5261 Eureka

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5261 Eureka

Discovery
Discovered by	David H. Levy
Discovery date	June 20, 1990
Designations
Alternative names	1990 MB
Minor planet category	Mars-crosser asteroid
Orbital characteristics
Epoch July 14, 2004 (JD 2453200.5)
Aphelion	242.684 Gm (1.622 AU)
Perihelion	213.132 Gm (1.425 AU)
Semi-major axis	227.908 Gm (1.523 AU)
Eccentricity	0.065
Orbital period	686.829 d (1.88 a)
Average orbital speed	24.11 km/s
Mean anomaly	104.086°
Inclination	20.280°
Longitude of ascending node	245.108°
Argument of perihelion	95.361°
Physical characteristics
Dimensions	~2-4 km
Mass	? kg
Mean density	? g/cm³
Equatorial surface gravity	? m/s²
Escape velocity	? km/s
Rotation period	? d
Albedo	?
Temperature	~225 K
Spectral type	?
Absolute magnitude	16.1

5261 Eureka (pronounced /jʊˈriːkə/) was discovered at Mt Palomar on June 20, 1990 and turned out to be the first known Mars Trojan asteroid. 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 the Earth, Venus and Jupiter are 0.5, 0.8 and 3.5 AU, respectively.

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: 1999 UJ₇ at the L₄ point, 1998 VF₃₁ at the L₅ point,[1] and 2007 NS₂, also at the L₅ point.[2] At least six other asteroids have been discovered which are in near 1:1 resonances with Mars, but fail to exhibit trojan behavior. They are 2001 FR₁₂₇, 2001 FG₂₄, 2001 DH₄₇, 1999 ND₄₃, 1998 QH₅₆ and 1998 SD₄.[3][4]

The infrared spectrum for 5261 Eureka is typical of an A-class 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.

[edit] References

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