Himalia
Himalia.png
Himalia as seen by spacecraft Cassini
Discovery [1]
Discovered byCharles D. Perrine
Discovery siteLick Observatory
Discovery date3 December 1904
Designations
Designation
Jupiter VI
Pronunciation/hɪˈmliə/ or /hɪˈmɑːliə/[2]
Named after
Ἱμαλία Himalia
AdjectivesHimalian[3]
Orbital characteristics[4]
Epoch 27 April 2019 (JD 2458600.5)
Observation arc114.32 yr (41,728 d)
0.0761287 AU (11,388,690 km)
Eccentricity0.1537860
+248.29 d
3.312 km/s
94.30785°
1° 26m 59.616s / day
Inclination29.90917° (to the ecliptic)
44.99935°
21.60643°
Satellite ofJupiter
GroupHimalia group
Physical characteristics
Dimensions205.6 × 141.4 km (occultation, projected)[5]
150±20 × 120±20 km (Cassini estimate)[6]
Mean diameter
170 km (ground-based estimate)[7][6]
139.6±1.7 km[8]
~ 91000 km2
Volume~ 2600000 km3
Mass(4.2±0.6)×1018 kg[9]
Mean density
2.6 g/cm3 (assumed)[7]
1.63 g/cm3 (assuming radius 85 km)[9][a]
~ 0.062 m/s2 (0.006 g)
~ 0.100 km/s
7.7819±0.0005 h[10]
Albedo0.057±0.008[8]
0.04[7][6]
Temperature~ 124 K
14.6[7]
7.9[4]

Himalia /hɪˈmliə/, or Jupiter VI, is the largest irregular satellite of Jupiter, with a diameter of at least 140 km (90 mi).[5] 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.[b]

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.[11][12]

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;[13] 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:

Unfortunately the numeration of Jupiter's satellites is now in precisely the same confusion as that of Saturn's system was before the numbers were abandoned and names substituted. A similar course would seem to be advisable here; the designation V for the inner satellite [Amalthea] was tolerated for a time, as it was considered to be in a class by itself; but it has now got companions, so that this subterfuge disappears. The substitution of names for numerals is certainly more poetic.[14]

The moon was sometimes called Hestia, after the Greek goddess, from 1955 to 1975.[15]

Orbit

Animation of Himalia's orbit.
   Jupiter ·    Himalia ·   Callisto

At a distance of about 11,400,000 km (7,100,000 mi) from Jupiter, Himalia takes about 250 Earth days to complete one orbit around Jupiter.[16] 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.[17] Their orbits are continuously changing due to solar and planetary perturbations.[18]

Physical characteristics

Himalia's rotational light curve from Earth-based observations taken between August and October 2010.[10]
Himalia observed by the Wide-field Infrared Survey Explorer (WISE) spacecraft in 2014

Himalia's rotational period is 7 h 46 m 55±2 s.[10] 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.[19] Measurements by Cassini confirm a featureless spectrum, with a slight absorption at 3 μm, which could indicate the presence of water.[20]

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).[6][7] In May 2018, Himalia occulted a star, allowing for precise measurements of its size.[5] The occultation was observed from the US state of Georgia.[5] 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.[5]

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.[9] 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.[7]

Himalia's density will depend on whether it has an average radius of about 67 km (geometric mean from Cassini)[9] or a radius closer to 85 km.[7]

Cassini image of Himalia, taken in December 2000 from a distance of 4.4 million kilometres
Source Radius
km
Density
g/cm³
Mass
kg
Emelyanov 67 3.33 4.2×1018
Emelyanov 85 1.63[a] 4.2×1018
JPL SSD 85 2.6 6.7×1018

Exploration

Phases of Himalia imaged by the LORRI instrument aboard New Horizons

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.[6]

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.[21]

Possible relationship with Jupiter's rings

New Horizons image of possible Himalia ring

The small moon Dia, 4 kilometres in diameter, had gone missing since its discovery in 2000.[22] 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.[23] However, the recovery of Dia in 2010 and 2011[24] disproves the link between Dia and the Himalia ring, although it is still possible that a different moon may have been involved since an impact by an object the size of Dia would produce far more material than the predicted lower limit volume of ejected material.[25]

See also