Minor-planet moon

Some space rocks have moons. These moons fly around them.

Most moons are small. But some are very big. One moon is half the size of Pluto. This is Charon.

Some rocks have many moons. One rock has three moons. It is named Elektra.

Finding moons helps us learn. We can learn how heavy a rock is. We can learn what it is made of.

Space is full of surprises!

Some space rocks have moons. These moons orbit a main object called a primary. The moon is called a secondary.

Most moons are small. But some are very big. For example, the moon Charon is half the size of Pluto. This makes Pluto and Charon a binary system. A binary system is when two objects are almost the same size.

Some rocks have many moons. The rock 130 Elektra has three moons. This makes it a quadruple asteroid. Other rocks have two moons. We call these triple systems. There are 18 known multiple systems in total.

Finding moons helps us learn about space. By watching how moons move, we can find the mass of the primary. Mass is how much matter is in an object. We can also find its density. Density tells us what the rock is made of.

How do these moons form? One idea is that a crash makes debris. This debris then forms a moon. Another idea is that a large object grabs a small one. This is called capture. Space is full of surprises!

Space is filled with small objects called minor planets. Many of these have their own natural satellites, which we call moons.

Finding these moons is a big deal for scientists. When we track how a moon orbits, we can figure out the mass of the primary object. Mass is a measure of how much matter is in something. Knowing the mass helps us find the density of the rock. Density tells us what the object is actually made of.

There are many different ways these moon systems can work. If a minor planet has one moon, it is sometimes called a binary. If it has two moons, it is called a triple. Some very small moons are even called moonlets.

Most moons are much smaller than their primary object. However, some are surprisingly large. For example, the moon Charon is about half the size of Pluto. Because they are so similar in size, they form a true binary system.

Scientists have been looking for these moons for a long time. Before we had the Hubble Space Telescope, people used Earth-based tools. In 1978, astronomers thought they saw a moon around the asteroid 532 Herculina. Later, the Hubble Telescope showed that the moon was not actually there. In 1993, we got a real confirmation. The Galileo probe found a small moon named Dactyl orbiting the asteroid 243 Ida.

This was the first confirmed asteroid moon discovered.

Today, we know of 457 minor planets that have moons. Some systems are very crowded with many moons. The asteroid 130 Elektra is a quadruple asteroid because it has three moons.

Pluto is even more crowded with a sextuple system. This means it has five known moons: Charon, Nix, Hydra, Kerberos, and Styx. Other objects like 87 Sylvia and 93 Minerva are also triple systems. There are 18 known multiple systems in total among minor planets.

We are still learning how these moons began. One idea is that a crash happens between two objects. This impact knocks off debris that eventually forms a moon. Another idea is called capture. This happens when a large object's gravity grabs a passing small object.

Near Earth, moons might form because of tidal stresses from planets. In the far reaches of space, many moons might be primordial. This means they have been there since the very beginning.

A minor-planet moon is a natural satellite that orbits a minor planet. These small celestial bodies are found throughout our solar system. Scientists have identified 457 minor planets that possess moons or suspected moons. Studying these systems is vital for understanding the universe. When researchers determine the orbit of a moon, they can calculate the mass of the primary object. Mass tells us how much matter is in the body. From mass, scientists can estimate density. Density helps reveal the physical properties and composition of the minor planet.

Astronomers use specific terms to describe these orbiting systems. If a minor planet has one moon, it is often called a binary. If it has two moons, the system is called a triple. Very small satellites are frequently referred to as moonlets. In systems where the two objects are nearly equal in size, they are called binary companions. The larger object is the primary, and the smaller is the secondary. Some scientists use the term double asteroid for systems with similar-sized components. This terminology helps clarify the relationship between the objects in orbit.

These moon systems vary greatly in their physical proportions. Most moons are much smaller than their primary bodies. However, some moons are remarkably large. For example, the moon Charon has about half the diameter of Pluto. Other systems like 90 Antiope, Mors–Somnus, and Sila–Nunam have mass ratios near 95%. The Lempo–Hiisi system features a moon that is 50% the mass of its primary. Some objects even possess ring systems, such as the rings found around Chariklo and Chiron.

Our understanding of these objects has changed through major discovery milestones. Before the Hubble Space Telescope and space probes, astronomers relied on optical observations from Earth. In 1978, researchers claimed to see a satellite around the asteroid 532 Herculina via stellar occultation. However, the Hubble Telescope later showed no such satellite existed. The first confirmed asteroid moon was Dactyl, discovered by the Galileo probe in 1993. Dactyl orbits the asteroid 243 Ida. In 1998, a second moon was found orbiting 45 Eugenia.

Multiple systems can become quite complex with many orbiting bodies. The asteroid 130 Elektra is a quadruple asteroid because it has three moons. Some systems are even more crowded. Pluto is a sextuple system with five known moons: Charon, Nix, Hydra, Kerberos, and Styx. Other triple systems include 87 Sylvia, 93 Minerva, and 216 Kleopatra. There are currently 18 known multiple systems among minor planets. These include the Pluto and Haumea systems.

Scientists have several hypotheses regarding how these moons originate. One model suggests moons form from debris. This debris is knocked off a primary body during a collision. Another theory involves gravitational capture. This occurs when a large object's gravity pulls in a passing small object. The distance between objects can help scientists test these ideas. For asteroids, separations can be as small as a few hundred kilometers. In the trans-Neptunian region, separations can reach 50,000 km.

The characteristics of a moon system often depend on its location. Near-Earth asteroids often have moons that orbit at 3 to 7 primary radii. Tidal stresses from passing planets may help form these satellites. In the main asteroid belt, moons are usually much smaller. They often orbit about 10 primary radii away. Many are fragments from collisions that disrupted a parent body. In the distant trans-Neptunian region, moons are often much larger relative to their primaries. Many of these are thought to be primordial, meaning they formed during the early solar system.