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.
Name
Himalia is named after the nymph Himalia, who bore three sons of Zeus. The moon did not receive its present name until 1975; before then, it was simply known as 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: The moon was sometimes called Hestia, after the Greek goddess, from 1955 to 1975.
Orbit
At a distance of about 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 to, 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 . Himalia appears neutral in color, 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. Resolved images of Himalia by Cassini have led to a size estimate of, while ground-based estimates suggest that Himalia is large, with a diameter around. In May 2018, Himalia occulteda 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, in agreement with earlier ground-based estimates.
Mass
In 2005, Emelyanov estimated Himalia to have a mass of , based on a perturbation of Elara on July 15, 1949. JPL's Solar System dynamics web site assumes that Himalia has a mass of with a radius of. Himalia's density will depend on whether it has an average radius of about or a radius closer to.
Source
Radius km
Density g/cm³
Mass kg
Emelyanov
67
3.33
4.2
Emelyanov
85
1.63
4.2
JPL SSD
85
2.6
6.7
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 and, 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.
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 since an impact by an object the size of Dia would produce far more material than the predicted lower limit volume of ejected material.
