Himalia (astronomy)

Himalia (astronomy), small satellite of the planet Jupiter. Himalia orbits Jupiter at an average distance of about 11.5 million km (about 6.89 million mi), completing one orbit in about 250 Earth days. Himalia’s orbit is shaped like an oval and is tilted about 28 degrees to Jupiter’s equator.

Scientists believe Himalia has an irregular shape, measuring about 190 km (about 110 mi) in diameter. The moon is the largest of eight small moons that orbit Jupiter beyond the large moon Callisto, but Himalia is still only about one-eighteenth as large as Earth’s moon. No spacecraft has flown past Himalia, so it has never been studied from a close distance. Little is known of Himalia’s interior structure or composition, though its density indicates that it is more rock than ice.

Himalia is the innermost of four moons that are called the a moons because their names end with the letter a. The a moons all have similar orbits around Jupiter and similar densities. Astronomers theorize that the moons started as an asteroid orbiting the sun about four billion years ago. The asteroid flew through the dense dust cloud surrounding Jupiter and collided with particles within the cloud. These collisions slowed down and shattered the asteroid. The four largest fragments slowed down enough to be captured by Jupiter’s gravity and become the a moons.

Argentine-American astronomer Charles Dillon Perrine discovered Himalia in 1904. The moon is named for a character in Roman mythology, Himalia, a nymph who bore three sons to the god Jupiter.