Elara was discovered on Jan. 5, 1905 by Charles Dillon Perrine in photographs taken with the Crossley 36-inch (0.9 meter) reflector of the Lick Observatory on Mount Hamilton at the University of California, San Jose.
Elara is the eighth largest moon of Jupiter. With a mean radius of 26.7 miles (43 km) assuming an albedo of 0.04, it's only about 2 percent the size of Europa, the smallest of the four Galilean moons. But it's about half as big as Himalia, which makes it the second biggest in the Himalia group, a family of Jovian satellites which have similar orbits and appearance, and are therefore thought to have a common origin.
Elara may be a chunk of an asteroid (a C- or D-class asteroid, judging by the fact that it reflects only about 4 percent of the light it receives), which was broken apart in a collision either before or after being captured by Jupiter's gravity. In this scenario, the other pieces became the other moons in the Himalia group: Leda, Himalia (the largest), and Lysithea. A fifth moon, called S/2000 J11, only about 1.2 miles (2 km) in radius, was considered a candidate for this group. However, it was lost before its orbit could be definitively determined. It may have crashed into Himalia, reuniting two pieces of the former asteroid, and perhaps creating a faint temporary ring of Jupiter near the orbit of Himalia.
At a distance of about 7.3 million miles (11.7 million km) from Jupiter, Elara takes nearly 260 Earth days to complete one orbit.
How Elara Got its Name
Elara is named for one of the lovers of Zeus, the Greek equivalent of the Roman god Jupiter. In Greek mythology, Zeus hid her from his wife, Hera, by placing Elara deep beneath the Earth, where she gave birth to their son, a giant called Tityas.
A name ending in "a" was chosen for this moon in keeping with the International Astronomical Union's policy for designating Jupiter's outer moons which have prograde orbits (orbiting in the same direction as Jupiter's rotation).