Himalia (moon)

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Himalia
Himalia.png
Himalia as seen by spacecraft Cassini
Discovery
Discovered by C. D. Perrine
Discovery dateDecember 3, 1904 [1]
Designations
Pronunciation /hˈmliə/ hy-MAY-lee-ə or /hɪˈmɑːliə/ hi-MAH-lee-ə
Adjectives Himalian
Orbital characteristics
Periapsis 9,782,900 km
Apoapsis 13,082,000 km
Mean orbit radius
11,460,000 km [2]
Eccentricity 0.16 [2]
250.56 d (0.704 a) [2]
Average orbital speed
3.312 km/s
Inclination
  • 27.50° (to the ecliptic)
  • 29.59° (to Jupiter's equator) [2]
Satellite of Jupiter
Physical characteristics
Mean radius
102.8 × 70.7 km (stellar occultation) [3]
85 km (ground-based estimate) [4] [5]
75±10 × 60±10 km (Cassini estimate) [5]
~90800 km2
Volume ~2570000 km3
Mass (4.2±0.6)×1018  kg [6]
Mean density
2.6 g/cm3 (assumed) [4]
1.63 g/cm3 (assuming radius 85 km) [6] [lower-alpha 1]
~0.062 m/s2 (0.006 g)
~0.100 km/s
Sidereal rotation period
7.782 h [7]
Albedo 0.04 [4] [5]
Temperature ~124 K
14.6 [4]

    Himalia (Jupiter VI) is the largest irregular satellite of Jupiter, with an estimated diameter of at least 205 km (127 mi). [3] It is the fifth largest Jovian satellite, after the four Galilean moons. It was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904 and is named after the nymph Himalia, who bore three sons of Zeus (the Greek equivalent of Jupiter). [1] It is one of the largest planetary moons in the Solar System not imaged in detail, and the largest within the orbit of Neptune. [lower-alpha 2]

    Contents

    Discovery

    Himalia was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904. [1] 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. [8] [9]

    Name

    Himalia is named after the nymph Himalia, who bore three sons of Zeus (the Greek equivalent of Jupiter). The moon did not receive its present name until 1975; [10] before then, it was simply known as Jupiter VI 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. [12]

    Orbit

    Animation of Himalia's orbit.
Jupiter *    Himalia *   Callisto Animation of Himalia orbit around Jupiter.gif
    Animation of Himalia's orbit.
       Jupiter ·   Himalia ·  Callisto

    At a distance of about 11,500,000 km (7,100,000 mi) from Jupiter, Himalia takes about 251 Earth days to complete one orbit around Jupiter. [13] It is the largest member of the Himalia group, which are a group of small moons orbiting Jupiter at a distance from 11,400,000 km (7,100,000 mi) to 13,000,000 km (8,100,000 mi), with inclined orbits at an angle of 27.5 degrees to Jupiter's equator. [14] Their orbits are continuously changing due to solar and planetary perturbations.[ citation needed ]

    Physical characteristics

    Himalia's rotational light curve from Earth-based observations taken between August and October 2010. Himalia Lightcurve2010.png
    Himalia's rotational light curve from Earth-based observations taken between August and October 2010.

    Himalia's rotational period is 7 h46 m55±2 s. [7] 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. [15] Measurements by Cassini confirm a featureless spectrum, with a slight absorption at 3 μm, which could indicate the presence of water. [16]

    Resolved images of Himalia by Cassini have led to a size estimate of 150 km × 120 km (93 mi × 75 mi), while ground-based estimates suggest that Himalia is large, with a diameter around 170 km (110 mi). [5] [4] In May 2018, Himalia occulted a star, allowing for precise measurements of its size. [3] The occultation was observed from the US state of Georgia. [3] From the occultation, Himalia was given a size estimate of 205.6 km × 141.3 km (127.8 mi × 87.8 mi), in agreement with earlier ground-based estimates. [3]

    Mass

    In 2005, Emelyanov estimated Himalia to have a mass of (4.2±0.6)×1018  kg (GM=0.28±0.04), based on a perturbation of Elara on July 15, 1949. [6] JPL's Solar System dynamics web site assumes that Himalia has a mass of 6.7×1018 kg (GM=0.45) with a radius of 85 km. [4]

    Himalia's density will depend on whether it has an average radius of about 67 km (geometric mean from Cassini) [6] or a radius closer to 85 km. [4]

    Cassini image of Himalia, taken in December 2000 from a distance of 4.4 million kilometres Cassini-Huygens Image of Himalia.png
    Cassini image of Himalia, taken in December 2000 from a distance of 4.4 million kilometres
    SourceRadius
    km
    Density
    g/cm³
    Mass
    kg
    Emelyanov 67 3.334.2×1018
    Emelyanov851.63 [lower-alpha 1] 4.2×1018
    JPL SSD852.66.7×1018

    Exploration

    In November 2000, the Cassini spacecraft, en route to Saturn, 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 150±20 and 120±20 km, close to the Earth-based estimations. [5]

    In February and March 2007, the New Horizons 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. [17]

    Possible relationship with Jupiter's rings

    New Horizons image of possible Himalia ring Himalia ring.jpg
    New Horizons image of possible Himalia ring

    The small moon Dia, 4 kilometres in diameter, had gone missing since its discovery in 2000. [18] One theory was that it had crashed into the much larger moon Himalia, 170 kilometres in diameter, creating a faint ring. This possible ring appears as a faint streak near Himalia in images from NASA's New Horizons mission to Pluto. This suggests that Jupiter sometimes gains and loses small moons through collisions. [19] However, the recovery of Dia in 2010 and 2011 [20] disproves the link between Dia and the Himalia ring, although it is still possible that a different moon may have been involved.[ citation needed ]

    See also

    Notes

    1. 1 2 Density = GM / G / (Volume of a sphere of 85km) = 1.63 g/cm3
    2. It is the largest with the exception of some of the moons of Neptune and several trans-Neptunian objects, particularly Dysnomia, the moon of Eris.

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