Phobos is so close to Mars that it orbits Mars much faster than Mars rotates. This means that it rises in the west and sets in the east, even though its orbit is prograde.

Are there any other known natural satellites that orbit their planet faster than the planet rotates, or is Phobos unique in this regard?

  • $\begingroup$ Which sources have you checked? There are only a handful of planets known to have satellites... $\endgroup$ – antispinwards Jul 7 at 16:25
  • $\begingroup$ @antispinwards and yet we keep discovering more moons of the outer planets. $\endgroup$ – Carl Witthoft Jul 7 at 16:58
  • $\begingroup$ @antispinwards That's already a couple hundred satellites $\endgroup$ – usernumber Jul 7 at 18:27
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    $\begingroup$ @usernumber - most of the lists you can find sort the satellites in order of increasing orbital period. No point considering the outer ones... $\endgroup$ – antispinwards Jul 7 at 18:53

Uranus rotates once every 0.718 days. 11 of its satellites have a shorter orbital period. These are inner satellites of Uranus which are roughly in the equatorial plane of Uranus. I don't quite understand what direction they orbit in compared to the direction in which Uranus spins.

Neptune rotates once every 0.671 days. 5 of its satellites have a shorter orbital period and orbit prograde.

Jupiter rotates once every 10 hours. 2 of its satellites have a shorter orbital period and orbit prograde.

Saturn rotates once every 0.4 days and none of its satellites orbit faster than that.

So Phobos is not alone in its case. Around Mars, Jupiter and Neptune (and maybe Uranus), there are moons that orbit so fast that they rise in the west and set in the east.

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  • $\begingroup$ All of the 18 innermost known moons of Uranus are in prograde orbits, as evidenced by their orbital inclinations of <=90° in the linked Moons of Uranus article. $\endgroup$ – notovny Jul 8 at 12:17

Jupiter has a sidereal rotation period or day of 9.925 hours.


The two innermost moons of Jupiter, Metis and Adrastea, have orbital periods of 7 hours 10 minutes 16 seconds and 7 hours 15 minutes 21 seconds respectively.


Saturn has a sidereal rotation period or day of 10 hours 33 minutes 38 seconds, or 0.4400231 Earth days.


The innermost confirmed moon, S/2009 S1, has an orbital period of 0.47 Earth days, which is longer than Saturn's day.


So I have checked the moons of two of the giant planets, leaving two giant planets, Uranus and Neptune, to be checked.

I note that a number of objects in the solar system which are not classified as planets, but as dwarf planets, Trans Neptunian Objects, asteroids, etc., have been discovered to have moons. If you are interested whether any of them orbit in less than a day of their primary, you should know that:

Of the objects within our Solar System known to have natural satellites, there are 76 in the asteroid belt (five with two each), four Jupiter trojans, 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers.2 There are also 84 known natural satellites of trans-Neptunian objects.2 Some 150 additional small bodies have been observed within the rings of Saturn, but only a few were tracked long enough to establish orbits.


So there would be a lot of checking to do to find which of them might orbit faster than one day of their primary.

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