S/2018 J 4

Lore & Background

S/2018 J 4 was discovered by Scott S. Sheppard on 11 May 2018 using the 4.0-meter Víctor M. Blanco Telescope at Cerro Tololo Observatory, Chile. It was announced by the Minor Planet Center on 20 January 2023, after observations were collected over a long enough time span to confirm the satellite's orbit. The satellite has a diameter of about 2 km (1.2 mi) for an absolute magnitude of 16.7.

S/2018 J 4 is an irregular moon of Jupiter on a highly inclined prograde orbit at an angle of 53° with respect to the ecliptic plane. It belongs to the same group as the similarly inclined moon Carpo, which was long thought to be an outlier until the discovery of S/2018 J 4. Like all irregular moons of Jupiter, S/2018 J 4's orbit is highly variable over time due to gravitational perturbations by the Sun and other planets. On average, S/2018 J 4's orbit has a semi-major axis of 16.3 million km (10.1 million mi), an eccentricity of 0.18, and a very high inclination of 50° with respect to the ecliptic.

Like Carpo, S/2018 J 4's very high inclination subjects it to the Lidov–Kozai resonance, where there is a periodic exchange between its orbital eccentricity and inclination while its argument of pericenter oscillates about a constant value without apsidally precessing. For example, the Lidov–Kozai resonance causes Carpo's eccentricity and inclination to fluctuate between 0.19–0.69 and 44–59°, respectively. S/2018 J 4's argument of pericenter oscillates about 270° with respect to the ecliptic.

Reader's Guide

S/2018 J 4 is significant as the second known member of a rare group of highly inclined prograde irregular moons of Jupiter, alongside Carpo. Its discovery resolved the long-standing outlier status of Carpo, which had been thought to be unique until S/2018 J 4 was found. The moon's orbit is subject to the Lidov–Kozai resonance due to its very high inclination, causing periodic exchanges between its eccentricity and inclination while its argument of pericenter oscillates about 270° with respect to the ecliptic. This resonance, also observed in Carpo, makes S/2018 J 4 an important object for studying dynamical processes in the Jovian system. Like all irregular moons, its orbit is highly variable over time due to gravitational perturbations by the Sun and other planets. The moon's small size—about 2 km in diameter—and faint absolute magnitude of 16.7 underscore the challenges of detecting such objects, requiring long observation spans to confirm its orbit before its announcement in January 2023.

Did You Know?

• It has a diameter of about 2 km (1.2 mi) and an absolute magnitude of 16.7.
• Its orbit is subject to the Lidov–Kozai resonance, with its argument of pericenter oscillating about 270° with respect to the ecliptic.