Jupiter: How Many Moons?
Jupiter has 67 confirmed moons, for now. It's complicated.
Galileo discovered the four big ones in 1609 and 1610. Today they're known as the Galilean Moons: Io, Europa, Ganymede, 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, then it got its current name: Amalthea. (Some people still call it Jupiter V though.)
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.
Capturing More Moons
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.
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).
The Bottom Line
Jupiter currently has 67 confirmed moons, but don't carve that number in stone. It will change.
The Jupiter Satellite and Moon Page by Scott S. Sheppard at the Department of Terrestrial Magnetism at Carnegie Institution of Washington.
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).