4959 Niinoama

4959 Niinoama (prov. designation: 1991 PA₁) is a dark background asteroid from the outer regions of the asteroid belt. It was discovered by Japanese astronomers Akira Natori and Takeshi Urata at JCPM Yakiimo Station on 15 August 1991.^[10] The presumed carbonaceous C-type asteroid has a rotation period of 4.7 hours and measures approximately 36 kilometers (22 miles) in diameter. It was named after Taira no Tokiko (1126–1185) of the Imperial House of Japan during the Heian period.

4959 Niinoama

Shape model of Niinoama from its lightcurve
Discovery[1]
Discovered by	A. Natori T. Urata
Discovery site	JCPM Yakiimo Stn.
Discovery date	15 August 1991
Designations
MPC designation	(4959) Niinoama
Named after	Taira no Tokiko (Imperial House of Japan)[2]
Alternative designations	1991 PA1 · 1958 TZ 1966 CB · 1968 MC 1972 EB · 1979 OU13 1980 TG1 · 1980 TS8 1984 OO · 1985 OD 1986 VS1 · 1989 FE1
Minor planet category	main-belt · (outer)[3] background
Orbital characteristics[1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 0
Observation arc	67.15 yr (24,528 days)
Aphelion	3.1759 AU
Perihelion	3.1272 AU
Semi-major axis	3.1516 AU
Eccentricity	0.0077
Orbital period (sidereal)	5.59 yr (2,044 days)
Mean anomaly	129.49°
Mean motion	0° 10m 34.32s / day
Inclination	8.9913°
Longitude of ascending node	128.79°
Argument of perihelion	320.53°
Physical characteristics
Mean diameter	26.50±0.73 km[4] 27.96±2.4 km[5] 35.842±0.117 km[6] 36.21±0.80 km[7] 42.507±0.576 km[8]
Synodic rotation period	4.73±0.01 h[9]
Geometric albedo	0.0468±0.0109[8] 0.066±0.003[7] 0.079±0.010[6] 0.1081 (derived)[3] 0.1082±0.021[5] 0.120±0.014[4]
Spectral type	C (assumed)[3]
Absolute magnitude (H)	10.8[3][4][5][7][8] · 10.9[1]

Orbit and classification

Niinoama is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements. It orbits the Sun in the outer main-belt at a distance of 3.1–3.2 AU once every 5 years and 7 months (2,044 days). Its orbit has an eccentricity of 0.01 and an inclination of 9° with respect to the ecliptic.^[1] A first precovery was taken at Palomar Observatory in May 1950, extending the body's observation arc by more than 41 years prior to its official discovery observation at Yakiimo.^[10]

Naming

This minor planet was named after Taira no Tokiko (1126–1185), second wife of military leader Taira no Kiyomori and grandmother of Emperor Antoku after whom 3686 Antoku is named. According to the Tale of the Heike, she drowned herself during the Battle of Dan-no-ura together with the boy-Emperor Antoku in her arms.^[2][11] The official naming citation was published by the Minor Planet Center on 10 November 1992 (M.P.C. 21132).^[12]

Physical characteristics

Niinoama is an assumed C-type asteroid.^[3]

Photometry

Photometric observations of Niinoama collected during 2008 show a rotation period of 4.73±0.01 hours with a brightness variation of 0.32 ± 0.04 magnitude (U=3),^[9] superseding an early measurement that gave 4.725±0.002 hours (U=1+).^[13]

Diameter and albedo

According to the surveys carried out by the Infrared Astronomical Satellite IRAS, the Japanese Akari satellite, and NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Niinoama measures between 26.50 and 42.51 kilometers in diameter and its surface has an albedo of 0.079.^[5][7][8][4] The Collaborative Asteroid Lightcurve Link adopts a diameter of 27.96 kilometers from IRAS, and derives an albedo of 0.1082 based on an absolute magnitude of 10.8.^[3]