter and its biggest moons

As mentioned in my last post, Mars has two moons. Jupiter has… well, it's complicated.

Galileo discovered the four big ones in 1609–1610. Today they're known as the Galilean Moons: IoEuropaGanymede, and Callisto.

It took 282 years for the next Jovian moon to be spotted, which is remarkable, because a lot was discovered in the meantime (including the moons of Mars). That next-discovered moon was known as "Jupiter V" until 1975, when it got the name Amalthea.


Two grayscale images of Amalthea


Many Jovian moons were discovered since then; the current count stands at 67. Two were discovered in 2010 and two more in 2011, so others probably remain to be found. The four Galilean moons are much, much bigger than all the others, which is why it took the others so long to be found.

The fact that moons continue to be discovered isn't the only complicating factor, however. Jupiter can capture asteroids or comets, turning them into moons. Actually, any planet can do that, but Jupiter's large mass makes it very good at capture. Most of Jupiter's small moons are thought to be captured asteroids (or captured asteroids that broke into pieces).

The best-known example of a captured comet was Shoemaker-Levy 9. It was captured by Jupiter and was a moon for some 20–30 years (maybe longer), but its orbit wasn't stable; it broke up into pieces in 1992, and it crashed into Jupiter in 1994.

On September 10, 2012, amateur astronomers spotted a bright flash on Jupiter. It's thought it came from the collision of a small comet or asteroid. As such, it's the fourth such impact reported since 2009.

The two moons closest to Jupiter, named Metis and Adrastea, are slowly getting closer to Jupiter and will eventually crash into Jupiter.

Captured comets and asteroids can also crash into one of Jupiter's moons, leaving a telltale chain of craters like seen in the photo below.

Chain of craters on Ganymede

A chain of impact craters on Ganymede


Not all captured comets and asteroids end up crashing. Some even escape. For example, comet 147p/Kushida-Muramatsu was captured, orbited Jupiter for about 12 years, and then escaped. The so-called Quasi-Hilda comets have a knack for getting temporarily captured by Jupiter.

Jupiter also has rings and Trojan asteroids, but those aren't considered moons (although Trojans can get captured by Jupiter to become moons).

So how many moons does Jupiter have? It's something like 67, but don't carve that number in stone.


Sheppard, Scott S. "The Giant Planet Satellite and Moon Page." Departament of Terrestrial Magnetism at Carniege Institution of Washington.

Image Credits

All photos in this post are courtesy of NASA.

About this Blog

Outer Spacing is a blog about space exploration and development. It's written by Troy McConaghy (@TroyMc on Twitter).

2 Responses to “Jupiter: How Many Moons?”

  1. Bill | Permalink

    Very interesting. Good for that one comet that escaped!


Comments are closed.