(9928) 1981 WE9

(9928) 1981 WE₉, provisional designation 1981 WE₉, is a stony Florian asteroid from the inner regions of the asteroid belt, approximately 3 kilometers in diameter. It was discovered on 16 November 1981, by astronomers at Perth Observatory in Bickley, Australia.^[2]

(9928) 1981 WE₉

Discovery[1]
Discovered by	Perth Obs.
Discovery site	Perth Obs.
Discovery date	16 November 1981
Designations
MPC designation	(9928) 1981 WE9
Alternative designations	1981 WE9 · 1971 TJ1 1993 FC43
Minor planet category	main-belt[1][2] · Flora[3]
Orbital characteristics[1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 0
Observation arc	65.74 yr (24,012 days)
Aphelion	2.6101 AU
Perihelion	1.8246 AU
Semi-major axis	2.2174 AU
Eccentricity	0.1771
Orbital period (sidereal)	3.30 yr (1,206 days)
Mean anomaly	340.29°
Mean motion	0° 17m 54.6s / day
Inclination	2.8472°
Longitude of ascending node	179.45°
Argument of perihelion	176.04°
Physical characteristics
Dimensions	2.441±0.327 km[4] 2.938±0.660 km[5] 3.00±0.42 km[6] 3.11 km (calculated)[3]
Synodic rotation period	5.547±0.005 h[7] 18.310±0.0034 h[8] 18.3980±0.0034 h[3][8]
Geometric albedo	0.24 (assumed)[3] 0.283±0.152[6] 0.3557±0.2289[5] 0.428±0.109[4]
Spectral type	S[3]
Absolute magnitude (H)	14.4[5] · 14.60[4][6] · 14.7[1][3]

Orbit and classification

Orbit of 1981 WE₉ (blue), with the inner planets and Jupiter

The stony S-type asteroid is a member of the Flora family, one of the largest groups of asteroids in the main-belt. It orbits the Sun at a distance of 1.8–2.6 AU once every 3 years and 4 months (1,206 days).

Its orbit has an eccentricity of 0.18 and an inclination of 3° with respect to the ecliptic.^[1] A first precovery was taken at Palomar in 1951, extending the body's observation arc by 30 years prior to its official discovery observation at Bickley.^[2]

Physical characteristics

In December 2014, astronomer Maurice Clark obtained a rotational lightcurve from photometric observations at Preston Gott Observatory. Lightcurve analysis gave an ambiguous rotation period of 18.3980 hours with a brightness variation of 0.41 magnitude, suggesting a non-spheroidal shape (U=2+). The alternative period solution is 9.14 hours with an amplitude of 0.32 magnitude.^[8] The results supersede a previously obtained period of 5.547 hours (U=2).^[7]

Diameter and albedo

According to the survey carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, the asteroid measures between 2.44 and 3.00 kilometers in diameter, and its surface has an albedo between 0.283 and 0.428.^[4][5][6] The Collaborative Asteroid Lightcurve Link assumes an intermediate albedo of 0.24 – derived from 8 Flora, the largest member and namesake of this asteroid family – and calculates a diameter of 3.11 kilometers with an absolute magnitude of 14.7.^[3]

Numbering and naming

This minor planet was numbered by the Minor Planet Center on 2 February 1999.^[9] As of 2018, it has not been named.^[2]