1512 Oulu

1512 Oulu, provisional designation 1939 FE, is a dark Hildian asteroid, slow rotator and possibly the largest known tumbler orbiting in the outermost region of the asteroid belt. With a diameter of approximately 80 kilometers, it belongs to the fifty largest asteroids in the outer main-belt. The body was discovered on 18 March 1939, by Finnish astronomer Heikki Alikoski at Turku Observatory in Southwest Finland and named for the Finnish town Oulu.^[2][12]

1512 Oulu

Hubble Space Telescope image of Oulu taken in 2012
Discovery[1]
Discovered by	H. Alikoski
Discovery site	Turku Obs.
Discovery date	18 March 1939
Designations
MPC designation	(1512) Oulu
Named after	Oulu (Finnish town)[2]
Alternative designations	1939 FE · 1938 CU 1957 TA · 1958 XS
Minor planet category	main-belt · Hilda[3]
Orbital characteristics[1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 0
Observation arc	78.06 yr (28,510 days)
Aphelion	4.5541 AU
Perihelion	3.3892 AU
Semi-major axis	3.9717 AU
Eccentricity	0.1466
Orbital period (sidereal)	7.92 yr (2,891 days)
Mean anomaly	333.83°
Mean motion	0° 7m 28.2s / day
Inclination	6.4785°
Longitude of ascending node	10.168°
Argument of perihelion	238.20°
Jupiter MOID	0.6287 AU
Physical characteristics
Dimensions	65.0 km[4] 65.000±4.137 km[5] 79.222±0.241 km[6] 82.72±2.5 km (IRAS:38)[7] 91.05±2.20 km[8]
Synodic rotation period	132.3±0.1 h[9]
Geometric albedo	0.031±0.001[8] 0.0366±0.002 (IRAS:38)[7] 0.038±0.005[6] 0.0536±0.0061[5] 0.0594[4] 0.06±0.03[10]
Spectral type	Tholen = P[1] · X[11] · P[3] B–V = 0.715[1] U–B = 0.190[1]
Absolute magnitude (H)	9.62[1][3][5][4][7][8] · 9.92±0.40[11]

Orbit and classification

Located in the outermost part of the main-belt, Oulu is a member of the Hilda family, a large orbital group of asteroids that are thought to have originated from the Kuiper belt. They orbit in a 3:2 orbital resonance with the gas giant Jupiter, meaning that for every 2 orbits Jupiter completes around the Sun, a Hildian asteroid will complete 3 orbits.^[1] As it does not cross the path of any of the planets, it will not be pulled out of orbit by Jupiter's gravitational field, and will likely remain in a stable orbit for thousands of years.

It orbits the Sun at a distance of 3.4–4.6 AU once every 7 years and 11 months (2,891 days). Its orbit has an eccentricity of 0.15 and an inclination of 6° concerning the ecliptic.^[1] In 1938, Oulu was first identified as 1938 CU at Bergedorf Observatory. Its observation arc, however, begins one month after its official discovery observation.^[12]

Physical characteristics

Oulu is characterized as a dark and reddish P-type asteroid in the Tholen taxonomy, of which only a few dozen bodies are currently known.^[13]

Slow rotator and likely tumbler

In May 2009, a rotational light curve of Oulu was obtained from photometric observations by Slovak astronomer Adrián Galád at Modra Observatory. Lightcurve analysis gave a rotation period of 132.3 hours with a brightness variation of 0.33 in magnitude (U=2+).^[9] It is among the top few hundred slow rotators.

Oulu is likely in a state of non-principal axis rotation, which is commonly known as tumbling. It is the largest such object ever observed (also see List of tumblers).^[3][9][14]

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, Oulu measures between 65.00 and 91.05 kilometers in diameter, and its surface has an albedo between 0.031 and 0.06.^{[5][6][7][8][10]}

The Collaborative Asteroid Lightcurve Link agrees with IRAS, that is, an albedo of 0.0366 and a diameter of 82.72 kilometers using an absolute magnitude of 9.62.^[3] In May 2002, Vasilij Shevchenko and Edward Tedesco observed an occultation by Oulu, that gave a diameter of 65.0 kilometers with an occultation albedo of 0.0594.^[4]

Naming

This minor planet was named for the northern Finnish town Oulu, the birthplace of the discoverer.^[2] The official naming citation was published by the Minor Planet Center on 30 January 1964 (M.P.C. 2278).^[15]