S/2018 J 4
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 Discovery images of S/2018 J 4 from the Cerro Tololo Observatory's Dark Energy Camera on 11 May 2018 | |
| Discovery[1][2] | |
|---|---|
| Discovered by | Scott S. Sheppard |
| Discovery site | Cerro Tololo Obs. |
| Discovery date | 11 May 2018 |
| Designations | |
| S/2018 J 4 | |
| Orbital characteristics[3] | |
| Epoch 1 January 2000 (JD 2451545.0) | |
| Observation arc | 6 years 2024-12-03 (last obs)[4] |
| Earliest precovery date | 2018-05-11 |
| Satellite of | Jupiter |
| Group | Carpo |
| Proper orbital elements | |
Proper semi-major axis | 16,328,500 km (0.109149 AU) |
Proper eccentricity | 0.177 |
Proper inclination | 50.2° (to ecliptic) |
Proper orbital period | 1.17 years (426.26 d) |
Precession of perihelion | N/A arcsec / yr |
Precession of the ascending node | 6638.868 arcsec / yr |
| Physical characteristics | |
| ≈2 km[5] | |
| Albedo | 0.04 (assumed)[5] |
| 23.5[5] | |
| 16.75 (28 obs)[4][1] | |
S/2018 J 4 is a small outer natural satellite of Jupiter 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.[1] The satellite has a diameter of about 2 km (1.2 mi) for an absolute magnitude of 16.7.[5]
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.[5] 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.[6] 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.[3]
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.[7] For example, the Lidov–Kozai resonance causes Carpo's eccentricity and inclination to fluctuate between 0.19–0.69 and 44–59°, respectively.[6] S/2018 J 4's argument of pericenter oscillates about 270° with respect to the ecliptic, which keeps its perijove always above Jupiter and apojove below Jupiter.[3]
