128 Nemesis

128 Nemesis is a large 180 km main-belt asteroid, of carbonaceous composition. It rotates rather slowly, taking about 78 hours to complete one rotation.^[3][9] Nemesis is the largest member of the Nemesian asteroid family bearing its name. It was discovered by J. C. Watson on 25 November 1872,^[3] and named after Nemesis, the goddess of retribution in Greek mythology.

Diagram of Nemesis's orbit

128 Nemesis

Discovery[1]
Discovered by	James Craig Watson
Discovery date	25 November 1872
Designations
MPC designation	(128) Nemesis
Pronunciation	/ˈnɛmɪsɪs/[2]
Named after	Nemesis
Alternative designations	A872 WA; 1952 HW1
Minor planet category	main-belt[1][3] · Nemesis
Adjectives	Nemesian /nəˈmiːʒ(i)ən/[4][5]
Orbital characteristics[3]
Epoch 23 March 2018 (JD 2458200.5)
Uncertainty parameter 0
Observation arc	144.93 yr (52,934 d)
Aphelion	3.10 AU (463.69 Gm)
Perihelion	2.40 AU (359.00 Gm)
Semi-major axis	2.75 AU (411.35 Gm)
Eccentricity	0.1272
Orbital period (sidereal)	4.56 years (1,665 d)
Mean anomaly	345.49°
Mean motion	0° 12m 58.32s / day
Inclination	6.2453°
Longitude of ascending node	76.243°
Argument of perihelion	303.82°
Physical characteristics
Mean diameter	163±5 km[6] 162.5±1.3 km[7] 184.2±5.2 km[8]
Flattening	0.17[a]
Mass	(3.4±1.7)×1018 kg[6] (6.0±2.6)×1018 kg[8]
Mean density	1.5±0.8 g/cm3[6] 1.82±0.79 g/cm3[8]
Synodic rotation period	77.81 h (3.242 d)[9] 38.9325 h (1.62219 d)[3]
Geometric albedo	0.067 (calculated)[6] 0.067±0.005[3]
Spectral type	Tholen = C[3] SMASS = C[3]
Absolute magnitude (H)	7.70[3]

This object is orbiting the Sun with a period of 4.56 years and an eccentricity (ovalness) of 0.13. The orbital plane is inclined by 6.2° to the plane of the ecliptic. It is categorized as a C-type asteroid,^[10] indicating a primitive carbonaceous composition. Based on IRAS data Nemesis is about 188 km in diameter and is around the 33rd largest main-belt asteroid,^[11] while WISE measurements yield a size of ~163 km.^[7] The 77.81‑hour^[12] rotation period is the second longest for an asteroid more than 150 km in diameter.^[13]

Between 2005 and 2021, 128 Nemesis has been observed to occult eight stars.

Notes

1. Flattening derived from the maximum aspect ratio (c/a): ${\displaystyle f=1-{\frac {c}{a}}}$, where (c/a) = 0.83±0.04.^[6]