![]() Himalia as seen by spacecraft Cassini | |
Discovery | |
---|---|
Discovered by | C. D. Perrine |
Discovery date | December 3, 1904[1] |
Designations | |
Pronunciation | /haɪˈmeɪliə/ 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 | |
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][a] |
~ 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.[b]
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]
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:
“ | 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.[11] | ” |
The moon was sometimes called Hestia, after the Greek goddess, from 1955 to 1975.[12]
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]
Himalia's rotational period is 7 h 46 m 55±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]
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]
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 |
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]
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]