Do Natural Satellites of Natural Satellites Exist?

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:

• The gravity of the primary planet must be weak relative to that of the secondary satellite so as not to disrupt its orbit.
• The orbital period of the secondary moon must be significantly shorter than that of the satellite around its planet.
• The tidal forces induced by the primary planet must be insufficient to deorbit the secondary satellite.

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.

• Source: Icarus, Hill sphere and stability analysis of moonlets (new window).

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:

• The gravity of the primary asteroid is weak but sufficient to retain a small satellite at close range.
• The satellite is close to the main body, and any attempt to form a moon around the satellite would be quickly disrupted by the gravitational forces of the Sun and nearby planets.
• Tidal forces and dynamic perturbations make any orbit around an asteroid's satellite extremely unstable in the long term.

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 (celestial bodies orbiting another body), they have fundamental differences related to their size, gravity, and orbital environment.

The main distinctions are as follows:

• Mass and gravity: Planetary moons, such as Ganymede or Titan, have significant mass and sufficient gravity to maintain a tenuous atmosphere or influence their own surface through tidal forces. Asteroid satellites are much smaller, with very weak gravity, unable to retain a significant atmosphere.
• Formation: Planetary moons can form by accretion in a circumplanetary disk, gravitational capture, or giant collision. Asteroid satellites generally form by collision or rapid fission due to rotation, with little material available for a stable circum-asteroidal disk.
• Orbital stability: Planetary moons can theoretically host secondary moons very close to the parent satellite, although rare, due to their significant mass and relatively large Hill spheres. Asteroid satellites cannot maintain secondary moons: solar and planetary perturbations quickly dominate any orbit around a small body.
• Observability: Planetary moons are visible from a distance with telescopes and space missions. Asteroid satellites are more difficult to observe and often require radar measurements or mission flybys for confirmation.

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.

• Source: Icarus, Asteroid Satellites: Observations and Dynamics (new window) and Minor Planet Center (new window).