Adrastea (), also known as , is the second by distance, and the smallest of the four inner moons of Jupiter. It was discovered in photographs taken by ''

Voyager 2 ''Voyager 2'' is a space probe launched by NASA on August 20, 1977, to study the outer planets and interstellar space beyond the Sun's heliosphere. As a part of the Voyager program, it was launched 16 days before its twin, ''Voyager 1'', on ...

'' in 1979, making it the first

natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

to be discovered from images taken by an interplanetary

spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...

, rather than through a telescope. It was officially named after the mythological

Adrasteia In ancient Greek religion and mythology, Adrasteia (; , ), also spelled Adrastia, Adrastea, Adrestea, Adastreia or Adrasta), originally a Phrygian mountain goddess, probably associated with Cybele, was later a Cretan nymph, and daughter of Meli ...

, foster mother of the

Greek god The following is a list of gods, goddesses, and many other divine and semi-divine figures from ancient Greek mythology and ancient Greek religion. Immortals The Greeks created images of their deities for many purposes. A temple would house the ...

Zeus Zeus or , , ; grc, Δῐός, ''Diós'', label= genitive Boeotian Aeolic and Laconian grc-dor, Δεύς, Deús ; grc, Δέος, ''Déos'', label= genitive el, Δίας, ''Días'' () is the sky and thunder god in ancient Greek reli ...

—the equivalent of the

Roman god Roman mythology is the body of myths of ancient Rome as represented in the literature and visual arts of the Romans. One of a wide variety of genres of Roman folklore, ''Roman mythology'' may also refer to the modern study of these representat ...

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandth t ...

. Adrastea is one of the few moons in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

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 Ring may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell :(hence) to initiate a telephone connection Arts, entertainment and media Film and ...

. Despite observations made in the 1990s by the ''Galileo'' spacecraft, very little is known about the moon's physical characteristics other than its size and the fact that it is

tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked bo ...

Discovery and observations

Adrastea was discovered by

David C. Jewitt David Clifford Jewitt (born 1958) is a British-American astronomer who studies the Solar System, especially its minor bodies. He is based at the University of California, Los Angeles, where he is a Member of the Institute for Geophysics and Pl ...

and G. Edward Danielson in ''

'' 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 There are 82 known moons of Jupiter, not counting a number of moonlets likely shed from the inner moons. All together, they form a satellite system which is called the Jovian system. The most massive of the moons are the four Galilean moons: ...

( Thebe and

Metis Metis or Métis may refer to: Ethnic groups * Métis, recognized Indigenous communities in Canada and America whose distinct culture and language emerged after early intermarriage between First Nations peoples and early European settlers, prima ...

) were observed in the images taken a few months earlier by ''

Voyager 1 ''Voyager 1'' is a space probe launched by NASA on September 5, 1977, as part of the Voyager program to study the outer Solar System and interstellar space beyond the Sun's heliosphere. Launched 16 days after its twin ''Voyager 2'', ''Voyag ...

''. 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

and his lover

Ananke In ancient Greek religion, Ananke (; grc, Ἀνάγκη), from the common noun , "force, constraint, necessity") is the personification of inevitability, compulsion and necessity. She is customarily depicted as holding a spindle. One of the ...

. Although the '' Juno'' orbiter, which arrived at Jupiter in 2016, has a camera called

JunoCam JunoCam (or JCM) is the visible-light camera/telescope onboard NASA's ''Juno'' spacecraft currently orbiting Jupiter. The camera is operated by the JunoCam Digital Electronics Assembly (JDEA). Both the camera and JDEA were built by Malin Space ...

, it is almost entirely focused on observations of Jupiter itself. However, if all goes well, it should be able to capture some limited images of the moons

and Adrastea.JunoCam: Science and Outreach Opportunities with Juno
Hansen, C. J.; Orton, G. S. American Geophysical Union 12/2015

Physical characteristics

Adrastea has an irregular shape and measures 20×16×14 km across. A surface area estimate would be between 840 and 1,600 (~1,200) km². 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 Amalthea, 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 is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...

with a

porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...

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

at a radius of about (1.806 Jupiter radii) at the exterior edge of the planet's main ring. The orbit has very small

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

and

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

—around 0.0015 and 0.03°, respectively. Inclination is relative to the equator of Jupiter. Due to

tidal locking Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...

, 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

synchronous orbit A synchronous orbit is an orbit in which an orbiting body (usually a satellite) has a period equal to the average rotational period of the body being orbited (usually a planet), and in the same direction of rotation as that body. Simplified meanin ...

radius (as does

's), and as a result,

tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...

s 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 In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forc ...

. However, since it is not breaking up, it must still lie outside its rigid Roche limit. Adrastea is the second-fastest-moving of Jupiter's moons, with an orbital speed of 31.378 km/s.

Relationship with Jupiter's rings

Adrastea is the largest contributor to material in Jupiter's rings. 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 due to the satellites' low density and their surfaces lying close to the edge of their

Hill sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sph ...

s. 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.

Notes

References

Cited sources * * * * * * (discovery) * (naming the moon) * *

External links

Adrastea Profile
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NASA's Solar System Exploration
{{DEFAULTSORT:Adrastea (Moon) Moons of Jupiter 19790708 Discoveries by David C. Jewitt Discoveries by G. Edward Danielson Moons with a prograde orbit