Elara

Verified Timeline

Lore & Background

Elara orbits Jupiter at an average distance of 11,703,130 km in 258.65 days, at an inclination of about 30° to the ecliptic, in a prograde direction and with an eccentricity of 0.196. Its orbit is continuously changing due to solar and planetary perturbations. Elara belongs to the Himalia group, a prograde group of moons orbiting between 11 and 13 million km from Jupiter at inclinations between 27 and 30°, and eccentricities between 0.11 and 0.24. Elara has a diameter of about 80 kilometers, with a measured albedo of about 4.6%, making it the eighth largest moon of Jupiter. It is about 2.6% of the size of Europa. It is half the size of Himalia, making it the second largest member of the Himalia group. Like the other members of the Himalia group, the satellite appears gray (color indices B−V=0.66, R−V=0.36), which is typical for C-type asteroids. A measured low beam value of 0.79 ± 0.03 indicates that Elara has considerable surface roughness. In 1987 Elara was observed in an attempt to determine a rotation period. The result was inconclusive as its light curve was found to be too flat, and the small variations in brightness were not periodic in nature. This is in contrast to some earlier studies which reported larger brightness variations. It was suggested that the low magnitude variation could be explained if Elara's shape was close to spherical or its pole of rotation was being viewed head-on, or it simply had too long of a rotation period to detect. Later observations in 2020 found a possible rotation period of 9.596 hours. Elara probably did not form near Jupiter but was captured by Jupiter later. Like the other members of the Himalia group, which have similar orbits, Elara is probably the remnant of a broken, captured heliocentric asteroid.

In Their Own Story

In February and March 2007, the New Horizons spacecraft to Pluto captured Elara in several LORRI images from a distance of 8 million km (5 million mi).

Reader's Guide

Elara is a real moon of Jupiter, not a character from fiction. It was discovered on January 6, 1905, by Charles Dillon Perrine at the Lick Observatory. Its orbit averages 11,703,130 km from Jupiter, taking 258.65 days, with an inclination of about 30° and eccentricity of 0.196. It belongs to the Himalia group of prograde irregular moons. Physically, Elara is about 80 km in diameter with an albedo of about 4.6%, making it Jupiter's eighth largest moon and the second largest in the Himalia group. Its gray color (B−V=0.66, R−V=0.36) is typical for C-type asteroids. A 2020 observation found a possible rotation period of 9.596 hours. Elara is thought to be a captured heliocentric asteroid, a remnant of a broken parent body. The New Horizons spacecraft imaged Elara in 2007 from 8 million km away.

Did You Know?

• Elara was discovered on January 6, 1905, the day after the discovery of Himalia was announced.
• It was sometimes called 'Hera' between 1955 and 1975 before receiving its present name in 1975.
• Elara has a diameter of about 80 kilometers and an albedo of about 4.6%.
• A 2020 observation found a possible rotation period of 9.596 hours for Elara.
• The New Horizons spacecraft captured Elara in images from 8 million km away in 2007.