General · EdgepediaOctober 21, 20263 min read

12238 Actor

12238 Actor is a Jupiter Trojan asteroid discovered on 17 December 1987 by Eric Elst and Guido Pizarro at the La Silla Observatory in Chile.⁵ A precovery observation from November 1986 at the Kiso Observatory predates the discovery by 13 months.⁵ It is named after Actor, father of the twin heroes Eurytus and Cteatus in Greek mythology.⁵ The asteroid orbits the Sun at distances of 4.5 to 5.8 AU once every 11 years and 9 months, with an eccentricity of 0.12 and an inclination of 21° with respect to the ecliptic.⁵

|---|---|

| Discovery | 17 December 1987, Eric Elst and Guido Pizarro, La Silla Observatory, Chile⁵ |

| Name | Actor, father of the heroes Eurytus and Cteatus in Greek mythology⁵ |

Discovery and observations

A precovery observation, a recorded sighting made before the official discovery, dates to November 1986 at the Kiso Observatory, 13 months earlier.⁵

Stellar occultations by Actor have been predicted for ground-based observers. A RECON event prediction for 3 April 2025, visible from southern Africa, gave a cross-track diameter estimate of 38.7 km assuming a 5% albedo, with an expected flux drop of 68% and a maximum central chord of 2.9 seconds.¹ A further event was predicted for 23 May 2026.²

Orbit and classification

It orbits the Sun at distances of 4.5 to 5.8 AU once every 11 years and 9 months (4,300 days; semi-major axis 5.17 AU), with an eccentricity of 0.12 and an inclination of 21° with respect to the ecliptic.⁵

These orbital parameters sit within the broader Trojan population. Trojan inclinations cover a fairly uniform range from 0° to 35° in both the L4 and L5 regions, with a median inclination of 10°, and Trojan eccentricities tend to be below 0.15, values required to remain within the stable zones.³ Actor's 21° inclination places it in the more highly excited part of that distribution.

Physical properties

Dark, low-albedo surfaces are typical of the Trojans, which are dominated by D- and P-type bodies with featureless visible and near-infrared spectra and albedos consistent with dormant comets.³

Naming

The minor planet is named after Actor (son of Phorbas and Hyrmine in the sources' account), father of the twin heroes Eurytus and Cteatus, who defeated Nestor, king of Pylos, in a chariot race in Greek mythology.⁵ Larger Trojan members are named after characters from the epic Greek poems describing the Trojan war, the Iliad and the Odyssey.⁴

Context within the Trojan swarms

The Jupiter Trojans are two swarms of small bodies trapped at the L4 and L5 Lagrange points of Jupiter, which has a semi-major axis of 5.2 AU.³ The largest known member is (624) Hektor at a diameter of about 250 km, while the smallest observed Trojans are roughly a kilometer across.³ Most Trojans many tens of kilometers in diameter or larger, a size class that includes Actor, are on stable orbits and are presumed survivors of the processes that created the population.³ The Trojans are thought to have been captured early in Solar System history, most likely from the primordial Kuiper belt during a giant planet instability, rather than having formed in situ near Jupiter.³

References

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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