We all know planets orbit stars and moons orbit planets. Have you ever wondered whether a moon can have a moon of its own (a submoon)? Theoretically, the answer is yes, but there is a good reason why we haven’t discovered any submoons around moons in our own solar system.
How Moons Work
In order for a body to have a moon, the moon has to be close enough to the other body to form a stable orbit. There is a sort of Goldilocks distance. If the potential moon is too close, it can crash into its host. If it’s too far away, gravity can’t hold the two bodies together and the potential submoon moves away.
The zone within which a moon can be held is called the Hill sphere. The size of the Hill sphere depends on the mass of the host and thus the gravity it can exert. The Hill sphere around Jupiter is much larger than the one around the Earth. Most moons in the solar system are much less massive than the Earth, so their Hill spheres are even smaller. This makes capturing a submoon less likely. For example, while the Earth’s Hill sphere has a radius of 1.5 million kilometers (235 Earth radii), the Moon’s Hill sphere is only about 60,000 kilometers. As far as moons go, this is a large Hill sphere. Other moons potentially massive enough to host submoons include Jupiter’s moon Callisto and Saturn’s moons Titan and Iapetus.
The Moon or another moon could capture a submoon, but it wouldn’t keep it very long. The reason is that most moons are in synchronous rotation around their host planet. Like the Moon, moons show the same face toward their planet at all times. This produces a tidal force on any object trying to orbit a moon. Eventually, a submoon’s orbit would decay and it would crash into its host moon or else the tidal forces would rip the submoon apart. In fact, astronomers think it’s possible the equatorial ridge on Saturn’s moon Iapetus is evidence of a submoon impact.
Planets tend not to orbit synchronously around their star, so stable lunar orbits are possible.
Man-Made Submoons
A spacecraft in orbit around a moon becomes a temporary satellite or submoon. These orbiters feel the tidal forces, but it’s easy to compensate using thrusters. In fact, even without rockets, a man-made satellite could last thousands or even millions of years before crashing into the moon.
Other Bodies With Submoons
While astronomers haven’t discovered any moons with submoons, they have identified asteroids with their own moons. In some cases, two asteroids of comparable mass form a binary system. An example is the 90 Antiope system. More commonly, satellites are much smaller than the asteroids they orbit. Scientists aren’t entirely certain how these systems form. One hypothesis is that an enormous collision breaks an asteroid into pieces that continue to travel together and are bound by gravity.
References
- Dombard, A. J. et al. (2012). “Delayed formation of the equatorial ridge on Iapetus from a subsatellite created in a giant impact.” Journal of Geophysical Research: Planets. Volume 117, Issue E3. doi:10.1029/2011JE004010
- Kollmeier, Juna A. & Sean N. Raymond (2019). Can Moons Have Moons? Monthly Notices of the Royal Astronomical Society: Letters, Volume 483, Issue 1, pp. L80–L84. doi:
10.1093/mnrasl/sly219 - Marchis, Franck; Enriquez, J. E.; Emery, J. P.; Berthier, J.; Descamps, P. (2009). The Origin of the Double Main Belt Asteroid (90) Antiope by Component-Resolved Spectroscopy. DPS meeting #41. American Astronomical Society.