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4797 Ako
Discovery
Discovered by	T. Nomura; K. Kawanishi
Discovery site	Minami-Oda Obs. (374)
Discovery date	30 September 1989
Designations
MPC designation	(4797) Ako
Named after	Akō, Hyōgo; (Japanese city)
Alternative designations	1989 SJ · 1978 VY9; 1985 QB4
Minor planet category	main-belt · Nysa
Orbital characteristics
	Epoch 4 September 2017 (JD 2458000.5)
	Uncertainty parameter 0
Observation arc	38.49 yr (14,057 days)
Aphelion	2.8553 AU
Perihelion	1.9714 AU
Semi-major axis	2.4133 AU
Eccentricity	0.1831
Orbital period (sidereal)	3.75 yr (1,369 days)
Mean anomaly	146.42°
Mean motion	0° 15m 46.44s / day
Inclination	1.8108°
Longitude of ascending node	320.82°
Argument of perihelion	78.111°
Physical characteristics
Dimensions	4.00 km (calculated); 6.000±0.496 km
Synodic rotation period	4.085±0.001 h
Geometric albedo	0.112±0.021; 0.21 (assumed)
Spectral type	S
Absolute magnitude (H)	14.1 · 14.3 · 14.31±0.27

4797 Ako, provisional designation 1989 SJ, is a stony Nysian asteroid from the inner regions of the asteroid belt, approximately 5 kilometers in diameter. It was discovered on 30 September 1989, by the Japanese astronomers Toshiro Nomura and Kōyō Kawanishi at the Minami-Oda Observatory (374), Japan.^[8] The asteroid was named for the Japanese city of Akō.^[2]

Orbit and classification

Ako is a member of the stony subgroup of the Nysa family, which is named after its largest member 44 Nysa. It orbits the Sun in the inner main-belt at a distance of 2.0–2.9 AU once every 3 years and 9 months (1,369 days). Its orbit has an eccentricity of 0.18 and an inclination of 2° with respect to the ecliptic.^[1]

In 1978, it was first identified as 1978 VY₉ at Palomar Observatory, extending the body's observation arc by 11 years prior to its official discovery observation at Minami-Oda Observatory.^[8]

Physical characteristics

Rotation period

A rotational lightcurve of Ako was obtained for the first time from photometric observations made at the U.S. Ricky Observatory, Missouri, in November 2008. It gave a well-defined rotation period of 4.085±0.001 hours with a relatively high brightness variation of 0.90 in magnitude (U=3), indicative of a non-spheroidal shape.^[6]

Diameter and albedo

According to NASA's space-based Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Ako measures 6.0 kilometers in diameter and its surface has an albedo of 0.11,^[4]^[5] while the Collaborative Asteroid Lightcurve Link (CALL) assumes a standard albedo for stony asteroids of 0.21, and calculates a diameter of 4.0 kilometers, as the higher the albedo (reflectivity), the smaller a body's diameter for a certain absolute magnitude (brightness).^[3]

Naming

This minor planet was named for the city of Akō in the Hyōgo Prefecture, Japan, and for its ancient castle on the Seto Inland Sea.^[2]

Known for its salt production, Ako is the birthplace of the fictional account of Chūshingura, a tale about the forty-seven Ronin who committed seppuku after avenging their master. The city is also the home of the second discoverer's private Minami-Oda observatory, where Kōyō Kawanishi observes small Solar System bodies.^[2] The official naming citation was published by the Minor Planet Center on 27 June 1991 (M.P.C. 18465).^[9]