So – You’re Still Interested in Astronomy? Jupiter5 min read

Hello again.

Last time, we looked at the two moons of Mars, Phobos and Deimos.

Today, I thought we’d start to look at 10 facts you might not have known about the moons of Jupiter. You notice I said, start? Well, that’s because Jupiter has 67 confirmed moons. Yes, I said sixty-seven, giving her the largest retinue of moons with ‘reasonably secure’ orbits of any planet in the Solar System.

So, what we’ll do, is dip into the minor moons first. Those are the ones that aren’t classed as Galilean.

1.How many minor moons are there?

Believe it or not, out of the 67 moons orbiting Jupiter’s vast circumference, 63 of them are classed as minor. Of these, there are two main groups. The Regular Satellites, and Irregular Satellites.


This is a little graphic of Jupiter, surrounded by the orbital paths of her many moons. She is in there somewhere…honestly!

2.What’s the difference?

The Regular Satellites – otherwise known as the Amalthea Group & the Galilean Group – are the closest moons to Jupiter’s surface, all have a circular orbit and they also spin in the same direction as their parent planet.
The Irregular Satellites are substantially smaller objects with more distant and eccentric orbits. In fact, they are divided into a further two groups themselves. The Prograde moons – meaning they spin in the same direction as Jupiter – and Retrograde moons, which spin in the opposite direction.

3. Tell us a bit more about the Amalthea Group.

This group is made up of 4 moons. In order of distance – with the closest first – they are: Metis, Adrastea, Amalthea, and Thebe. These little moons orbit very close to Jupiter. (The innermost two orbit in less than a Jovian day. The latter two are respectively the fifth and seventh largest moons in the Jovian system.)

4. Do we know any interesting facts about these moons?

We certainly do. Observations suggest that Amalthea did not form on its present orbit, but farther out instead. Some think it to be a captured Solar System body, and that this might be true of its three closest neighbors.


Interesting fact:
These moons, along with a number of as-yet unseen inner moonlets, replenish and maintain Jupiter’s faint ring system. Metis and Adrastea in particular, help to maintain Jupiter’s main ring, whereas Amalthea and Thebe each maintain their own faint outer rings

5. So, what do we know about the irregular satellites then?

As we mentioned, these moons are substantially smaller than their brothers and sisters, and they also have more distant and eccentric orbits.

Interesting Fact:
These irregular bodies form five main smaller sub-families. Each have shared similarities in orbit and composition. It is believed that these particular moons were created when larger – but still small – parent bodies were shattered by impacts from asteroids captured by Jupiter’s gravitational field. Now, each family bears the names of their largest members.

6. Which is the first irregular satellite?

The very first irregular satellite is only about 8 kilometers (5 miles) in diameter, and is called, Themisto.


7. Are there any interesting facts we might like to know about this little moon?

Of course. Themisto was first discovered by Charles T. Kowal on September 30, 1975, although he didn’t report the sighting until October 3, 1975. At first, its designation was S/1975J 1. However, not enough observations were made to establish an orbit and it was subsequently lost.
Then, in 2000, a seemingly new satellite was discovered by Scott S. Sheppard, David C. Jewitt, Yanga R. Fernández and Eugene A. Magnier, and was designated S/2000J 1.
However, it was soon confirmed that this was the same as the 1975 object.  After much wrangling and debate, the finding was officially called “Themisto” daughter of the river god Inachus and lover of Zeus (Jupiter) in Greek mythology.
Another interesting fact is that Themisto’s orbit is unusual. Unlike most of Jupiter’s moons, which orbit in distinct groups, Themisto orbits alone.

8. What comes after Themisto?

 Next comes the Himilaya Group, a selection of four moons which are thought to be the remnants of a larger, single body from the Main Asteroid Belt.

9. Does the Himilaya group contain any little gems?

It certainly does. Himalia, the largest irregular satellite of Jupiter, was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904. Himalia is Jupiter’s most easily observed small satellite.
At a distance of about 11.5 million km (7 million miles) from Jupiter, she takes about 251 Earth days to complete one orbit, and is the largest member of the group that bears its name.

10. We’ve got to know. What is the very last moon?

Actually, the last moon is quite boring. Called, S/2003J 2, it was discovered by a team of astronomers from the University of Hawaii led by Scott S. Sheppard and David C. Jewitt, on March 4, 2003. [
This tiny moon is only about 2 km (1.2 mi) in diameter, and orbits Jupiter at an average distance of 1,880,000 km (1,168,171 miles)
Although she is currently the last known moon, it is possible that even more distant moons of Jupiter may be discovered in the future.

So there you go. With so many tiny moons surrounding Jupiter, you’d think there would be more to say. But don’t forget, although large by quantity, these 63 satellites only account for 0.0003% of the mass of all 67 moons. As you will note, the Galilean moons make up all the rest.

So – you know what that means?

Yup! Come back next month, where we’ll take a look at 10 interesting facts that make the Galilean Moons so special.

See you then…


Andrew Weston
Andrew Weston

Andrew P. Weston is Royal Marine and Police veteran from the UK who now lives on the beautiful Greek island of Kos with his wife, Annette, and their growing family of rescue cats. An astronomy and law graduate, he is the creator of the international number one bestseller, The IX, and also has the privilege of being a member of the Science Fiction and Fantasy Writers of America and the British Fantasy Society. When not writing, Andrew devotes some of his spare time to assisting NASA with one of their remote research projects, and writes educational articles for and Amazing Stories. He also enjoys Greek dancing and language lessons, being told what to do by his wife, and drinking Earl Grey Tea. If you would like to find out more, visit his blog or website at:

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