What Is The Fastest Moon In The Solar System?

What Is The Fastest Moon In The Solar System?
21 October 2021

I was asked on Twitter on behalf of a very curious four-year-old what was the fastest moon in the Solar System. And I need to admit that I was a bit stumped. Which moon moves the fastest around its planet? Well, the answer might surprise you. Or maybe not. 

My reasoning is that the bigger the planet, the closer the moon, the faster it ought to be going so the closest satellites of Jupiter and maybe some of the shepherd moons that are found in the rings of Saturn ought to be the fastest. 

And indeed they are. So here’s the Top 10 list of moons that orbit their planet in order of speed:

1. Metis (31.5 km/s)

2. Adrastea (31.37 km/s)

3. Amalthea (26.57 km/s)

4. Thebe (23. 92 km/s)

5. S/ 2009 S1( about 18 km/s )

6. Io (17.3 km/s)

7. Pan (16.89 km/s) 

8. Daphnis (16.71 km/s)

9. Atlas (16.63 km/s)

10. Prometheus (16.53 km/s)

The kid suggested the Galilean satellites of Jupiter and they were absolutely on the right track. Within that top 10, moons 1-4 and 6 belong to Jupiter, while the remaining four are found around Saturn!

We don’t get to a moon belonging to another planet until we reach position number 21, with Naiad of Neptune which is whizzing at 11.9 kilometers per second. 

The first one from Uranus, Cordelia, is at number 28 just after Jupiter’s Ganymede. It moves at 10.8 kilometers per second. Mars’ phobos is at position 130 and it goes at about 2 kilometers per second. 

And our own Moon is at position 192 orbiting our planet at the (comparatively) leisure pace of 1 kilometer per second.

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Image Credit: The Galileo ProjectNASA