(121514) 1999 UJ₇

Discovery[1]
Discovered by	30 October 1999 by LINEAR
Discovery site	Socorro
Discovery date	30 October 1999
Designations
Alternative designations	2002 AC180[1]
Minor planet category	Martian L4
Orbital characteristics[1]
Epoch 13 January 2016 (JD 2457400.5)
Uncertainty parameter 0
Observation arc	21861 days (59.85 yr)
Aphelion	1.5843517 AU (237.01564 Gm)
Perihelion	1.4644704 AU (219.08165 Gm)
Semi-major axis	1.5244110 AU (228.04864 Gm)
Eccentricity	0.0393206
Orbital period (sidereal)	1.88 yr (687.47 d)
Mean anomaly	229.9277°
Mean motion	0° 31m 25.183s / day
Inclination	16.75044°
Longitude of ascending node	347.37714°
Argument of perihelion	48.35771°
Earth MOID	0.47372 AU (70.868 Gm)
Jupiter MOID	3.49473 AU (522.804 Gm)
Physical characteristics
Dimensions	~1 km[2]
Spectral type	X[3]
Absolute magnitude (H)	16.9[1]

(121514) 1999 UJ₇ is a small asteroid orbiting near the L₄ point of Mars (60 degrees ahead Mars on its orbit).^[2] As of September 2011, it is the only known asteroid to orbit the leading L₄ point of Mars, although at least three other asteroids orbit Mars's trailing L₅ point: 5261 Eureka, (101429) 1998 VF31, and 2007 NS2.^[2] Not only does (121514) 1999 UJ₇ orbit on the other side of Mars from other similar asteroids, its spectrum is different as well, which is puzzling because all of the Martian trojans seem to be in very stable orbits.^[3]

Orbit

(121514) 1999 UJ₇ orbits around the L₄ point of Mars in a very stable orbit and is large enough that the Yarkovsky effect will not affect its orbit.^[2]

Physical characteristics

Due to similarity in the measured brightness of (121514) 1999 UJ₇ with other Martian trojans, it is thought to be a small asteroid with an effective diameter on the order of 1 kilometer (0.62 mi).^[2] Its spectrum suggests that it is an X-type asteroid, which is different from 5261 Eureka and 1998 VF₃₁,^[3] and is somewhat puzzling since different mineral compositions suggest different origins for the two groups of asteroids. The long lifetime of the orbits for these asteroids makes the possibility of one or more of them being interlopers unlikely, however. This suggests that either one or more of the Martian trojans was captured in such a way as to give it a long-term stable orbit (and it is therefore not a primordial Martian asteroid), or that some fusion or combination of previous asteroids resulted in the presently observed ones.^[3] The Yarkovsky effect may provide a potential capture mechanism but not enough is known about the shapes of these objects to provide a useful Yarkovsky model.^[3]