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Last update: October 16, 2025

Do Natural Satellites of Natural Satellites Exist?

Hypothetical moon of a moon

Natural Satellites or Moons

Natural satellites, commonly called moons, orbit planets. An intriguing scientific question arises: Can a satellite itself have a natural satellite, which could be called a "secondary moon"?

A secondary moon is a natural satellite that orbits another natural satellite, which itself orbits a planet. The stability of such an orbit is extremely limited by several factors:
- The dominant gravity of the primary planet, which tends to disrupt the trajectories of small satellites.
- The tidal forces exerted on the secondary satellite by the primary planet, which can cause its disintegration or ejection.
- The Hill sphere of the primary satellite, defining the region around the satellite where its gravity dominates over that of the planet, severely limits the maximum distance for a stable secondary moon.
Currently, no secondary moon has been observed in our solar system, but numerical simulations suggest they could exist around very massive satellites like Ganymede or Titan, provided they remain close to the parent satellite.

Physical Constraints

For such a system to be stable, several conditions must be met:

According to George Darwin (1845-1912), tidal effects limit the maximum size and orbital distance of any secondary satellite. The Hill sphere defines the volume around a satellite in which its gravity dominates over that of the planet.

Examples and Simulations

So far, no secondary moon has been observed in our solar system.

Potential for Secondary Satellites in the Solar System
Primary SatelliteMassMax Distance for Secondary SatelliteComment
Ganymede1.48 × 1023 kg~500 kmNumerical simulation indicates possible stable orbit close to the satellite
Titan1.35 × 1023 kg~400 kmSaturn's gravity strongly limits outer orbits
Callisto1.08 × 1023 kg~300 kmInner orbits possible but not detected

Source: Icarus, Hill sphere and stability analysis of moonlets.

Asteroids and Their Satellites

Unlike the large moons of the giant planets, many asteroids have their own natural satellites. These systems, called binary or multiple asteroid systems, are relatively common in the main belt and among near-Earth asteroids.

However, these asteroids do not have "secondary moons" in the strict sense. The main reason is related to the very small gravitational scale:

Thus, although asteroids can have moons, the formation of secondary moons around these satellites is practically impossible. This contrast illustrates the importance of mass and central gravity in the stability of multi-level systems.

Differences Between Planetary Moons and Asteroid Satellites

Although planetary moons and asteroid satellites are all natural satellites, they have fundamental differences related to their size, gravity, and orbital environment.

The main distinctions are as follows:

This comparison illustrates how gravity and orbital environment control the complexity of natural satellite systems and explains why hierarchical systems with "moons of moons" are extremely rare in the solar system.

Examples of Asteroids with Satellites
Main AsteroidSatelliteAverage DistanceComment
243 IdaDactyl~90 kmFirst asteroid satellite discovered by the Galileo mission
87 SylviaRomulus~1350 kmTriple system with Remus, stable in the main belt
87 SylviaRemus~710 kmSecond satellite of the same system
22 KalliopeLinjia~1100 kmExample of a binary system with a circular orbit
90 AntiopeAntiope B~170 kmBinary system nearly identical in mass
130 ElektraP4~500 kmTriple system with very close satellites
107 CamillaS/2001 (107) 1~1200 kmDiscovered by recent telescopic observations
3749 BalamS/2002 (3749) 1~70 kmExample of a near-Earth asteroid with a satellite

Source: Icarus, Asteroid Satellites: Observations and Dynamics and Minor Planet Center.

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