When we think of moons we also think of planets. It makes sense because among the six planets with moons in our solar system they accumulate more than 200 satellites . And yet there are many more moons that orbit around asteroids or smaller bodies in the solar system, than those that orbit around planets. This is a matter of numbers. Although the planets are the largest bodies in the solar system (after the Sun) they do not accumulate more than a small percentage of the mass that is not in the Sun and they are only a tiny part of the very large number of bodies that populate it.
In the asteroid belt alone that occupies the region between the orbits of Mars and Jupiter, we can find about a million objects with sizes over a kilometer , a number similar to what we think we could find among Jupiter’s Trojan asteroids . In addition, more than two hundred objects with sizes greater than 100 kilometers are known in the asteroid belt, compared to ten among the Trojans. Beyond Neptune, in the region known as the Kuiper belt, many more objects dwell.
The Kuiper belt is estimated to contain about 50 times more mass than the asteroid belt and also contains several considerably larger objects. While Ceres is the largest body in the asteroid belt, with 940 kilometers in diameter, in the Kuiper belt that place is occupied by Pluto , with its 2,376 kilometers, with up to 6 objects larger than Ceres. In addition, it is estimated that there could be up to one hundred thousand objects with a diameter greater than 100 kilometers in this region.
With such a large number of objects of considerable size it is not surprising that some of them have moons orbiting them . All these objects smaller than a planet and with a stable and not excessively elliptical orbit (like that of comets) are known as “ minor planets ”. This group includes dwarf planets, but also asteroids of many types: main- belt asteroids, trans-Neptunian objects, NEOs , etc. Well, today 468 minor planets are known that have or could have at least one moon , adding a total of 488 moons of minor planets. Let’s visit some of these moons, differentiating them according to the group of objects to which they belong.
Near Earth Objects (NEOs)
These objects, which are any asteroid or comet in orbit around the Sun that passes close to the Earth’s orbit, have several very interesting examples of moons of minor planets. Asteroid 3122 Florence , just under 5 kilometers in diameter, has two moons orbiting it. The closest one has a size of about 200 meters, while the farthest exceeds 300 meters. Both orbit between 5 and 10 kilometers above the surface of Florence, taking just hours to complete an orbit. On the other hand (410777) 2009 FD would be a double asteroid, that is, an asteroid whose moon is so large that both bodies can be considered to orbit around the system’s barycenter . The main body has a diameter of about 150 meters, while the secondary would be around 90 meters.
Asteroid belt
Here is located the first asteroid to which a natural satellite was identified, 243 Ida , an elongated object 60 kilometers long and just over 20 kilometers wide. Its moon, called Dactyl , is a roughly spherical object 1.5 kilometers in diameter and orbiting the main asteroid about 90 kilometers away. Thanks to the fact that this system was visited by NASA’s Galileo probe in 1993, we have detailed photos of the surface of 243 Ida and even of its satellite.
Also in the asteroid belt is 130 Elektra , an asteroid about 200 kilometers in size orbited by three natural satellites . These range in size from just 1.6 kilometers for the innermost moon to 6 kilometers for the asteroid’s most distant moon, which orbits 1,300 kilometers from Elektra’s surface in a roughly circular orbit. The asteroid was discovered in 1873 , although the moons had to wait until this century to be discovered.
Jupiter Trojans
Fewer large objects or objects with moons are known among Jupiter’s Trojans, but among them 624 Hektor stands out, the largest Trojan with an elongated shape that makes it about 400 kilometers long by 200 kilometers wide. It is believed in fact that this asteroid could be the result of the merger of the two members of a double asteroid. Two similarly sized asteroids , about 200 kilometers across, are thought to have merged to form 624 Hektor. The body is also orbited by a satellite of just over 10 kilometers called Skamandrios , which could have formed during the same process that gave rise to the central object.
Trans-Neptunian objects
This is the largest and richest family of objects and here we can find bodies like Pluto , which is considered a double dwarf planet (because its “moon” Charon has half its radius) which is also orbited by four other moons, the largest number of any minor planet in the solar system. This also includes objects such as Haumea , with its two moons and its ring system, or the triple asteroid (47171) Lempo–Hiisi . This system would consist of a central pair, Lempo and Hiisi, 270 and 250 kilometers in diameter respectively, orbiting each other just 870 kilometers apart. This central couple would in turn be orbited by Paha , a body 130 kilometers in diameter and located about 7,400 kilometers from them. It is believed that this configuration could have been reached after a capture of the third body, which would have separated the initial pair from an orbit closer to the current one.
References:
Pitjeva, EV et al, 2018, Masses of the Main Asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft, Astronomy Letters. 44, doi:10.1134/S1063773718090050
Johnston, Wm. Robert, 2022, Asteroids with Satellites, Johnston’s Archive, accessed 06/22/2022
