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Simulated animation of 1999 KW4's rotation and moon.
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Discovery
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| Discovered by | LINEAR |
| Discovery date | May 20, 1999 |
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Designations
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| Alternate name(s) | none |
| Minor planet category |
Aten asteroid, Mercury-crosser asteroid, Venus-crosser asteroid |
| Epoch July 14, 2004 (JD 2453200.5) | |
| Aphelion | 162.228 Gm (1.084 AU) |
| Perihelion | 29.943 Gm (0.200 AU) |
| Semi-major axis | 96.085 Gm (0.642 AU) |
| Eccentricity | 0.688 |
| Orbital period | 188.017 d (0.51 a) |
| Average orbital speed | 37.16 km/s |
| Mean anomaly | 168.533° |
| Inclination | 38.890° |
| Longitude of ascending node | 244.934° |
| Argument of perihelion | 192.597° |
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Physical characteristics
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| Dimensions | 1.5 x 1.5 x 1.34 km (primary) |
| Mass | 2.4×1012 kg |
| Mean density | 2.0 g/cm³ |
| Equatorial surface gravity | 0 - 0.000 36 m/s² (variable) |
| Escape velocity | 0.000 72 km/s |
| Rotation period | 0.1152 d |
| Albedo | 0.15 |
| Temperature | 250 - 600 K |
| Spectral type | ? |
| Absolute magnitude (H) | 16.5 |
(66391) 1999 KW4 (also written (66391) 1999 KW4) is an Aten and Mercury-crosser asteroid discovered by LINEAR in 1999.
1999 KW4 has a moon orbiting it. The moon, designated S/2001 (66391) 1 or '1999 KW4 Beta' is ~360 m in diameter, and orbits 1999 KW4 'Alpha' in 0.758 d (16 hours) at a distance of 2.6 km. The presence of a companion was suggested by photometric observations made June 19-27, 2000 by Petr Pravec and Lenka Šarounová at Observatoř Ondřejov (Ondřejov Observatory) and 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.
The shapes of the two bodies and their dynamics are complex.[1] Among other bizarre properties, the equatorial regions of Alpha are very close to breakup: raising a particle a meter above the surface would put it into orbit around the object.
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