Himalia (moon)
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Himalia (), also known as Jupiter VI, is the largest irregular satellite of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
. With a diameter of at least , it is the sixth largest Jovian satellite, after the four
Galilean moons The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede (moon), Ganymede, Callisto (moon), Callisto, Io (moon), Io, and Europa (moon), Europa. They are the most apparent m ...
and Amalthea. It was discovered by Charles Dillon Perrine at the
Lick Observatory The Lick Observatory is an astronomical observatory owned and operated by the University of California. It is on the summit of Mount Hamilton (California), Mount Hamilton, in the Diablo Range just east of San Jose, California, United States. The ...
on 3 December 1904 and is named after the
nymph A nymph (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...
Himalia, who bore three sons of
Zeus Zeus (, ) is the chief deity of the List of Greek deities, Greek pantheon. He is a sky father, sky and thunder god in ancient Greek religion and Greek mythology, mythology, who rules as king of the gods on Mount Olympus. Zeus is the child ...
(the Greek equivalent of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...
). It is one of the largest planetary moons in the Solar System not imaged in detail, and the third largest not imaged in detail within the orbit of
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
.


Discovery

Himalia was discovered by Charles Dillon Perrine at the
Lick Observatory The Lick Observatory is an astronomical observatory owned and operated by the University of California. It is on the summit of Mount Hamilton (California), Mount Hamilton, in the Diablo Range just east of San Jose, California, United States. The ...
on 3 December 1904 in photographs taken with the 36-inch Crossley reflecting telescope which he had recently rebuilt. ;
;
;
Himalia is Jupiter's most easily observed small satellite; though Amalthea is brighter, its proximity to the planet's brilliant disk makes it a far more difficult object to view.


Name

Himalia is named after the
nymph A nymph (; ; sometimes spelled nymphe) is a minor female nature deity in ancient Greek folklore. Distinct from other Greek goddesses, nymphs are generally regarded as personifications of nature; they are typically tied to a specific place, land ...
Himalia, who bore three sons of
Zeus Zeus (, ) is the chief deity of the List of Greek deities, Greek pantheon. He is a sky father, sky and thunder god in ancient Greek religion and Greek mythology, mythology, who rules as king of the gods on Mount Olympus. Zeus is the child ...
(the Greek equivalent of Jupiter). The moon did not receive its present name until 1975; before then, it was simply known as or Jupiter Satellite VI, although calls for a full name appeared shortly after its and Elara's discovery; A.C.D. Crommelin wrote in 1905: The moon was sometimes called Hestia, after the Greek goddess, from 1955 to 1975.


Orbits


At a distance of about from Jupiter, Himalia takes about 250
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
day A day is the time rotation period, period of a full Earth's rotation, rotation of the Earth with respect to the Sun. On average, this is 24 hours (86,400 seconds). As a day passes at a given location it experiences morning, afternoon, evening, ...
s to complete one orbit around Jupiter. It is the largest member of the Himalia group, which are a group of small moons orbiting Jupiter at a distance from to , with inclined orbits at an angle of 27.5 degrees to Jupiter's
equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...
. Their orbits are continuously changing due to solar and planetary perturbations.


Physical characteristics

Himalia's rotational period is . Himalia appears neutral in color (grey), like the other members of its group, with colour indices B−V=0.62, V−R=0.4, similar to a
C-type asteroid C-type (carbonaceous ) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...
. Measurements by ''Cassini'' confirm a featureless
spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
, with a slight absorption at , which could indicate the presence of water. Although Himalia is the sixth-largest moon of Jupiter, it is the fifth-most massive. Amalthea is only a few km bigger, but less massive. Resolved images of Himalia by ''Cassini'' have led to a size estimate of , while ground-based estimates suggest that Himalia is large, with a diameter around . In May 2018, Himalia occulted a star, allowing for precise measurements of its size. The occultation was observed from the US state of
Georgia Georgia most commonly refers to: * Georgia (country), a country in the South Caucasus * Georgia (U.S. state), a state in the southeastern United States Georgia may also refer to: People and fictional characters * Georgia (name), a list of pe ...
. From the occultation, Himalia was given a size estimate of , in agreement with earlier ground-based estimates.


Mass

In 2005, Emelyanov estimated Himalia to have a mass of ( GM=0.28±0.04), based on a perturbation of Elara on July 15, 1949 (when the distance between them became a mere 64246.04 kilometers). JPL's Solar System dynamics website assumes that Himalia has a mass of (GM=0.15) with a radius of . Himalia's density will depend on whether it has an average radius of about (
geometric mean In mathematics, the geometric mean is a mean or average which indicates a central tendency of a finite collection of positive real numbers by using the product of their values (as opposed to the arithmetic mean which uses their sum). The geometri ...
from ''Cassini'') or a radius closer to .


Exploration

In November 2000, the '' Cassini'' spacecraft, en route to
Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...
, made a number of images of Himalia, including photos from a distance of 4.4 million km. Himalia covers only a few pixels, but seems to be an elongated object with axes and , close to the Earth-based estimations. In February and March 2007, the ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
'' spacecraft en route to Pluto made a series of images of Himalia, culminating in photos from a distance of 8 million km. Again, Himalia appears only a few pixels across.


Himalia ring

In September 2006, as NASA's ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
'' mission to
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
approached Jupiter for a
gravity assist A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby (spaceflight), flyby which makes use of the relative movement (e.g. orbit around the Sun) and gra ...
, it photographed what appeared to be a faint new planetary ring parallel with and slightly inside Himalia's orbit. Because the small (4-km) moon Dia, which had a similar orbit to Himalia, had gone missing since its discovery in 2000, there was some speculation that the ring could be debris from an impact of Dia into Himalia, suggesting that Jupiter continued to gain and lose small moons through collisions. However, an impact by an object the size of Dia would produce far more material than the calculated amount of ejected material needed to form the ring, although it is possible that a smaller, unknown moon may have been involved instead. The recovery of Dia in 2010 and 2011 disproved any connection between Dia and the Himalia ring.


Notes


See also

* Moons of Jupiter


References


External links


"Himalia: Overview"
b
NASA's Solar System Exploration




(by Scott S. Sheppard)
Two Irregular Satellites of Jupiter
(Himalia & Elara: Remanzacco Observatory: November 23, 2012) {{DEFAULTSORT:Himalia (Moon) Himalia group Moons of Jupiter Irregular satellites 19041203 Discoveries by Charles D. Perrine Moons with a prograde orbit