Himalia

Himalia

Himalia as seen by Cassini-Huygens
Discovery
Discovered by	C. D. Perrine
Discovery date	December 16, 1904
Orbital characteristics
Periapsis	9,782,900 km
Apoapsis	13,082,000 km
Mean orbit radius	11,460,000 km[1]
Eccentricity	0.16[1]
Orbital period	250.56 d (0.704 a)[1]
Average orbital speed	3.312 km/s
Inclination	27.50° (to the ecliptic) 29.59° (to Jupiter's equator)[1]
Satellite of	Jupiter
Physical characteristics
Mean radius	85 km[2]
Surface area	~90,800 km²
Volume	~2,570,000 km³
Mass	6.7 × 1018 kg[2] 4.19 × 1018 kg[3]
Mean density	2.6 g/cm³ (assumed)[2] 1.63 g/cm³ (assuming radius 85km)[3][4]
Equatorial surface gravity	~0.062 m/s2 (0.006 g)
Escape velocity	~0.100 km/s
Sidereal rotation period	~0.4 d (10 h)
Albedo	0.04[2][5]
Temperature	~124 K
Apparent magnitude	14.6[2]

Himalia (pronounced /haɪˈmeɪliə/ hye-MAY-lee-ə, or /hɪˈmɑːliə/ hi-MAH-lee-ə as in Greek ‘Ιμαλíα) is the largest irregular satellite of Jupiter, and the sixth largest overall. It was discovered by Charles Dillon Perrine at the Lick Observatory on 1904 December 3^[6] and is named after the nymph Himalia who bore three sons of Zeus (the Greek equivalent of Jupiter).

Discovery

Himalia, the largest irregular satellite of Jupiter, was discovered by Charles Dillon Perrine at the Lick Observatory on 1904 December 3.^[6]

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;^[7] 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 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.^[8]

The moon was sometimes called Hestia, after the Greek goddess, from 1955 to 1975.^[9]

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°.^[10] The orbital elements are as of January 2000.^[1] They are continuously changing due to Solar and planetary perturbations.

Physical characteristics

Himalia appears neutral (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.^[11] Measurements by Cassini confirm a featureless spectrum, with a slight absorption at 3 μm which could indicate the presence of water.^[12]

Mass

Image of Himalia taken by the New Horizons spacecraft. The moon covers only a few pixels.

In 2005, Emelyanov estimated Himalia to have a mass of 4.19 × 10¹⁸ kg (GM=0.28), based on a perturbation of Elara on July 15, 1949.^[3] JPL's Solar System Dynamics assumes that Himalia has a mass of 6.7 × 10¹⁸ (GM=0.45) with a radius of 85 km.^[2]

Himalia's density will depend on whether it has an average radius of about 67 km (geometric mean from Cassini 2000)^[3] or a radius closer to 85 km.^[2]

Exploration

Cassini-Huygens faint image of Jupiter's moon Himalia in 2000

In November 2000, the Cassini spacecraft, enroute to Saturn, made a number of images of Himalia, including photos from a distance as close as 4.4 million km. The moon 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 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.

See also

• Irregular satellites

References

External links

• Himalia Profile by NASA's Solar System Exploration
• David Jewitt pages
• Jupiter's Known Satellites (by Scott S. Sheppard)