9799 Thronium, provisional designation: 1996 RJ, is a large Jupiter trojan from the Greek camp and the parent body of a small, unnamed asteroid family (006), approximately 68 kilometers (42 miles) in diameter. It was discovered on 8 September 1996, by American astronomer Timothy Spahr at the Catalina Station of the Steward Observatory near Tucson, Arizona, in the United States.^[1] The assumed C-type asteroid belongs to the 50 largest Jupiter trojans and has a relatively long rotation period of 21.52 hours.^[4] It was named for the ancient Greek city of Thronium mentioned in the Iliad.^[2]

Thronium is a dark Jovian asteroid orbiting in the leading Greek camp at Jupiter's L₄ Lagrangian point, 60° ahead of the Gas Giant's orbit in a 1:1 resonance (see Trojans in astronomy). It orbits the Sun at a distance of 4.9–5.4 AU once every 11 years and 10 months (4,320 days; semi-major axis of 5.19 AU). Its orbit has an eccentricity of 0.05 and an inclination of 31° with respect to the ecliptic.^[3] The body's observation arc begins in December 1986, with its first observation as 1986 XP₅ at the Observatory of the University of St Andrews (482), Scotland, almost 10 years prior to its official discovery observation at Catalina Station.^[1]

Thronium is also the parent body of a small, unnamed asteroid family with the family identification number 006.^[6][7] The family seems to be young, compact and consist of only 7 known members.^[14]: 23 Only a few families have been identified among the Jovian asteroids; four of them in the Greek camp. This potentially collisional family was first characterized by Jakub Rozehnal and Miroslav Brož in 2014.^[8]

The other members of this family include the unnamed Jovian asteroids (89938), (226027), (243316), (254691), (388876) and (392189).^[14]

This minor planet was numbered on 8 December 1998 after its orbit had been sufficiently secured (M.P.C. 33211).^[15] On 14 May 2021, the object was named by the Working Group for Small Bodies Nomenclature (WGSBN), after the ancient Greek city of Thronium. In Greek mythology and mentioned in the Iliad (Catalogue of Ships), it was one of the places from which the Locrians joined the Achaeans.^[2]

1996 RJ is an assumed, carbonaceous C-type asteroid.^[4] Nesvorný does not give an overall spectral type for this unnamed family, but derives an albedo of 0.06 (see below), which is also typical for carbonaceous C-types.^[14]: 23

In October 2009, a rotational lightcurve of 1996 RJ was obtained from photometric observations by Stefano Mottola using a 1.2-meter telescope at the Calar Alto Observatory in Spain. Lightcurve analysis gave a longer-than average rotation period of 21.52 hours with a brightness amplitude of 0.16 magnitude (U=2+).^[4][12]

According to the surveys carried out by the Infrared Astronomical Satellite IRAS, the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, 1996 RJ measures between 64.87 and 72.42 kilometers in diameter and its surface has an albedo between 0.037 and 0.060.^[9][10][11]

The Collaborative Asteroid Lightcurve Link derives an albedo of 0.0603 and a diameter of 65.06 kilometers based on an absolute magnitude of 9.6.^[4]

• Long-term evolution of asteroid families among Jovian Trojans, Jakub Rozehnal and Miroslav Brož (2014)
• Asteroid Lightcurve Database (LCDB), query form (info Archived 16 December 2017 at the Wayback Machine)
• Discovery Circumstances: Numbered Minor Planets (5001)-(10000) – Minor Planet Center
• 9799 Thronium at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info
• 9799 Thronium at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit viewer · Orbit parameters · Physical parameters