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Adrastea (moon)
Encyclopedia
Adrastea also known as , is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in Voyager 2
probe photographs taken in 1979, making it the first natural satellite
to be discovered from images taken by an interplanetary spacecraft
, rather than through telescopic photography. It was officially named after the mythological Adrastea, foster mother of Greek god Zeus
—the equivalent of Roman god Jupiter
. Adrastea is one of the few moons in the Solar System
known to orbit its planet in less than the length of that planet's day. It orbits at the edge of Jupiter's Main Ring and is thought to be the main contributor of material to the Rings of Jupiter
. Despite observations made in the 1990s by the Galileo spacecraft
, very little is known about the moon's physical characteristics outside its size and the fact that it is tidally locked to Jupiter
.
Adrastea was discovered by David C. Jewitt and G. Edward Danielson in Voyager 2
probe photographs taken on July 8, 1979, and received the designation . Although it appeared only as a dot, it was the first moon to be discovered by an interplanetary spacecraft. Soon after its discovery, two other of the inner moons of Jupiter (Thebe
and Metis
) were observed in the images taken a few weeks earlier by Voyager 1
. The Galileo spacecraft
was able to determine the moon's shape in 1998, but the images remain poor. In 1983, Adrastea was officially named after the Greek nymph Adrastea, the daughter of Zeus
and his lover Ananke
.
, around 0.86 g/cm³, its mass can be estimated at about 2 kg. Amalthea's density implies that the moon is composed of water ice
with a porosity
of 10–15%, and Adrastea may be similar.
No surface details of Adrastea are known, due to the low resolution of available images.
at a radius of about 129,000 km (1.806 Jupiter radii) at the exterior edge of the planet's Main Ring.
Adrastea is one of only three moons in the Solar System
known to orbit its planet in less than the length of that planet's day—the other two being Jupiter's innermost moon Metis
, and Mars
' moon Phobos
. The orbit has very small eccentricity
and inclination
—around 0.0015 and 0.03°, respectively. Inclination is relative to the equator of Jupiter
.
Due to tidal locking
, Adrastea rotates synchronously with its orbital period, keeping one face always looking toward the planet. Its long axis is aligned towards Jupiter
, this being the lowest energy configuration.
The orbit of Adrastea lies inside Jupiter
's synchronous orbit
radius (as does Metis
’s), and as a result, tidal forces are slowly causing its orbit to decay so that it will one day impact Jupiter. If its density is similar to Amalthea's then its orbit would actually lie within the fluid Roche limit
. However, since it is not breaking up, it must still lie outside its rigid Roche limit.
. This appears to consist primarily of material that is ejected from the surfaces of Jupiter's four small inner satellites by meteorite impacts. It is easy for the impact ejecta to be lost from these satellites into space. This is because, due to the satellites' low density, their surfaces lie rather close to the edge of their Roche spheres.
It seems that Adrastea is the most copious source of this ring material, as evidenced by the densest ring (the Main Ring) being located at and within Adrastea's orbit. More precisely, the orbit of Adrastea lies near the outer edge of Jupiter's Main Ring. The exact extent of visible ring material depends on the phase angle of the images: in forward-scattered light
Adrastea is firmly outside the Main Ring, but in back-scattered light (which reveals much bigger particles) there appears to also be a narrow ringlet outside Adrastea's orbit.
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
probe photographs taken in 1979, making it the first natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
to be discovered from images taken by an interplanetary spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
, rather than through telescopic photography. It was officially named after the mythological Adrastea, foster mother of Greek god Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...
—the equivalent of Roman god Jupiter
Jupiter (mythology)
In ancient Roman religion and myth, Jupiter or Jove is the king of the gods, and the god of the sky and thunder. He is the equivalent of Zeus in the Greek pantheon....
. Adrastea is one of the few moons in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
known to orbit its planet in less than the length of that planet's day. It orbits at the edge of Jupiter's Main Ring and is thought to be the main contributor of material to the Rings of Jupiter
Rings of Jupiter
The planet Jupiter has a system of rings, known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe and thoroughly investigated in the 1990s...
