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.