Natural satellite

A natural satellite is an astronomical body that orbits a planet, dwarf planet, or small Solar System body. These objects are often called moons, just like Earth's moon, which is also known as Selene or Luna. The word Moon with a capital M refers to Earth's moon, while moon with a lowercase m is used for any natural satellite.

In our Solar System, six planets have moons. Even some dwarf planets, like Pluto and Haumea, have their own moons. Scientists have found that many smaller planets also have natural satellites. These moons come in many shapes and sizes, and they help us learn more about space.

Terminology

The first known natural satellite was the Moon. Long ago, people thought the Moon was a planet until Copernicus changed that idea. Later, Galileo discovered four objects orbiting Jupiter, which we now call the Galilean satellites.

The word "satellite" was first used by Johannes Kepler in 1610. He took the word from the Latin language. After that, the word "satellite" came to mean any object that orbits a planet. When the first human-made object, Sputnik, was launched, people needed a new way to talk about natural satellites. Today, we use the word "moon" for natural satellites.

Origin and orbital characteristics

Further information: Circumplanetary disk

Natural satellites that orbit close to their planet in steady paths likely formed from the same cloud of material that created the planet. Others, especially those with distant or tilted orbits, might be captured asteroids. Most large natural satellites have steady orbits, but some smaller ones have unusual paths. The Moon and the Moons of Pluto are special; they may have formed from a big crash early in the Solar System. Another example is Triton, which orbits backward, suggesting it was captured from space.

Trojan satellites

Two natural satellites have small companions at special points in their orbits. These companions are called trojan moons, like certain asteroids that share Jupiter's path. The trojan moons are Telesto and Calypso, which follow the moon Tethys, and Helene and Polydeuces, which follow the moon Dione.

Temporary satellites

Main article: Temporary satellite

Sometimes, an asteroid can temporarily orbit a planet before moving away. As of 2023^[update], a few small objects have done this with Earth, including 1991 VG, 2006 RH120, 2020 CD3, and 2022 NX1.

2006 RH120 was a temporary satellite of Earth for nine months in 2006 and 2007.

Tidal locking

Main article: Tidal locking

Most moons in our Solar System always show the same face to the planet they orbit. This happens because the planet's gravity slows the moon's rotation until it stops. One exception is Saturn's moon Hyperion, which spins unpredictably because of the pull from Titan. Pluto's small moons also spin chaotically due to the influence of Charon.

Farther-out moons, like Jupiter's Himalia, Saturn's Phoebe, and Neptune's Nereid, do not always face the same way because they are too far from their planets to be slowed down.

Satellites of satellites

Main article: Subsatellite

We do not know of any natural satellites that orbit another natural satellite, which we call "moons of moons" or subsatellites. This is because the pull of the planet would usually make such a system unstable. However, some large moons far from their planets, like Titan, Iapetus, Callisto, and Earth's Moon, might be exceptions. But other forces could still prevent submoons from staying in orbit.

After finding a possible ring system around Saturn's moon Rhea in 2008, scientists thought satellites might orbit Rhea safely. They also thought the rings might be kept narrow by small moons called shepherd moons. But later pictures from the Cassini spacecraft did not show any rings around Rhea.

Some ideas suggest that Iapetus, another moon of Saturn, might have had a satellite in the past. This is one idea to explain a strange ridge along its middle. Studies also suggest that Saturn's irregular moon Kiviuq has a very stretched shape, and it might even be made of two pieces orbiting each other.

Shape

See also: Satellite planet

Some moons, like Proteus around Neptune, have unusual shapes. As moons get bigger, their gravity can make them more round over time.

Big moons that always face their planet the same way often look like eggs — squashed at the poles and stretched a bit at the sides. For example, Saturn's moon Mimas is a little wider along one direction than another. Tiny moons like Saturn's Methone show this effect clearly because they are small.

Geological activity

Some big natural satellites in our Solar System are still active today. Io is the most active volcano in the Solar System. Europa, Enceladus, and Triton show signs of movement and cold volcanoes. This activity is often caused by the pull of gravity from the planets they orbit.

Many other natural satellites, like Earth's Moon, Ganymede, Titan, Tethys, and Miranda, had geological activity in the past. This was powered by things like the breaking down of special materials inside them or changes in their orbits. Enceladus and Triton have features like geysers. Titan and Triton have atmospheres, and Titan even has lakes made of special liquids. Europa, Ganymede, Callisto, Titan, and Enceladus are believed to have oceans of water deep underneath their surfaces.

Occurrence in the Solar System

Of the inner planets, Mercury and Venus have no natural satellites. Earth has one large natural satellite, called the Moon. Mars has two tiny natural satellites, Phobos and Deimos. The giant planets have many natural satellites. Some are about the size of Earth's Moon. These include the four Galilean moons, Saturn's Titan, and Neptune's Triton. Saturn has six more mid-sized natural satellites, and Uranus has five.

Some smaller objects in space, like Ceres and Sedna, have no known natural satellites. Pluto has a large natural satellite named Charon and four smaller ones. Haumea has two natural satellites. Other small objects like Orcus, Quaoar, Makemake, Gonggong, and Eris each have one natural satellite. The Pluto–Charon system is special because the center of mass is between them, like a double-planet system.