Himalia, also known as Jupiter VI, is the largest irregular satellite of Jupiter. With a diameter of at least 140 km, it is the sixth largest Jovian satellite, after the four Galilean moons and Amalthea. 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 third largest not imaged in detail within the orbit of Neptune.

Discovery

Himalia was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904 in photographs taken with the 36-inch Crossley reflecting telescope which he had recently rebuilt. 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.

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

Orbit

At a distance of about 11400000 km from Jupiter, Himalia takes about 250 Earth days to complete one orbit around Jupiter. It is the largest member of the Himalia group, which are a group of small moons orbiting Jupiter at a distance from 11400000 km to 13000000 km, with inclined orbits at an angle of 27.5 degrees to Jupiter's equator. Their orbits are continuously changing due to solar and planetary perturbations.

Physical characteristics

Himalia's rotational period is 7 h 46 m 55 s. 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 3 μm, which could indicate the presence of water. Although Himalia is the sixth-largest moon of Jupiter, it is the fifth-most massive. Amalthea is only a few km bigger, but less massive. Resolved images of Himalia by Cassini have led to a size estimate of 150 x, while ground-based estimates suggest that Himalia is large, with a diameter around 170 km. In May 2018, Himalia occulted a star, allowing for precise measurements of its size. The occultation was observed from the US state of Georgia. From the occultation, Himalia was given a size estimate of 205.6 x, in agreement with earlier ground-based estimates.

Mass

In 2005, Emelyanov estimated Himalia to have a mass of 4.2 kg (GM=0.28±0.04), based on a perturbation of Elara on July 15, 1949 (when the distance between them became a mere 64246.04 kilometers). JPL's Solar System dynamics website assumes that Himalia has a mass of 2.3 kg (GM=0.15) with a radius of 85 km. Himalia's density will depend on whether it has an average radius of about 67 km (geometric mean from Cassini) or a radius closer to 85 km.

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 and 120 km, close to the Earth-based estimations. 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.

Himalia ring

In September 2006, as NASA's New Horizons mission to Pluto approached Jupiter for a gravity assist, it photographed what appeared to be a faint new planetary ring parallel with and slightly inside Himalia's orbit. Because the small (4-km) moon Dia, which had a similar orbit to Himalia, had gone missing since its discovery in 2000, there was some speculation that the ring could be debris from an impact of Dia into Himalia, suggesting that Jupiter continued to gain and lose small moons through collisions. However, an impact by an object the size of Dia would produce far more material than the calculated amount of ejected material needed to form the ring, although it is possible that a smaller, unknown moon may have been involved instead. The recovery of Dia in 2010 and 2011 disproved any connection between Dia and the Himalia ring.