Himalia as seen by Cassini–Huygens
|Discovered by||C. D. Perrine|
|Discovery date||December 3, 1904|
Mean orbit radius
|250.56 d (0.704 a)|
Average orbital speed
|±10 × 75±10 km (Cassini estimate) 60
85 ± ?? km (ground-based estimate)
|2.6 g/cm3 (assumed)
1.63 g/cm3 (assuming radius 85 km)
|~0.062 m/s2 (0.006 g)|
Sidereal rotation period
Himalia (// hy-MAY-lee-ə or // hi-MAH-lee-ə; Greek: ‘Ιμαλíα) is the largest irregular satellite of Jupiter, the sixth largest overall in size, and only the four Galilean moons of Jupiter have greater mass. 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). It is one of the largest planetary moons in the Solar System not imaged in detail, and the largest not including the moons of Neptune and several trans-Neptunian objects, particularly that of the second-largest-but-most-massive dwarf planet, Eris.
Himalia was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904. 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.
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; 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.
At a distance of about 11.5 million km from Jupiter, Himalia takes about 251 Earth days to complete one orbit. It is the largest member of the group that bears its name, the moons orbiting between 11.4 and 13 million kilometers from Jupiter at an inclination of about 27.5°. The orbital elements are as of January 2000. They are continuously changing due to solar and planetary perturbations.
Himalia's rotational period is 7 h 46 m±2 s. 55 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. Measurements by Cassini confirm a featureless spectrum, with a slight absorption at , which could indicate the presence of water. 3 µm
In 2005, Emelyanov estimated Himalia to have a mass of ×1018 kg (GM=0.28), based on a 4.19perturbation of Elara on July 15, 1949. JPL's Solar System dynamics web site assumes that Himalia has a mass of ×1018 kg (GM=0.45) with a radius of 6.7. 85 km
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 ±20 and 150±20 km, close to the Earth-based estimations. 120
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 eight million km. Again, Himalia appears only a few pixels across.
Possible relationship with Jupiter's rings
The small moon Dia, 4 kilometres in diameter, had gone missing since its discovery in 2000. 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. However, the recovery of Dia in 2010 and 2011 disproves the link between Dia and the Himalia ring, although it is still possible that a different moon may have been involved.
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- Emelyanov, N.V.; Archinal, B. A.; a'Hearn, M. F.; et al. (2005). "The mass of Himalia from the perturbations on other satellites". Astronomy and Astrophysics. 438 (3): L33–L36. Bibcode:2005A&A...438L..33E. doi:10.1051/0004-6361:200500143.
- Density = GM / G / (Volume of a sphere of 85km) = 1.63 g/cm3
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