5261 Eureka

Discovery
Discoverer	David H. Levy
Discovery Date	June 20, 1990
Alternate Designations	1990 MB
Category	Mars-crosser asteroid
Orbital Elements Epoch July 14, 2004 (JD 2453200.5)
Eccentricity (e)	0.065
Semi-Major Axis (a)	227.908 Gm (1.523 AU)
Perihelion (q)	213.132 Gm (1.425 AU)
Aphelion (Q)	242.684 Gm (1.622 AU)
Orbital Period (P)	686.829 d (1.88 a)
Mean Orbital Speed	24.13 km/s
Inclination (i)	20.280°
Longitude of the Ascending Node (Ω)	245.108°
Argument of Perihelion (ω)	95.361°
Mean Anomaly (M)	104.086°
Physical Characteristics
Dimensions	? km
Mass	? kg
Density	? g/cm³
Surface Gravity	? m/s²
Escape Velocity	? km/s
Rotation Period	? d
Spectral Class	?
Absolute Magnitude	16.1
Albedo	?
Mean Surface Temperature	~311 K

5261 Eureka 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 then, other Mars Trojans have been identified; namely 1999 UJ₇ at the L₄ point and 1998 VF₃₁ , 2001 DH₄₇ , 2001 FG₂₄ , and 2001 FR₁₂₇ at the L₅ point. The co-orbitals 1998 QH₅₆ and 1998 SD₄ are not destined to remain as Trojans —they'll be perturbed away by Mars within the next 500,000 years or so.

References

• IAUC 5045 http://cfa-www.harvard.edu/iauc/05000/05045.html#Item1
• IAUC 5047 http://cfa-www.harvard.edu/iauc/05000/05047.html#Item3
• IAUC 5067 http://cfa-www.harvard.edu/iauc/05000/05067.html#Item1
• IAUC 5075 http://cfa-www.harvard.edu/iauc/05000/05075.html#Item2