12923 Zephyr

12923 Zephyr (prov. designation: 1999 GK₄) is a stony asteroid, classified as potentially hazardous asteroid and near-Earth object of the Apollo group, approximately 2 kilometers (1.2 miles) in diameter. It was discovered on 11 April 1999, by astronomers of the Lowell Observatory Near-Earth Object Search at Anderson Mesa Station near Flagstaff, Arizona.^[4] The asteroid was named after the deity Zephyrus from Greek mythology.^[3]

12923 Zephyr

Discovery[1]
Discovered by	LONEOS
Discovery site	Anderson Mesa Stn.
Discovery date	11 April 1999
Designations
MPC designation	(12923) Zephyr
Pronunciation	/ˈzɛfər/[2]
Named after	Zephyrus[3] (Greek mythology)
Alternative designations	1999 GK4
Minor planet category	NEO Apollo[1][4] PHA[1][4]
Orbital characteristics[1]
Epoch 4 September 2017 (JD 2458000.5)
Uncertainty parameter 0
Observation arc	61.75 yr (22,554 days)
Earliest precovery date	21 April 1955
Aphelion	2.9267 AU
Perihelion	0.9964 AU
Semi-major axis	1.9615 AU
Eccentricity	0.4920
Orbital period (sidereal)	2.75 yr (1,003 days)
Mean anomaly	199.23°
Mean motion	0° 21m 31.68s / day
Inclination	5.3045°
Longitude of ascending node	168.21°
Argument of perihelion	147.06°
Earth MOID	0.0211 AU (8.2 LD)
Physical characteristics
Mean diameter	1.86±0.46 km[5] 2.060±0.013 km[6][7] 2.062 km[8]
Synodic rotation period	3.891 h[a]
Geometric albedo	0.1764[8] 0.199±0.034[6][7] 0.20±0.16[9] 0.21±0.17[5]
Spectral type	SMASS = S:[1] S[10][11]
Absolute magnitude (H)	15.40±0.1 (R)[a] 15.77±0.24[12] 15.8[1][7] 15.93±0.078[8][11]

Orbit and classification

Zephyr orbits the Sun at a distance of 1.0–2.9 AU once every 2 years and 9 months (1,003 days). Its orbit has an eccentricity of 0.49 and an inclination of 5° with respect to the ecliptic.^[1] The body's observation arc begins with a precovery taken at Palomar Observatory in April 1955, almost 44 years prior to its official discovery observation at Anderson Mesa.^[4]

Close encounters

This near-Earth asteroid has an Earth minimum orbital intersection distance of 0.0211 AU (3,160,000 km), which corresponds to 8.2 lunar distances.^[1] This short distance as well as its sufficiently large size makes it a potentially hazardous asteroid. In September 2010, the asteroid approached Earth at 0.2546 AU (38,100,000 km); it will make close encounters with Earth again in 2021, 2032 and 2043.^[1]

Naming

This minor planet was named after the god of the west wind, Zephyrus, from Greek mythology. The name was suggested by M. Smitherman.^[3] The official naming citation was published by the Minor Planet Center on 28 September 2004 (M.P.C. 52768).^[13]

Physical characteristics

In the SMASS classification, Zephyr is a common S-type asteroid.^[1] The body is also characterized as a stony asteroid by the Infrared Telescope Facility,^[10] and in the Tholen classification (noisy spectrum).^[11]

Rotation period

In April 1999, a rotational lightcurve of Zephyr was obtained from photometric observations by Czech astronomer Petr Pravec at Ondřejov Observatory. Lightcurve analysis gave a well-defined rotation period of 3.891 hours with a brightness amplitude of 0.18 magnitude (U=3).^[a]

Diameter and albedo

According to the survey carried out by the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer and the ExploreNEOs survey of the Spitzer Space Telescope, Zephyr measures between 1.86 and 2.062 kilometers in diameter and its surface has an albedo between 0.1764 and 0.21.^{[5][6][7][8][9]}

The Collaborative Asteroid Lightcurve Link adopts Petr Pravec's revised WISE data, with albedo of 0.1764 and a diameter of 2.06 kilometers based on an absolute magnitude of 15.93.^[11]

Notes

1. Pravec (1999) web: rotation period 3.891 hours with a brightness amplitude of 0.18 mag. Summary figures for (12923) Zephyr at LCDB and Ondrejov Asteroid Photometry Project (data sheet)