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5261 Eureka
Discovered by David H. Levy
Discovery date June 20, 1990
Alternate name(s) 1990 MB
Minor planet
Martian L5 Martian L5
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 (H) 16.1

5261 Eureka (pronounced /jʊˈriːkə/) was discovered by Palomar Observatory on June 20, 1990, and turned out to be the first known Lagrangian asteroid of Mars.[1] 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,[2] and 2007 NS2, also at the L5 point.[3] 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.

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.


Further reading
  • IAUC 5045
  • IAUC 5047
  • IAUC 5067
  • IAUC 5075
  • A. S. Rivkin, R. P. Binzel, S. J. Bus, and J. A. Grier, "Spectroscopy and Classification of Mars Trojan Asteroids", Bulletin of the American Astronomical Society 34, 2002, p. 840.
  • S. Tabachnik and N. W. Evans, "Cartography for Martian Trojans", The Astrophysical Journal 517, 1999, pp. L63-L66.

External links



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