|Discovery site||Lincoln Lab's ETS|
|Discovery date||20 May 1999|
|Aten · NEO · PHA |
|Orbital characteristics |
|Epoch 4 September 2017 (JD 2458000.5)|
|Uncertainty parameter 0|
|Observation arc||19.01 yr (6,942 days)|
|0.51 yr (188 days)|
|1° 54m 54s / day|
|Earth MOID||0.0138 AU · 5.4 LD|
|Dimensions||1.532 km × 1.495 km × 1.347 km|
|SMASS = S: · S |
(66391) 1999 KW4, provisional designation 1999 KW4, is a binary asteroid, classified as a near-Earth object and potentially hazardous asteroid of the Aten group, approximately 1.3 kilometers in diameter. It was discovered on 20 May 1999, by Lincoln Near-Earth Asteroid Research (LINEAR) at the Lincoln Laboratory's Experimental Test Site in Socorro, New Mexico, United States. It is also a Mercury-crosser and the closest known binary system to the Sun with a perihelion of just 0.2 AU.
The asteroid orbits the Sun at a distance of 0.2–1.1 AU once every 6.18 months (188 days). Its orbit has an eccentricity of 0.69 and an inclination of 39° with respect to the ecliptic. A first precovery was taken by 2MASS at the Fred Lawrence Whipple Observatory in 1998, extending the body's observation arc by one year prior to its official discovery observation at Socorro.
As a potentially hazardous asteroid, it has an Earth minimum orbital intersection distance of 0.0138 AU (2,060,000 km) which corresponds to 5.4 lunar distances. On 25 May 2036, it will pass 0.0155 AU (2,320,000 km) from Earth.
1999 KW4 has a minor-planet moon orbiting it. The moon, designated S/2001 (66391) 1 is approximately 360 metres in diameter, and orbits its primary in every 16 hours at a mean-distance of 2.6 kilometers. The presence of a companion was suggested by photometric observations made by Pravec and Šarounová and was confirmed by radar observations from Arecibo observations and announced on 23 May 2001 (also see below).
According to radiometric observations from Arecibo Observatory, the asteroid has a mean diameter of 1.317 kilometers. The observations were taken from May 21–23, 2001, by Lance A. M. Benner, Steven J. Ostro, Jon D. Giorgini, Raymond F. Jurgens, Jean-Luc Margot and Michael C. Nolan.
The shapes of the two bodies and their dynamics are complex. With a dimension of 1.5 x 1.5 x 1.34 kilometers for a simple triaxial ellipsoid, the asteroid has an oblate shape, which is dominated by an equatorial ridge at the body's potential-energy minimum. This bizarre property of the equatorial region means that it is close to breakup: raising a particle a meter above the surface would put it into orbit. As seen in the image at above right, the gravitational effects between the moon and the asteroid create a gigantic mountain extending in the equatorial plane around the entire asteroid. It was the first asteroid to be described as "muffin-shaped", which is now understood to be a very common shape for asteroids in critical rotation, including 101955 Bennu and 162173 Ryugu.
During 19–27 June 2000, a rotational lightcurve of this asteroid was obtained from photometric observations by Petr Pravec and Lenka Šarounová at Ondřejov Observatory. Lightcurve analysis gave a rotation period of 2.7650 hours with a brightness variation of 0.12 magnitude (U=3).
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