# 5284 Orsilocus

**5284 Orsilocus** (provisional designation 1989 CK\(_2\)) is a Jupiter Trojan asteroid, one of the small bodies trapped in a 1:1 mean-motion resonance with Jupiter around its L\(_4\) Lagrange point, 60 degrees ahead of the planet in its orbit. It was discovered on 1 February 1989 at [Palomar Observatory](https://www.edgepedia.org/palomar-observatory) by the American planetary scientists Carolyn S. Shoemaker and Eugene M. Shoemaker.<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup>

| Key fact | Detail |
|---|---|
| Discovery | 1 February 1989, Palomar Observatory, by C. S. Shoemaker and E. M. Shoemaker<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup> |
| Orbit | Semi-major axis 5.2268 au, eccentricity 0.0842, inclination 20.2227°, period 11.950 years<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup> |
| Size | Diameter 50.159 ± 0.462 km from NEOWISE thermal infrared data<sup>[2](https://neoexchange.lco.global/target/36521/)</sup> |
| Surface | Geometric albedo 0.07 ± 0.009; D-type spectrum (SDSS-MOC)<sup>[2](https://neoexchange.lco.global/target/36521/)</sup> |
| Rotation | Synodic rotation period 10.31 hours, based on less than full light-curve coverage<sup>[2](https://neoexchange.lco.global/target/36521/)</sup> |

## Discovery and orbit

The Shoemakers found Orsilocus at Palomar during their Trojan search program, and the object received the provisional designation 1989 CK\(_2\) before being numbered 5284.<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup> Its orbit has a semi-major axis of 5.2268 au, an eccentricity of 0.0842, and an inclination of 20.2227° to the ecliptic, giving perihelion and aphelion distances of 4.786 au and 5.667 au and an orbital period of 11.950 years (4,364.7 days).<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup> The Tisserand parameter (orbital value used to tell asteroids from comets) with respect to Jupiter, 2.9, is in the range typical of Trojan librators rather than comets.<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup>

The observation record is long: the asteroid has been observed over 4,630 times across 31 oppositions, with positions recovered back to 1953, more than three decades before its official discovery.<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup> Orsilocus also appears in the catalog of resonant secular frequencies computed for observed Trojans by Robutel and Gabern, with a listed inclination of 18.8 degrees, placing it among the objects whose long-term dynamical behavior has been analyzed individually.<sup>[3](https://academic.oup.com/mnras/article/372/4/1463/1184958)</sup>

## Physical properties

Thermal infrared measurements from NASA's NEOWISE survey give a diameter of 50.159 ± 0.462 km and a geometric albedo of 0.07 ± 0.009, consistent with the dark, matte surfaces that WISE found characteristic of the Jovian Trojans as a population.<sup>[2](https://neoexchange.lco.global/target/36521/)</sup><sup> • </sup><sup>[4](https://www.jpl.nasa.gov/news/nasas-wise-colors-in-unknowns-on-jupiter-asteroids/)</sup> Spectral classification from the [Sloan Digital Sky Survey](https://www.edgepedia.org/sloan-digital-sky-survey) assigns a probability score of 91 to a D-type taxonomy, the reddish, carbon-rich class that dominates the Trojan swarms.<sup>[2](https://neoexchange.lco.global/target/36521/)</sup>

The rotation period is 10.31 hours, but the Lightcurve Database entry is based on less than full coverage, so the period may be wrong by about 30 percent.<sup>[2](https://neoexchange.lco.global/target/36521/)</sup> Published absolute magnitudes differ slightly between catalogs: the Czech asteroid database lists H = 10.12 mag, while the LCO follow-up page gives H = 10.47 (E2021B10).<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup><sup> • </sup><sup>[2](https://neoexchange.lco.global/target/36521/)</sup>

## Name

Like other members of the leading L\(_4\) swarm, Orsilocus carries the name of a Greek hero from the [Trojan War](https://www.edgepedia.org/trojan-war) cycle; the convention assigns names of Greek characters to L\(_4\) asteroids and Trojan characters to L\(_5\) asteroids.<sup>[5](https://academic.oup.com/mnras/article/504/2/1571/6207955)</sup> The twins fought under [Agamemnon](https://www.edgepedia.org/agamemnon) and Menelaus, and Orsilocus was killed by Aeneas.

## Context among the Trojans

The Jovian Trojans are two swarms of small bodies librating around Jupiter's stable L\(_4\) and L\(_5\) Lagrange points, 60 degrees ahead of and behind the planet, and are thought to have been captured into these orbits early in solar system history, most likely from the trans-Neptunian region during a giant-planet instability.<sup>[6](https://arxiv.org/abs/2312.02864)</sup> Their orbits are stable over the lifetime of the [Solar System](https://www.edgepedia.org/solar-system), though simulations show that roughly a quarter of the population escapes over 4.5 billion years.<sup>[7](https://academic.oup.com/mnras/article/495/4/4085/5837576)</sup> Orsilocus's moderately high inclination of about 20 degrees reflects the excited inclination distribution of the captured population, in which some objects reach 35 degrees.<sup>[1](https://astronomia.zcu.cz/planety/planetka-5284)</sup><sup> • </sup><sup>[6](https://arxiv.org/abs/2312.02864)</sup>

## References

1. [Planetka Orsilocus (5284), Planety.astro.cz / Astronomia ZČU](https://astronomia.zcu.cz/planety/planetka-5284)
2. [5284 Orsilocus (1989 CK2) details, LCO NEOx](https://neoexchange.lco.global/target/36521/)
3. [Robutel & Gabern (2006), The resonant structure of Jupiter's Trojan asteroids – I, MNRAS 372, 1463](https://academic.oup.com/mnras/article/372/4/1463/1184958)
4. [NASA's WISE Colors in Unknowns on Jupiter Asteroids, JPL](https://www.jpl.nasa.gov/news/nasas-wise-colors-in-unknowns-on-jupiter-asteroids/)
5. [Holt et al. (2021), Astrocladistics of the Jovian Trojan Swarms, MNRAS 504, 1571](https://academic.oup.com/mnras/article/504/2/1571/6207955)
6. [Bottke et al. (2023), Origin and Evolution of Jupiter's Trojan Asteroids, Space Science Reviews](https://arxiv.org/abs/2312.02864)
7. [Holt et al. (2020), Stability of Jovian Trojans and their collisional families, MNRAS 495, 4085](https://academic.oup.com/mnras/article/495/4/4085/5837576)

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*Topic: Encyclopedia › Places and geography › Extraterrestrial places › Small bodies of the Solar System › Jupiter Trojans and other dynamical groups › Greek-camp Jupiter Trojans › Lesser Achaean warriors*

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

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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license