S/2018 J 4
A small irregular moon of Jupiter in the Carpo group.
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S/2018 J 4 is a small, irregular moon of Jupiter, roughly 2 km across, first spotted by Scott S. Sheppard on May 11, 2018, using the Víctor M. Blanco Telescope in Chile. Its discovery was officially announced on January 20, 2023, after enough orbital data had been gathered. This moon follows a highly inclined prograde orbit, tilted about 53° to the ecliptic, and shares its orbital region with Carpo—a moon once considered an oddball until S/2018 J 4 turned up.
On average, its orbit has a semi-major axis of 16.3 million km, an eccentricity of 0.18, and an inclination of 50°. Like Carpo, its extreme tilt puts it in the Lidov–Kozai resonance, where its eccentricity and inclination trade off over time while its argument of pericenter stays fixed, oscillating around 270° relative to the ecliptic. For reference, Carpo’s own eccentricity and inclination swing between 0.19–0.69 and 44–59°, respectively. All irregular Jovian moons, including this one, have orbits that shift noticeably due to gravitational tugs from the Sun and other planets.
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.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: S/2018 J 4 (CC BY-SA 4.0).
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