Himalia: Jupiter’s largest irregular prograde moon

Physical characteristics and orbit

Himalia is noticeably elongated rather than round, indicating that its gravity is not strong enough to shape it into a sphere. Its diameter is on the order of a few hundred kilometres, making it substantially smaller than Jupiter’s major moons but large for an irregular satellite. It orbits far from Jupiter, with a long orbital period and a significant orbital inclination and eccentricity compared with the regular satellites.

• Shape: non-spherical, elongated.
• Surface: dark, spectrally similar to carbonaceous (C-type) bodies.
• Orbit: distant, prograde, and inclined relative to Jupiter’s equator.

Discovery and name

Himalia was discovered by astronomer Charles Dillon Perrine at the Lick Observatory on December 3, 1904. The name comes from a nymph of Greek myth, Himalia, who was associated with Zeus; names from classical mythology are commonly used for Jupiter’s moons. The satellite’s discovery was one of the earliest finds of small Jovian companions beyond the four large Galilean satellites.

Origin and group membership

Himalia is the largest member of a cluster of prograde irregular moons known collectively as the Himalia group. The similar orbits and physical properties of the group’s members suggest a common origin—most likely the breakup of a larger parent body captured by Jupiter’s gravity. Collisional fragmentation explains the spread of sizes and slight differences in orbital elements among the group.

Observations and significance

Himalia has been examined by telescopes on the ground and observed at a distance by spacecraft missions, providing limited imaging and spectral data that help constrain its composition and rotation. Studying Himalia and its kin sheds light on capture processes, collisional evolution in the outer Jupiter system, and the population of primitive small bodies in the early Solar System.

For additional context see entries on irregular satellites, the discovery site at Lick Observatory, the mythological nymph Himalia, the god Zeus, and the planet Jupiter.