. Despite observations made in the 1990s by the Galileo spacecraft
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...
, very little is known about the moon's physical characteristics outside its size and the fact that it is tidally locked to Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
.
Discovery and observations
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
probe photographs taken on July 8, 1979, and received the designation . Although it appeared only as a dot, it was the first moon to be discovered by an interplanetary spacecraft. Soon after its discovery, two other of the inner moons of Jupiter (Thebe
Thebe (moon)
Thebe also known as ', is the fourth of Jupiter's moons by distance from the planet. It was discovered by Stephen P. Synnott in images from the Voyager 1 space probe taken on March 5, 1979, while orbiting around Jupiter...
and Metis
Metis (moon)
Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...
) were observed in the images taken a few weeks earlier by Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
. The Galileo spacecraft
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...
was able to determine the moon's shape in 1998, but the images remain poor. In 1983, Adrastea was officially named after the Greek nymph Adrastea, the daughter of Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...
and his lover Ananke
Ananke (mythology)
In Greek mythology, Ananke, also spelled Anangke, Anance, or Anagke , was the personification of destiny, necessity and fate, depicted as holding a spindle. She marks the beginning of the cosmos, along with Chronos...
.
Physical characteristics
Adrastea has an irregular shape and measures 20×16×14 km across. This makes it the smallest of the four inner moons. The bulk, composition and mass of Adrastea are not known, but assuming that its mean density is like that of AmaltheaAmalthea (moon)
Amalthea is the third moon of Jupiter in order of distance from the planet. It was discovered on September 9, 1892, by Edward Emerson Barnard and named after Amalthea, a nymph in Greek mythology. It is also known as '....
, around 0.86 g/cm³, its mass can be estimated at about 2 kg. Amalthea's density implies that the moon is composed of water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
with a porosity
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of 10–15%, and Adrastea may be similar.
No surface details of Adrastea are known, due to the low resolution of available images.
Orbit
Adrastea is the smallest and second closest member of the inner Jovian satellite family. It orbits JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
at a radius of about 129,000 km (1.806 Jupiter radii) at the exterior edge of the planet's Main Ring.
Adrastea is one of only three moons in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
known to orbit its planet in less than the length of that planet's day—the other two being Jupiter's innermost moon Metis
Metis (moon)
Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...
, and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
' moon Phobos
Phobos (moon)
Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...
. The orbit has very small eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
and inclination
Inclination
Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...
—around 0.0015 and 0.03°, respectively. Inclination is relative to the equator of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
.
Due to tidal locking
Tidal locking
Tidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, the same side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner...
, Adrastea rotates synchronously with its orbital period, keeping one face always looking toward the planet. Its long axis is aligned towards Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, this being the lowest energy configuration.
The orbit of Adrastea lies inside Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
's synchronous orbit
Synchronous orbit
A synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...
radius (as does Metis
Metis (moon)
Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...
’s), and as a result, tidal forces are slowly causing its orbit to decay so that it will one day impact Jupiter. If its density is similar to Amalthea's then its orbit would actually lie within the fluid Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
. However, since it is not breaking up, it must still lie outside its rigid Roche limit.
Relationship with Jupiter's rings
Adrastea is the largest contributor to material in Jupiter's ringsRings of Jupiter
The planet Jupiter has a system of rings, known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe and thoroughly investigated in the 1990s...
. This appears to consist primarily of material that is ejected from the surfaces of Jupiter's four small inner satellites by meteorite impacts. It is easy for the impact ejecta to be lost from these satellites into space. This is because, due to the satellites' low density, their surfaces lie rather close to the edge of their Roche spheres.
It seems that Adrastea is the most copious source of this ring material, as evidenced by the densest ring (the Main Ring) being located at and within Adrastea's orbit. More precisely, the orbit of Adrastea lies near the outer edge of Jupiter's Main Ring. The exact extent of visible ring material depends on the phase angle of the images: in forward-scattered light
Light scattering
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
Adrastea is firmly outside the Main Ring, but in back-scattered light (which reveals much bigger particles) there appears to also be a narrow ringlet outside Adrastea's orbit.