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(66391) 1999 KW4
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(66391) 1999 KW4

(66391) 1999 KW4
When May 20 1999
Alternate Designation(s)  
Orbital Characteristics
Category Aten asteroid,
Mercury-crosser asteroid,
Venus-crosser asteroid
Semi-Major Axis 96.085 Gm (0.642 AU)
Perihelion 29.943 Gm (0.200 AU)
Aphelion 162.228 Gm (1.084 AU)
Eccentricity 0.688
Revolution Period 188.017 d (0.51 yr)
Inclination 38.890°
Mean Orbital Speed 37.16 km/s
Physical Characteristics
Dimensions 1.2 km
Mass 2.33×1012 kg
Density 2.6 g/cm³
Surface Gravity 0.000 43 m/s²
Escape Velocity 0.000 72 km/s
Rotation Period 0.1152 d
Spectral Class ?
Absolute Magnitude 16.5
Albedo 0.3
Mean Surface Temperature ~449 K
(66391) 1999 KW4 is an Aten and Mercury-crosser asteroid discovered by LINEAR in 1999. It is interesting for two reasons.

First, 1999 KW4 is one of the few asteroids whose orbit crosses that of Mercury, the innermost planet (another is 1566 Icarus). Its unusual, eccentric orbit suggests that the asteroid may in fact be a comet that has lost its surface ice and can no longer produce a tail.

Second, despite its small size (about 1.2 km in diameter), 1999 KW4 has a small moon orbiting it. The moon, designated S/2001 (66391) 1 is only 360 m in diameter, and orbits 1999 KW4 in 0.758 d (16 hours) at a distance of 2.6 km and a leisurely speed of 0.25 m/s (0.9 km/h). The companion was first suggested by an apparent eclipsing binary signal in photometric observations made June 19-27, 2000 by Petr Pravec and Lenka Šarounová at Ondrejov Observatory. It was confirmed by radar observations from Arecibo Observatory 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, announced on May 23, 2001.


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