Natural Satellite

I

Introduction

Natural Satellite, a body in space that orbits a larger body. The larger body is referred to as the satellite’s primary. Natural satellites that orbit planets are often called moons. Other objects in the solar system that sometimes have satellites include dwarf planets, Kuiper Belt Objects, centaurs, and asteroids. The term satellite is also used to refer to small galaxies that orbit larger galaxies.

Natural satellites are of special interest to astronomers and planetary scientists because these objects provide clues to how the planets and the solar system formed. Studying the orbits of satellites also allows scientists to determine the mass and density of planets and other objects. A number of moons are important to astrobiologists as places where conditions might permit extraterrestrial life to exist. The International Astronomical Union (IAU) gives official names to natural satellites.

II

Solar System Moons

The best-known natural satellite is Earth’s moon. The Moon is unusually large relative to the size of its primary (Earth) and has a diameter about one-fourth the diameter of the planet. The Moon’s surface, like the surfaces of most of the natural satellites in the solar system, is heavily cratered and geologically inactive.

Neither Mercury nor Venus has any natural satellites, but Mars has two small moons: Phobos and Deimos. Jupiter has more than 60 natural satellites, four of which are quite large: Io, Ganymede, Callisto, and Europa. These large moons were discovered by Galileo with an early telescope in 1610—the first moons detected around another planet. Active volcanoes cover Io, and scientists believe that oceans of water may hide beneath the icy crusts of Ganymede, Callisto, and Europa. All four of these moons are larger than the dwarf planet Pluto, and Ganymede is larger than the planet Mercury, as well. Saturn also has more than 60 natural satellites, the largest of which is Titan. Titan is bigger than Mercury, and is the only moon with a thick atmosphere. Enceladus, one of Saturn’s smaller moons, has active volcanism in the form of geysers that send out vast plumes of liquid water from the south polar region. Uranus has at least 27 moons, none of which is as large as Earth’s moon. Miranda, one of Uranus’s smaller moons, shows signs of terrific upheavals on its surface. Neptune’s largest natural satellite, Triton, is slightly larger than Pluto. Its surface appears to be continually reshaped by the freezing and thawing of nitrogen.

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The dwarf planet Pluto has three moons. Its largest moon, Charon, is half as large as Pluto itself and was discovered in 1978. Some astronomers consider the pair a double dwarf planet. Eris, a dwarf planet larger than Pluto, has a small moon named Dysnomia. The odd football-shaped dwarf planet 2003 EL61 has two small moons.

Astronomers have detected satellites around other types of solar system bodies. At least 100 asteroids are thought to possibly have satellites. Confirmed asteroid moons are given official names and catalog designations by the IAU. The first asteroid moon was discovered in 1993 when the Galileo space probe photographed the asteroid Ida and its moon Dactyl. In the outer solar system a number of centaurs and Kuiper Belt Objects are known to have satellites. To date, satellites have not been confirmed around any comets.

III

Satellite Orbits

The motion of most of the solar system’s natural satellites about their planets is direct: west to east, in the same direction as the rotation of their planets. Such moons are known as regular satellites. Most regular satellites are thought to have formed in place from debris that orbited the particular planet. One large moon (Triton) and many of the small outer moons of the giant planets revolve in the retrograde direction: east to west, opposite the direction of the rotation of their planets. These retrograde satellites tend to orbit far from their primaries and were probably captured by the planets’ gravitational fields some time after the formation of the solar system. Moons that orbit their planets in a retrograde direction are sometimes called irregular satellites.

Most satellites are tidally locked, meaning they keep the same face toward their primary at all times. For such satellites, the period of rotation on their axes matches the period of their orbits. Most satellites around planets also orbit in roughly the same plane as their planet’s equator. However, retrograde satellites, including Triton, often have steeply inclined orbits—another indication that such moons are captured bodies.

Orbiting satellites exert gravitational pulls on their primary and on other orbiting moons or objects. The Moon causes ocean tides to rise and fall on Earth in conjunction with the gravitational pull of the Sun. It also keeps the tilt of Earth on its axis relatively stable. Early in Earth’s history, the Moon helped slow the rapid rotation of our planet, making our periods of night and day more comfortable for complex life forms. The moons that orbit Jupiter and Saturn interact with other moons circling around each planet, creating tidal stresses that heat the interiors of some moons. Saturn’s complex system of rings and moons includes moons that share the same orbit and moons that control the shape of the planet’s rings.

IV

Special Features of Satellites

Most of the satellites in the solar system are ancient objects that probably formed shortly after the planets took shape about 4.6 billion years ago. Moons around planets and dwarf planets range in size from planetlike bodies that have a core, a mantle, and a crust to small chunks of debris. Satellites around asteroids, centaurs, and KBOs may be the result of collisions or the breakup of larger objects, or even the gravitational capture of passing objects.

Earth’s moon is made of rocky material thought to have come from a collision between Earth and another object the size of Mars. Except for Triton, most of the major moons that orbit the giant planets Jupiter, Saturn, Uranus, and Neptune most likely condensed from debris left in a disk around each of these planets. Moons in the outer solar system are made of water ice and other frozen material with a mixture of rock. In the cold temperatures from Jupiter outward, ice is hard as rock. Only volcanic Io is completely rocky, likely having lost its original water long ago. Triton is thought to be a large planetlike body from the Kuiper Belt that was captured by Neptune, so its composition may be more like that of the dwarf planet Pluto.

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