Want to stay on top of all the space news? Follow @universetoday on TwitterA moon is any solar body that orbits a larger body called the primary. As of September 2008, 335 bodies are formally classified as moons. They include 167 orbiting six of the eight planets, 6 orbiting three of the five dwarf planets, 104 asteroids with moons, and 58 satellites of Trans-Neptunian objects, some of which will likely turn out to be dwarf planets. There are another 150 objects that are within Saturn’s rings, but have not been tracked enough to establish orbits. Since there are so many asteroids with moons, we will not discuss all of them. What we will do is discuss a few of the more interesting ones and information about moons in general.
The regular natural satellites in the solar system are tidally locked to their primaries, meaning that the same side of the moon always faces the planet. The same holds true for most asteroids with moons, but there are exceptions. No moons of moons are known. In most cases, the tidal effects of the primary would make such a system unstable. Two moons are known to have small companions at their L4 and L5 Lagrangian points. These companions are called Trojan moons, as their orbits are analogous to the Trojan asteroids of Jupiter. The Trojan moons are Telesto and Calypso, which are the leading and following companions respectively of Tethys; Helene and Polysdueces, the leading and following companions of Dione.
It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size. Discoveries of asteroids with moons are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights into their physical properties that is generally not otherwise possible. The origin of asteroid moons is not currently known with certainty, and a variety of theories exist. A widely accepted theory is that asteroid moons are formed from debris knocked off of the primary asteroid by an impact. Other pairings may be formed when a small object is captured by the gravity of a larger one.
Research and observation suggests that the Earth may have a second moon. According to NASA Asteroid 2003 YN107 is looping around our planet once a year. Measuring only 20 meters across, the asteroid is too small to see with the unaided eye—but it is there. Most near-Earth asteroids, when they approach Earth, simply fly by. They come and they go, occasionally making news around the date of closest approach. 2003 YN107 is different: It came and it stayed. The asteroid is not a real moon, but the fact that it has stayed for a while is quite interesting.
Asteroids with moons are very new to astronomy. There existence was only proven around 20 years ago. As telescopes improve and scientists bring their studies to bear, more light will be shed on these celestial pairings.