5261 Eureka

5261 Eureka is the first Mars trojan discovered. It was discovered by David H. Levy and Henry Holt at Palomar Observatory on 20 June 1990. It trails Mars (at the L₅ point) at a distance varying by only 0.3 AU during each revolution (with a secular trend superimposed, changing the distance from 1.5–1.8 AU around 1850 to 1.3–1.6 AU around 2400). Minimum distances from Earth, Venus, and Jupiter, are 0.5, 0.8, and 3.5 AU, respectively.

5261 Eureka
Discovery
Discovered by	David H. Levy and Henry Holt
Discovery date	20 June 1990
Designations
Pronunciation	/jʊˈriːkə/ yuurr-EE-kə
Named after	Eureka
Alternative designations	1990 MB
Minor planet category	Martian L5
Orbital characteristics
Epoch 13 January 2016 (JD 2457400.5)
Uncertainty parameter 0
Observation arc	13267 days (36.32 yr)
Aphelion	1.6222 AU (242.68 Gm)
Perihelion	1.4249 AU (213.16 Gm)
Semi-major axis	1.5236 AU (227.93 Gm)
Eccentricity	0.064766
Orbital period (sidereal)	1.88 yr (686.89 d)
Average orbital speed	24.11 km/s
Mean anomaly	145.29°
Mean motion	0° 31^m 26.76^s / day
Inclination	20.280°
Longitude of ascending node	245.057°
Argument of perihelion	95.456°
Known satellites	1
Earth MOID	0.497052 AU (74.3579 Gm)
Jupiter MOID	3.52162 AU (526.827 Gm)
T_Jupiter	4.428
Physical characteristics
Dimensions	1.3 km ~2–4 km^H
Synodic rotation period	2.6902 h (0.11209 d)
Geometric albedo	0.39
Temperature	~250 K
Spectral type	S(I) (Gaffey)
Absolute magnitude (H)	16.0, 16.1

Long-term numerical integration shows that the orbit is stable. Kimmo A. Innanen and Seppo Mikkola note that "contrary to intuition, there is clear empirical evidence for the stability of motion around the L₄ and L₅ points of all the terrestrial planets over a timeframe of several million years".

Since the discovery of 5261 Eureka, the Minor Planet Center has recognized three other asteroids as Martian trojans: (121514) 1999 UJ7 at the L₄ point, (101429) 1998 VF31 at the L₅ point, and (311999) 2007 NS2, also at the L₅ point. At least five other asteroids in near-1:1 resonances with Mars have been discovered, but they do not exhibit trojan behavior. They are 2001 FR127, 2001 FG24, (36017) 1999 ND₄₃, 1998 QH56 and (152704) 1998 SD₄. Due to close orbital similarities, most of the other, smaller, members of the L₅ group are hypothesized to be fragments of 5261 Eureka that were detached after it was spun up by the YORP effect (consistent with its rotational period of 2.69 h).

The infrared spectrum for 5261 Eureka is typical for an A-type asteroid, but the visual spectrum is consistent with an evolved form of achondrite called an angrite. A-class asteroids are tinted red in hue, with a moderate albedo. The asteroid is located deep within a stable Lagrangian zone of Mars, which is considered indicative of a primordial origin—meaning the asteroid has most likely been in this orbit for much of the history of the Solar System.^{[citation needed]}

Satellite

Satellite
Discovery
Discovery date	28 November 2011
Designations
Designation	S/2011 (5261) 1
Orbital characteristics
Epoch 9 December 2014
Semi-major axis	2.1 km
Orbital period (sidereal)	0.7054 ± 0.0004 d
Physical characteristics
Dimensions	0.46 km

On 28 November 2011, a natural satellite of 5261 Eureka was found. This unnamed moon is about 0.46 km in diameter and orbits 2.1 km from Eureka. The satellite's existence was announced in September 2014.

Satellite

See also