5511 Cloanthus

5511 Cloanthus (provisional designation 1988 TH1) is a large Jupiter Trojan asteroid, one of the bodies trapped in a 1:1 mean-motion resonance with Jupiter near its L4 and L5 Lagrangian points.

Orbit and dynamical setting

Jupiter Trojans follow essentially the same orbit as Jupiter, leading or trailing the planet by about 60 degrees in co-orbital motion.³ Cloanthus fits this pattern with a semi-major axis of 5.255 AU, close to Jupiter's own 5.2 AU, and an orbital period of 4,400 days, or 12.05 years.¹ Its eccentricity of 0.119 carries it from 4.63 AU at perihelion to 5.88 AU at aphelion, and its orbit is inclined 11.17 degrees to the ecliptic.¹

The orbit is well determined. Observations used for its orbit solution extend back to a precovery from July 30, 1951, and the Minor Planet Center records 3,135 observations, with the last official observation on July 3, 2023.¹ A Czech astronomical database lists 4,831 observations across 35 oppositions, observed from 1951 onward.⁴

Because the orbit never approaches Earth's orbit closer than 3.63 AU, JPL's CNEOS simulations show no close approaches, and the asteroid is not classified as potentially hazardous. It is also not considered a viable target for human exploration by the NHATS study.¹

Physical properties

The two estimates bracket the true size, which depends on how reflective the surface actually is.

Rotation is unusually slow. Photometric monitoring gives a rotation period of 336.00 hours, about 14 Earth days, and the asteroid is cataloged as a tumbling body rotating in non-principal-axis rotation rather than about a single fixed axis.^{2 • 1} Slow, complex rotation of this kind is found among a subset of larger asteroids and is consistent with the long rotation periods reported for many Jupiter Trojans.⁵

Trojans as a population are thought to be more primitive than main-belt asteroids, believed to have formed at a range of heliocentric distances in the trans-Neptunian region, been scattered inward, and been captured during giant planet migration.⁵ Cloanthus shares this origin story with the rest of the Trojan clouds.

Occultations and observation opportunities

Because Cloanthus is large and its orbit is well known, it occasionally occults background stars. On 2019 May 3 UT, the roughly 40 km asteroid occulted a magnitude 12.1 star in the constellation Virgo for observers along a narrow path across northern Queensland and the Northern Territory of Australia; at that time the asteroid was 4.41 AU from Earth.⁶ Stellar occultations by Trojans are used to constrain sizes and shapes beyond what photometry alone provides.

References

1. Asteroid Cloanthus (5511), Space Reference
2. Non-principal axis rotation (tumbling) asteroids table, IAA RAS
3. Morbidelli et al. (2005). Chaotic capture of Jupiter's Trojan asteroids in the early Solar System. Nature 435, 462–465.
4. Planetka Cloanthus (5511), Astronomia ZČU
5. Mottola et al. (2024). Shapes, Rotations, Photometric and Internal Properties of Jupiter Trojans. Space Science Reviews 220, 17.
6. RASNZ Occultation Section: Cloanthus Occultation Update, 2019 May 3

Initially written Oct 21, 2026 · Edited: Oct 25, 2026

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