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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. 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 an 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, which keeps its perijove always above Jupiter and apojove below Jupiter.

References

1. ^ "MPEC 2023-B51 : S/2018 J 4". Minor Planet Electronic Circulars. Minor Planet Center. 20 January 2023. Retrieved 20 January 2023.
2. "Planetary Satellite Discovery Circumstances". JPL Solar System Dynamics. NASA. Retrieved 20 January 2023.
3. ^ "Planetary Satellite Mean Elements". JPL Solar System Dynamics. NASA. Retrieved 13 February 2023.
4. ^ Sheppard, Scott S. "Moons of Jupiter". Earth & Planets Laboratory. Carnegie Institution for Science. Retrieved 20 January 2023.
5. ^ Brozović, Marina; Jacobson, Robert A. (March 2017). "The Orbits of Jupiter's Irregular Satellites". The Astronomical Journal. 153 (4): 10. Bibcode:2017AJ....153..147B. doi:10.3847/1538-3881/aa5e4d. S2CID 125571053. 147.
6. Nesvorný, David; Alvarellos, Jose L. A.; Dones, Luke; Levison, Harold F. (July 2003). "Orbital and Collisional Evolution of the Irregular Satellites". The Astronomical Journal. 126 (1): 398–429. Bibcode:2003AJ....126..398N. doi:10.1086/375461. S2CID 8502734.

• Carpo group
• Moons of Jupiter
• Irregular satellites
• Astronomical objects discovered in 2018
• Discoveries by Scott S. Sheppard
• Moons with a prograde orbit
• Kozai mechanism