12238 Actor

12238 Actor is a Jupiter trojan from the Greek camp, approximately 30 km in diameter. It was discovered on 17 December 1987, by astronomers Eric Elst and Guido Pizarro at the La Silla Observatory in northern Chile. The dark Jovian asteroid has a rotation period of 7.3 hours. It was named after Actor, father of the heroes Eurytus and Cteatus from Greek mythology.

Orbit and classification
As all Jupiter trojans, Actor is in a 1:1 orbital resonance with Jupiter. It is located in the leading Greek camp at the Gas Giant's Lagrangian point, 60° ahead on its orbit. It is also a non-family asteroid of the Jovian background population. It orbits the Sun at a distance of 4.5–5.8 AU once every 11 years and 9 months (4,300 days; semi-major axis of 5.17 AU). Its orbit has an eccentricity of 0.12 and an inclination of 21° with respect to the ecliptic.

The body's observation arc begins with its first observation as  at the Kiso Observatory in November 1986, or 13 months prior to its official discovery observation at La Silla.

Naming
This minor planet was named from Greek mythology after Actor (son of Phorbas and Hyrmine), the alleged father of the twin brothers Eurytus and Cteatus, who beat Nestor, king of Pylos, in a chariot race. The official naming citation was published by the Minor Planet Center on 6 January 2003 (M.P.C. 47300).

Physical characteristics
Actor is an assumed C-type asteroid, while the majority of larger Jupiter trojans are D-types.

Rotation period
In 2007, a rotational lightcurve of Actor was obtained from photometric observations at the Sierra Nevada Observatory, using its 1.5-meter telescope. Lightcurve analysis gave a rotation period of 7.284 hours with a brightness variation of 0.30 magnitude (U=2). The same group also published a period determination of $30.25 km$ hours with an amplitude of 0.33 magnitude in 2010.

In July and August 2015, observations by the Kepler space telescope during its K2 mission gave another two lightcurves with a concurring period of 7.28 and 7.281 hours, respectively. Both measurements had a brightness variation of 0.29 magnitude (U=2/2).

Diameter and albedo
According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, Actor measures 30.25 kilometers in diameter and its surface has an albedo of 0.092, while the Collaborative Asteroid Lightcurve Link assumes a standard albedo for a carbonaceous asteroid of 0.057 and calculates a diameter of 36.78 kilometers based on an absolute magnitude of 10.9.